Patent Application
20060246460
The present invention relates to methods and materials for detecting or identifying particular nucleic acid sequences in a sample using modified molecular beacons.
The ability to detect specific DNA sequences or individual DNA bases within a sequenced genome is key to exploiting the data provided from the sequencing. This data can be used in a number of ways including monitoring gene expression and evaluation or diagnosis of specific disease states including infectious and hereditary disease.
In recent years a number of ingenious methods for detecting specific DNA sequences have been reported. The most widely used are based on fluorescence detection during PCR amplification and are closed tube homogenous assays. These include Molecular Beacons,1 Taqman,2 Scorpions3 and Hybridisation Probes. 4 Whilst widely accepted as being the best currently available, these methods rely solely on fluorescence detection which has it's own advantages and disadvantages.
The advantages are that it is a widely accepted and used detection technique, the addition of the labels has been well established and instrumentation and designer probes are commercially available. The main disadvantages are that due to the detection limit of fluorescence a large number of PCR cycles need to be used to generate sufficient numbers of the target and that the homogeneous multiplexing is at best four and three if an internal standard used. One of the problems in the use of quenched probes such as Molecular Beacons is that there is always background fluorescence due to the quenching of the fluorophore being less than 100%. This reduces the signal to noise ratio and hence the sensitivity.
In an attempt to overcome this problem Dubertret et al. used a gold nanoparticle attached to the 3′-end of a Molecular Beacon via a phosphine ligand with a 5′-FAM label (in which fluorescein was linked via an amide label). 5
In that study the quenching was improved due to the presence of the gold nanoparticle. The probes used could also detect a single base mismatch at room temperature.
It will be clear from the above that novel formats for detecting or identifying particular nucleic acid sequences in a sample, particularly those which have one or more advantages over those in current use, would provide a contribution to the art.
The present inventors have devised a new class of biomolecular probe which can be used in assays which are flexible, capable of single base resolution, and have a massive capacity for “one pot” multiplexing. In preferred embodiments the assays combine the detection techniques of fluorescence and surface enhanced Raman scattering (SERS) to provide an ultra sensitive probe.
As is known in the art, a Raman spectrum arises because light incident on an analyte is scattered due to nuclear motion and excitation of electrons in the analyte. Where the analyte whose spectrum is being recorded is closely associated with an appropriate surface, such as a roughened metal surface, this leads to a large increase in detection sensitivity, the effect being more marked the closer the analyte sits to the “active” surface (the optimum position is in the first molecular layer around the surface, i.e., within about 2 nm of the surface). This is termed ‘SERS’.
A further increase in sensitivity can be obtained by operating at the resonance frequency of the analyte (in this case usually a dye attached to the target of interest). Use of a coherent light source, tuned to the absorbance maximum of the dye, gives rise to a 103-105-fold increase in sensitivity (the laser excitation may also be set to the maximum of the surface plasmon resonance, which may or may not coincide with the dye maxima). The surface enhancement effect and the resonance effect may be combined to give SERRS and a range of excitation frequencies will still give a combined enhancement effect.
Thus the technique of SERRS provides a vibrational fingerprint of the analyte when two conditions are met. These are (i) the adsorption onto a suitable metal surface and (ii) the presence of a visible chromophore6,7. The use of a metal additionally means that fluorescence is efficiently quenched.7 This means that a large range of coloured molecules, including standard fluorophores, give excellent SERRS signals.
Since SERRS is preferred, in general this term will be used herein for brevity. However it will be understood the invention can also be practised with SERS if that is desired, for example when minimising background fluorescence by using an excitation frequency in the infra-red region.
The SERRS Beacon is a dual labelled probe with a different dye at each of its two ends. In conventional Beacons a quencher such as DABCYL is used with a dye. In the present invention, one of the dyes is specifically designed such that it is capable of immobilising the oligonucleotide probe onto an appropriate metal surface. In use, the SERRS Beacon is immobilised in the “closed state” on the metal surface, and this has the effect that due to the closeness to the surface of the coloured species a SERRS spectrum corresponding to both dyes is detectable.
When the complementary sequence hybridises, the SERRS Beacon opens up and one of the dyes is removed from the surface—this causes the SERRS signals to change to show only the dye on the surface, not the other dye. The wide combination of different dyes offers a massive coding potential for simultaneous multiplexed analysis of DNA sequences.
A preferred embodiment of the invention is shown in
In preferred embodiments of the invention, a “binary code” can be constructed by using combinations of fluorophores and dyes. The same fluorophore can be used with different dyes, as can different fluorophores with the same dye or two or more different dyes.
SERS has previously been applied in a number of nucleic acid detection formats (see e.g. U.S. Pat. No. 5,721,102 (Vo Dinh et al); WO 97/05280 (University of Strathclyde; and WO 99/60157 (Zeneca)). However its application to molecular beacon technology, and the advantages discussed above had not previously been recognised.
Thus in a first aspect of the present invention there is disclosed a method for determining the presence or absence of a target nucleotide sequence in a sample nucleic acid, the method comprising:
(a) providing a detection agent, which agent comprises:
As with other molecular beacons, the stem duplex has a melting temperature above the detection temperature under the assay conditions. Hybridisation of the TCS to any target sequence present in the sample causes the separation of the first and second oligonucleotide arms of the duplex. In the present case this causes the second label moiety to no longer be in close proximity to the enhancing surface. The reduction in surface enhancement resulting from reduced proximity to the enhancing surface results in a change of the SERS spectrum of second label moiety, and hence the overall SERS spectrum.
By “terminating” is meant that the label moiety is at or sufficiently close to the terminus of the arm, such that the arm does not sterically prevent the label moiety approaching the metal surface when the probe is in its “closed” state. Generally it is preferred that the labels are at the actual termini of the arms, although this is not a necessity—in particular where the first label moiety is detectable by fluorescence, including Surface Enhanced Fluorescence (SEF—discussed in more detail hereinafter) it may be preferred that it is present internally within the arm of the agent.
In a preferred embodiment, the first label moiety is detectable by both fluorescence and SERS. In this case the reduced proximity to the enhancing surface also causes an increase in fluorescence of said first label moiety resulting from reduced quenching of the fluorescence by the enhancing surface and\or proximity of the second label moiety.
Thus in this embodiment step (c), which is detection of hybridisation of the TCS to any target sequence present in the sample, can additionally or alternately be carried out by detecting a change in the fluorescence of the agent.
The exposure of the sample to the agent can take any form which brings the two into sufficient contact to allow binding of the agent to the target sequence of the sample. Generally this will be mixing of solutions of these components.
Some embodiments and aspects of the present invention will now be discussed in more detail.
Target and Sample Nucleic Acids
The “sample nucleic acid” can be any nucleic acid, including DNA (from any source e.g. genomic, cDNA, synthetic etc.), RNA (e.g. mRNA, tRNA, rRNA, synthetic etc.) or derivatives of these. Generally it will be at least 10 nucleotides in length, more preferably at least 20, 30, 40, 50, 100 or 200 nucleotides in length. The sample can represent all or only some of the nucleic acid present in a given source. The sample may be prepared prior to testing in order to make the sample nucleic acid therein more available for the testing process. For instance the sample nucleic acid may be fully or partially purified and/or fragments may be produced and separated. As an alternative to, or in addition to, using the nucleic acid in the sample directly, copies may be prepared and used (e.g. using PCR). The term “sample nucleic acid” covers all of these possibilities.
Generally the sample nucleic acid will be prepared as single strand nucleic acid prior to the sequence detection.
If desired the sample may be blotted, tethered or otherwise immobilised on a solid phase, optionally in the form of an array (e.g. a so called nucleic acid chip—see e.g. Marshall & Hodgson (1998) Nature Biotechnology 16: 27-31).
The “target” sequence itself may be any sequence of any length within the sample which it is desired to investigate. Thus it may be any sequence found in a genome, or subgenomic nucleic acid, chromosome, extrachromasomal vector, or gene, or motif, or non-coding sequence, or a sequence tagged site, or expressed sequence tag. The sequence may be derived from any source e.g. published material on a database.
The sequence may be unique within a given genome, or may have multiple occurrences within it (the methods of the present invention may be used to determine its frequency of occurrence). Likewise the sequence may be unique to a particular individual, or population, or species, genus, family etc. or be present within more than one of these groupings. The length of the target sequence may be selected on the basis of its statistical likelihood of chance occurrence within a given size of genome. For instance it has been suggested that a sequence of up to 16 bases in yeast, and a few more in humans (e.g. 17-24), may be sufficient to indicate a unique sequence in these organisms.
Particularly envisaged is the detection of nucleic acid “variants”. These may include single nucleotide variants (mutations or polymorphisms) or variable number tandem repeats, or other satellite or microsatellite repeats. Thus the target sequence in these cases may be characterised by only a single base, or numbers of pairs of bases, within a given longer sequence.
As set out in more detail below, it may be desirable to probe several target sequences simultaneously using appropriate, distinctive agents.
Target Complement Sequence
The TCS of the agent will generally be based on a nucleic acid (DNA or RNA) or modified nucleic acid, or nucleic acid analog, which is complementary to all or part of the target sequence. Under certain circumstances (when the TCS is not being synthesised to order for instance) it may not be necessary to know its sequence. For instance nucleic acid may be taken from a (known) source, cleaved, and the cleaved portions can be used to prepare the detection agent of the present invention. Thus the target (original source) is predetermined, even if the sequence is not established.
By “complementary” is meant capable of specific base pairing with the target sequence whereby A is the complement of T (and U); G is the complement of C. Generally complementary nucleic acids run anti-parallel i.e. one runs 5′ to 3′, while the other 3′ to 5′. Where modified nucleic acid, or nucleic acid analog is used, the base pairing is between corresponding modified or analog bases and the complementary target sequence as appropriate.
It is known that normally nucleic acid hybridisation conditions require the presence of salt to prevent the repulsion of the negative phosphate backbones. However if it is preferred not to add salt, this may be avoided by use of modified nucleic acid, or use of a nucleic acid analog. For instance it is known that DNA forms which are neutral, or at least zwitterionic, do not require high salt concentrations for hybridisation to occur. One possible example of this is propargyl amino modified base as described by Cruickshank & Stockwell (1988) Tetrahedron Letters 29: 5221-5224, and later in Graham et al (1997) Anal Chem 69: 4703-4707. This particular modification is also believed to promote greater specificity of base pairing (see Wagner et al (1993) Science 260: 1510-1513). In another embodiment peptide nucleic acid (PNA) is used for the probe which assemble the colloid.
The TCS can be any length appropriate to the target sequence (see above), but will generally be between from 10 to about 140 nucleotides, more preferable from 20 to 60 nucleotides.
In one embodiment the TCS are based on so called “siRNAs” which are typically between 19-23 nucleotides in length. The present invention may, for example, be used to detect the binding of siRNAs to the genes which they silence (see e.g. S. M. Elbashir, W. Lendeckel, T. Tuschl, “RNA interference is mediated by 21- and 22-nucleotide RNAs.” Genes Dev.,15, p188-200 (2001); S. M. Elbashir, J. Harborth, W. Lendeckel, A. Yalcin, K. Weber, T. Tuschl, “Duplexes of 21-nucleotide RNAs mediate RNA interference in cultured mammalian cells.” Nature, 411, p494-498 (2001)).
The Duplex
Preferably the stem duplex is between 3-25 nucleotides in length, more preferably between 6 and 12 nucleotides in length. Generally it will be formed by complement sequences (the first and second arms) which conform a unitary probe into a hairpin i.e. preferably the first arm will comprise a sequence of between 6-12 nucleotides immediately adjacent to its terminus that are complementary to nucleotides of the second arm sequence immediately adjacent to its terminus. Preferably the melting temperature of the duplex will be at least 5° C. above the chosen detection temperature (which detection temperature will generally be room temperature).
One of said arm sequences may be at least partially complementary to the target nucleic acid sequence.
Detection by SERS
This can be by conventional methods, for instance as disclosed in WO 97/05280 (University of Strathclyde).
Thus in SERS the primary measurements are of the intensity of the scattered light and the wavelengths of the emissions. Neither the angle of the incident beam nor the position of the detector is critical. With flat surfaces an incident laser beam is often positioned to strike the surface at an angle of 600 with detection at either 90° or 180° to the incident beam. With colloidal suspensions detection can be at any angle to the incident beam, 90° again often being employed.
The detection step of the assay may comprise measuring the SERS spectra, and this measurement may be quantitative. As discussed above, in all aspects and embodiments of the invention it is preferred that the SERS is SERRS, and that the label moieties are appropriate for SERRS.
Several devices are suitable for collecting SERS signals, including wavelength selective mirrors, holographic optical elements for scattered light detection and fibre-optic waveguides. Gratings, prisms and filters can for example be used to discriminate between the different scattering frequencies. The intensity of a SERS signal can be measured using a charge coupled device (CCD), a silicon photodiode, or photomultiplier tubes arranged either singly or in series for cascade amplification of the signal. Photon counting electronics can be used for sensitive detection. The choice of detector will largely depend on the sensitivity of detection required to carry out a particular assay.
In one embodiment it may be preferred to collect the total Raman scattering using a filter and photo multiplier tube as opposed to a spectrometer.
When multiplexing (see below) a complex SERS spectrum across a range of wavelengths will generally be obtained. Although analysis by eye may be possible, methods for obtaining and/or analysing a SERS spectrum will preferably include the use of some form of data processor such as a computer. Methods for statistically analysing and resolving complex spectral data are well known to those skilled in the art and do not per se form part of the present invention. Such methods may, for example, utilise neural networks, partial least squares analysis, density function algorithms and so on (see e.g. S. D. Brown, “Chemical Measurements and Data Reduction from a Systems Analysis Perspective”, TrAC, 6 (10) 260-266 (1987); S. D. Brown, “Chemometrics”, Encyclopedia of Statistical Sciences, Update, Wiley-Interscience: New York, N.Y., Update Vol. 3, 77-87 (1998); S. D. Brown, “Chemometrics.” In Encyclopedia of Analytical Chemistry, R. A. Myers, Ed., Wiley Interscience, pp 9669-9671 (2000); K. L. Mello and S. D. Brown, “Novel ‘Hybrid’ Classification Method Employing Bayesian Networks,” J. Chemom., 13, 579-591 (1999); K. L. Mello and S. D. Brown, “Combining Recursive Partitioning and Uncertain Reasoning for Data Exploration and Characteristic Prediction,” Proceedings, AAAI Symposium on Predictive Toxicology, 119-122 (1999); K. L. Mello and S. D. Brown, “System for Discovering Implicit Relationships in Data and a Method of Using the Same,” U.S. Pat. No. 6,466,929, Issued Oct. 15, 2002; N. A. Woody and S. D. Brown, “Partial Least Squares Modeling of Continuous Nodes in Bayesian Networks,” Analytica Chimica Acta, 2003, 490, 355-63; H. W. Tan, C. R. Mittermayr, and S. D. Brown, “Robust Calibration with Respect to Background Variation,” Applied Spectrosc., 55, 827-33 (2001); R. N. Feudale, H.-W. Tan, S. D. Brown, “Piecewise Orthogonal Signal Correction,” Chemom. Intell. Lab. Syst., 63, 129-38 (2002)).
Detection by Fluorescence
Where the first label is a fluorophore, fluorescence detection may be used as an initial rapid screen (for example in a microtitre plate to find the ‘hot’ wells). Samples giving a positive fluorescence in the initial screen can then be interrogated by SERS.
The nature of the present invention is such that phenomenon of surface enhanced fluorescence (SEF) may be used to detect the probe in its “open” conformation. SEF requires appropriate spacing (e.g. c. 60 angstrom depending on environmental factors) of the fluorophore from the metal surface in order to demonstrate enhancement—see Strekal, N., Maskevich, A., Maskevich, S., Jardillier, J. C., Nabiev, I., Biopolymers 2000, 57, 325-328; Antunes, P. A., Constantino, C. J. L., Aroca, R. F., Duff, J., Langmuir 2001, 17, 2958-2964.
Thus the fluorophore may be derivatised at an appropriate point in the probe (not necessarily at the terminus of the first arm) so that once opened by hybridisation the fluorophore is a controlled distance away from the surface.
Label Moieties
As discussed in the examples below, it is preferable that the first label moiety (detectable by both fluorescence and SERS) is conjugated to the 5′ arm sequence and a second label moiety conjugated to the 3′ arm sequence. However the converse (i.e. fluorophore at the 3′ terminus may also be employed.
The label moieties may be linked through a spacer, e.g., an alkyl spacer, to said arms.
More than one first label moiety may be incorporated into the first arm. As discussed above, preferably the or each first label moiety is incorporated at or near the terminus of the first arm.
Examples of suitable SERS-active (more preferably, SERRS active) species include:
fluorescein dyes, such as 5- (and 6-) carboxy-4′,5′-dichloro-2′,7′-dimethoxy fluorescein, 5-carboxy-2′,4′,5′,7′-tetrachlorofluorescein and 5-carboxyfluorescein;
rhodamine dyes such as 5- (and 6-) carboxy rhodamine, 6-carboxytetramethyl rhodamine and 6-carboxyrhodamine X;
phthalocyanines such as methyl, nitrosyl, sulphonyl and amino phthalocyanines;
azo dyes such as those listed in C H Munro et al, Analyst (1995), 120, p993;
azomethines;
cyanines and xanthines such as the methyl, nitro, sulphano and amino derivatives; and
succinylfluoresceins.
As discussed in the Examples below, fluorescein is particularly efficiently quenched by the preferred SERS dyes of the present invention, such as those based on the azo-benzotriazoles (see below).
Each of these examples may be substituted in any conventional manner, giving rise to a large number of useful labels.
The choice of label in any given case will depend on factors such as the resonance frequency of the label, the other species present, label availability, choice or laser excitation equipment etc. It may be preferred that the second label moiety is an azo group, which can be very easily derivatised (see below). The skilled person will appreciate, however, that other SAS may also be readily employed in the invention.
The dye may be associated with the metal surface using either covalent or non-covalent interactions. Particular preferred is the use of SSGs as described above
SSGs
“Surface seeking groups” (SSGs) bind extremely tightly to metal surfaces, and are discussed in WO 97/05280 (University of Strathclyde). SSGs are generally either complexing or chelating in nature, or will comprise bridging ligands or polymer forming groups. The interaction between the SSG and the metal surface will typically be by chemisorption of the complex onto the surface, or by chemical bonding of the complex with a coating on the surface.
Naturally the choice of the SSG will depend on the nature of the surface (e.g. its charge and the presence or absence of an oxide or other layer) and of any surface coatings or other species (such as citrate reducing agents) associated with it, and also on the nature of the probe. For most useful surfaces, the functional group preferably comprises a Lewis base. Ideally, it is actively attracted to the surface in use. For gold surfaces phosphorus and sulphur containing groups, along with soft nitrogen ligands, may be particularly preferred.
Thus suitable groups by which the agent may be bound to the active surface include complexing groups such as nitrogen, oxygen, sulphur and phosphorous donors; chelating groups; bridging ligands and polymer forming ligands.
SSGs—Triazoles and Benzotriazoles
The triazole group (Formula A1) is rich in nitrogen lone pairs and seems to have a particular affinity for certain metal colloids. Thus, incorporation of this group in the agent is particularly preferred, since it can increase the proximity of the label to the surface, and thereby the surface enhancement effect which occurs when the probe binds the target sequence.
Preferably, the agent incorporates a benzotriazole (BT) group (Formula A2), particularly when the metal surface is silver- or copper-based. BT groups have a high degree of conjugation (especially when deprotonated) and are thus particularly amenable to SERRS detection which relies on label resonance.
Benzotriazole derivatives (such as those shown in Formula A3) may be readily obtained and can be coupled with existing labels (such as azo dyes) to give appropriately unitary label moieties\SSGs.
In preferred forms, the SSG is modified to be SERRS active and this is used to conjugate the probe to the metal surface. Examples of such groups include azo-benzotriazoles (Formula A4) and substituted derivatives thereof, typically formed by combining azo substrates with benzotriazole derivatives. Examples of suitable azo-benzotriazoles include 4-, 5-, 6-, and 7-azo-1H-benzotriazoles, and substituted derivatives thereof. The compounds comprise an azo chromophore which increases the wavelength of the absorbance maximum of the label.
In one embodiment, the compound is selected from compounds of the following formula:
wherein:
RAZ is independently an azo substituent;
n is independently 0, 1, 2, or 3; and,
each RB is independently a benzo substituent.
In one embodiment, the compound is selected from compounds of the following formulae:
wherein:
RAZ is independently an azo substituent; and,
each of RB4, RB5, RB6, and RB7 is independently —H or a benzo substituent.
In one embodiment, the compound is selected from compounds of Formula A5-3 and Formula A5-4.
In one embodiment, the compound is selected from compounds of Formula A5-3 (“6-position” azo)
In a preferred embodiment, the compound is selected from compounds of Formula A5-4 (“7-position” azo).
Probes and Linkers
In all cases, the compound is linked to the probe, e.g., a nucleotide arm of the probe, either directly or via a linker (e.g., an alkyl linker). See, e.g., below under the heading “Synthesis.”
Generally a linker of less than 5, 10 or 15 carbons in length will be preferred. Different linkers (e.g., different lengths of linkers) can also provide the agents with molecularly specific SERRS spectra as described below.
For example, one of the benzo substituents (RB, e.g., RB4, RB5, RB6, RB7) or one of the azo substituents (RAZ) (e.g., one of the phenyl substituents (RP, e.g., RP2, RP3, RP4, RP5, RP6) or one of the naphthyl substituents (e.g., RN1, RN2, RN3, RN4, RN5, RN6, RN7, RN8)) is, or comprises, a probe or a linker linked to a probe.
In one embodiment, one of the benzo substituents (RB, e.g., RB4, RB5, RB6, RB7) is, or comprises, a probe or a linker linked to a probe.
Alternatives for the remaining substituents, i.e., other than the one which is, or comprises a probe or a linker linked to a probe, are set out below.
The Benzo Substituents (RB)
Each of the benzo substituents (RB, RB4, RB5, RB6, RB7) is independently —H or as defined below for the substituents on the phenyl/naphthyl azo substituent, RP and RN.
In one embodiment, one of the n RB groups, or one of RB4, RB5, RB6, and RB7 is an amino group (—NH2) or a maleimido group.
In one embodiment, one of the n RB groups, or one of RB4, RB5, RB6, and RB7 is an amino group (—NH2) or a maleimido group; and the remaining groups are —H.
In a preferred embodiment RB6 is an amino group (—NH2) or a maleimido group, which can be derivatised to a linker or nucleic acid, for example by standard Diels Alder or other amide linkage chemistry.
The Azo Substituent (RAZ)
The azo substituent, RAZ, is independently C6-20carboaryl or C5-20heteroaryl, and is optionally substituted.
In one embodiment, RAZ, is independently monocyclic C6carboaryl or C5-7heteroaryl, and is optionally substituted.
In one embodiment, RAZ, is independently bicyclic C9-10carboaryl or C8-14heteroaryl, and is optionally substituted.
In one embodiment, RAZ, is independently bicyclic C10carboaryl or C9-10heteroaryl.
In one embodiment, RAZ, is a group of the formula:
wherein:
each of RP2, RP3, RP4, RP5, and RP6 is independently —H or a phenyl substituent;
and optionally, two adjacent groups selected from RP2, RP3, RP4, RP5, and RP6 (e.g. RP2 & RP3, RP3 & RP4, RP4 & RP5, RP5 & RP6), together with the carbon atoms to which they are attached, form a fused ring (fused to the phenyl ring), having 5 or 6 ring atoms, including one or more heteroatoms selected from O, S, and N.
For example, in one embodiment, RAZ, is a group of the formula:
wherein:
each of RH2, RH3, and RH4 is independently —H or as defined below for the substituents on the phenyl/naphthyl azo substituent, RP and RN; and, each of W, Y, Y, and Z is independently —CH═, —CR═, —N═, —O—, or —S—.
In one embodiment, RAZ is independently phenyl, naphth-1-yl, or naphth-2-yl, and is optionally substituted.
In one embodiment, RAZ, is a group of one of the following formulae:
wherein:
each of RP2, RP3, RP3, RP5, and RP6 is independently —H or a phenyl substituent; and,
each of RN1, RN2, RN3, RN4, RN5, RN6, RN7, and RN8 is independently —H or a naphthyl substituent.
The Substituents on the Phenyl/Naphthyl Azo Substituent (RP and RN)
In one embodiment, each of RP2, RP3, RP4, RP5, and RP6, and each of RN1, RN2, RN3, RN4, RN5, RN6, RN7, and RN8 is independently —H or a group selected from:
C1-7alkyl (e.g., -Me, -Et, -nPr, -iPr, -tBu);
C2-7alkenyl (e.g., vinyl, allyl);
C2-7alkynyl (e.g., propargyl);
C3-10cycloalkyl (e.g., cyclohexyl);
C3-10cycloalkenyl (e.g., cyclohexenyl);
C6-20carboaryl (e.g., phenyl, napthyl);
C5-20heteroaryl (e.g., pyridyl);
halogen (e.g., —F, —Cl, —Br, —I);
carboxy (—COOH);
formyl (—CHO);
acyl (—C(═O)R) (e.g., —C(═O)Me, —C(═O)Ph);
amino-acyl (—C(═O)NH2, —C(═O)NHR, —C(═O)NR2) (e.g., —C(═O)NHMe);
amino-C1-7alkyl (e.g., —(CH2)nNH2, —(CH2)nNHR, —(CH2)nNR2, n=1-6);
amino-methyl (—CH2NH2, —CH2NHR, —CH2NR2) (e.g., —CH2NHMe);
carboxy-C1-7alkyl (e.g., —(CH2)nCOOH, n=1-6);
carboxy-methyl (—CH2COOH);
hydroxy-C1-7alkyl (e.g., —(CH2)nOH, n=1-6);
hydroxy-methyl (—CH2OH);
oxy-C1-7alkyl (e.g., —(CH2)nOR, n=1-6);
oxy-methyl (—CH2OR) (e.g., —CH2OMe);
halo-methyl (—CH2X, —CHX2, —CX3) (e.g., —CH2F, —CF3);
amino (—NH2, —NHR, —NR2) (e.g., —NMe2);
cycloamino (e.g., morpholino, piperidino, piperazino);
azido (—N3);
nitroso (—NO);
nitro (—NO2);
ethynyl (—C≡CH, —C≡CR) (e.g., —C≡CH, —C≡CMe);
imino-methyl (—CH═NH, —CH═NR) (e.g., —CH═NMe);
ketone-oxime (—C═N(OH)R);
cycloalkyl (—CHR(—(CH2)n—), n=2-7) (e.g., 1-methyl-cyclohex-1-yl);
urea (—NHCONHR) (e.g., —NHCONH2, —NHCONHMe);
thiourea (—NHCSNHR) (e.g., —NHCSNH2, —NHCSNHMe);
acyl-amino (—NHCOR) (e.g., —NHCOMe);
amino (—NH2, —NHR, —NR2) (e.g., —NH2, —NHMe, —NMe2);
hydroxy (—OH);
ether (—OR) (e.g., —OMe, —OPh);
phosphate (—OP(═O)(OR)2) (e.g., —OP((═O) (OH)2);
silyl (—SiR3) (e.g., —SiMe3, —SiPh3);
sulfhydryl (—SH);
thioether (—SR) (e.g., —SMe);
disulfide (—SSR) (e.g., —SSMe);
sulfonic acid (—SO3H);
selenide (—SeR) (e.g., —SeMe);
stannyl (—SnR3) (e.g., —SnMe3, —SnPh3);
plumbyl (—PbR3) (e.g., —PbMe3, —PbPh3);
wherein X is —F, —Cl, —Br, or —I; and,
wherein each R is independently H, C1-7-alkyl, C2-7alkenyl, C2-7alkynyl, C3-10cycloalkyl, C3-10cycloalkenyl, C6-20carboaryl, or C5-20heteroaryl.
In one embodiment, each of RP2, RP3, RP4, RP5, and RP6, and each of RN1, RN2, RN3, RN4, RN5, RN6, RN7, and RN8 is independently —H or a group selected from: hydrogen; hydroxy; C1-4alkoxy; amino; C1-4alkyl-amino; nitro; cyano; C1-7alkyl; C3-10cycloalkyl; C6carboaryl; C6heteroaryl; halogen; carboxy; sulfonate; phosphate; and sulfhydryl.
In one embodiment, each of RP2, RP3, RP4, RP5, and RP6, and each of RN1, RN2, RN3, RN4, RN5, RN6, RN7, and RN8 is independently —H or a group selected from: hydrogen; hydroxy; C1-4alkoxy; amino; C1-4alkyl-amino; nitro; and cyano.
In one embodiment, each of RP2, RP3, RP4, RP5, and RP6, and each of RN1, RN2, RN3, RN4, RN5, RN6, RN7, and RN8 is independently selected from: —H, —OH, —OMe, —NH2, —NO2, and —CN.
In one embodiment, each of RP2, RP3, RP4, RP5, and RP6, and each of RN1, RN2, RN3, RN4, RN5, RN6, RN7, and RN8 is independently selected from: —H, —OH, —OMe, and —NH2.
Phenyl-Azo-1H-Benzotriazoles
In one embodiment, RAZ, is a group formula (A8-1).
In one embodiment, exactly 4 of RP2, RP3, RP4, RP5, and RP6 are —H.
In one embodiment, exactly 3 of RP2, RP3, RP4, RP5, and RP6 are —H.
In one embodiment, exactly 2 of RP2, RP3, RP4, RP5, and RP6 are —H.
In one embodiment, exactly 4 of RP2, RP3, RP4, RP5, and RP6 are —H; and the remaining 1 is —OMe.
In one embodiment, exactly 2 of RP2, RP3, RP4, RP5, and RP6 are —H; and each of the remaining 2 is —OMe.
In one embodiment, RAZ, is selected from:
In one preferred embodiment, RP3 and RP5 are both —OMe; and, RP2, RP4, and RP6 are —H:
Naphthyl-Azo-1H-Benzotriazoles
In one embodiment, RAZ, is a group formula (A8-2) or formula (A8-3).
In one embodiment, RAZ, is a group formula (A8-2).
In one embodiment, exactly 6 of RN2, RN3, RN4, RN5, RN6, RN7, and RN8 are —H.
In one embodiment, exactly 5 of RN2, RN3, RN4, RN5, RN6, RN7, and RN8 are —H.
In one embodiment, exactly 6 of RN1, RN3, RN4, RN5, RN6, RN7, and RN8 are —H.
In one embodiment, exactly 5 of RN1, RN3, RN4, RN5, RN6, RN7, and RN8 are —H.
In one embodiment, exactly 6 of RN1, RN3, RN4, RN5, RN6, RN7, and RN8 are —H; and the remaining 1 is —OMe, —CN, or —NO2.
In one embodiment, exactly 6 of RN1, RN3, RN4, RN5, RN6, RN7, and RN8 are —H; and the remaining 1 is —OMe.
In one embodiment, RAZ, is selected from:
Some Preferred Compounds
A preferred compound (before being linked to the probe) is as follows:
In this and similar compounds, a methoxy group is preferred to nitro or nitrile groups (being an electron-donating group versus electron-withdrawing groups) and dimethoxy is preferred to monomethoxy. 3′,5′-substitution is preferred to 2′,4′-substitution for the disubstituted ring system. This dye has the amine group for facilitating linkage to a probe (see below).
Synthesis
Appropriate chemistry to derivatise the fluorophore and/or dye to the probe will be well known to those skilled in the art and does not per se form a part of the present invention.
The dyes and SSGs of the present invention may also be conjugated to the second arm using other standard chemistry, e.g., addition of a maleimide dervivative to a butadiene phosphoramidite group. For example, the synthesis of the SERRS Beacon is illustrated in Scheme 1 (
Alternative SSGs
An alternative to the triazole- and benzotriazole-based SSGs is shown in Formula A12, wherein R9 may be a substituent as defined for R10 below, or is, or comprises, a probe or a linker linked to a probe.
Functional groups on the surface seeking portion of the agents may include charged polar groups (e.g., amine, carboxyl, phosphate, thiol, hydroxyl), attracted to the surface or surface coating (e.g., to free amine groups in a polyamine coating).
Examples of these are shown in Formula A13, wherein R9 is as defined above, and each R10 is independently selected from —H and —(CH2)n-Q, wherein Q is —COOH, —PPh2, —SH, —NH2, or —OH, and n is 0, 1, 2, 3, or 4, with no more than 3 of the R10 groups in the formula being —H. Preferably the R10 groups in the formula, other than those which are H, are all the same, as exemplified by Formulae A14 and A15.
Further alternative surface seeking groups (SSGs) are shown in Formulae A16, A17, and A18, in which X is N or C, and R11 is —Ph—N═N—RAZ or —CH2—Ph—N═N—RAZ:
Other suitable surface seeking groups for the agent include the calixerenes and the mercapto benzotriazoles.
Enhancing Surface
The “enhancing surface” is any surface suitable for carrying out SERS. A range of metal surfaces have been used in the art to provide the surface enhancement, including citrate reduced silver nanoparticles8,9 and a PVA/silver film.10 The silver/PVA films are robust, easily handled, and allow accurate spotting of the SERS Beacon due to the hydrophilic nature of the PVA surface preventing uncontrollable spreading of the spot.
Preferably the surface is provided by colloidal metal particles, or a solid surface i.e. the probes can be used both in solution state probes or in a surface array
Processes for Producing Agents
In a further aspect of the invention there is disclosed a method of producing a detection agent for use in a method as described herein comprising:
(i) providing a probe comprising
Preferably said first arm incorporate a first label moiety being detectable by fluorescence and surface enhanced Raman spectroscopy (SERS).
In a further aspect of the invention there is disclosed a method of producing a probe for use in a detection agent as described herein comprising:
(i) synthesising a nucleic acid comprising
Preferably first label moiety being is detectable by fluorescence and surface enhanced Raman spectroscopy (SERS)
The first and second label moieties may be any discussed herein e.g. the first label moiety may be FAM, and the second label moiety may be an azo benzotriazole (see e.g. Scheme 1).
The conjugation to the 5′ terminus may employ standard derivatisation chemistry e.g. phosphoramidite chemistry commonly used in nucleic acid synthesis and\or Diels Alder cycloaddition e.g. based on butadiene\maleimide addition (see refs 18-20).
In an alternate embodiment the 3′ terminus of the nucleic acid is tethered to a solid support via the second label moiety, and the first label moiety (being detectable SERS and optionally also by fluorescence) is added at the 5′ terminus. In this case it is preferred that a diene solid support is used to attach the first label moiety (e.g. benzotriazole dye) to the 3′ terminus.
Multiplexing
In preferred embodiments more than one target sequence is determined using multiple detection agents having distinguishable detection characteristics.
The fact that hybridisation of the beacon gives rise to two different signals (the SERRS of the second label moiety, without the influence of the first label moiety, and the fluorescence of the first label moiety) means it is particularly effective in multiplexing—by utilising combinations of different labels, or use of different labelling chemistries as discussed below.
Thus in one embodiment of the invention there is provided a method as described herein for determining the presence or absence of two or more target nucleic acid sequences in a sample nucleic acid, the method comprising:
providing a plurality of detection agents as described above, where each agent comprises a target complement sequence (TCS) complementary to one of said target sequences;
and wherein detection of hybridisation of each TCS to its corresponding target sequence present in the sample is detected by a change in the SERS spectra (and optionally fluorescence), of each agent,
which change is distinctive to each agent.
Thus in general the agents of the method not only have specific TCSs, but also have distinguishable (by SERS spectra and\or fluorescence) first label moieties, or second label moieties, or both first and second label moieties. As discussed herein, the present invention is particularly effective in multiplexing since the same fluorophore can be used with different dyes, as can different fluorophores with the same dye.
In particular, multiplexing capacity can be increased based on the chemistry of the preferred embodiments discussed herein. Thus (by way of example only), Scheme 1 shows a 3′ FAM was used with one benzotriazole azo dye. The nature of the 5′-labelling means that once the diene modified beacon sequence has been synthesised a range of different maleimide dye quenchers can be used to increase the multiplexing capacity of the SERRS Beacons due to the different SERRS signals from structurally similar dyes. A further degree of multiplexing can be obtained by changing the diene used to generate a different cycloadduct with the range of dye maleimides.22 Cycloadducts made from either different dyes and the same diene or different dienes and the same maleimide give distinctly different SERRS signals thus allowing a high degree of multiplexing by using only the one fluorescent dye. Obviously changing the identity of the fluorescent dye can increase this degree of multiplexing further as this also gives different SERRS codes.
In a further aspect of the invention there is disclosed a detection agent, as described herein, comprising:
(i) a probe comprising
In a further aspect there is disclosed a composition comprising two or more detection agents as described above, each having distinctive TCSs and distinctive first and\or second label moieties.
The agents or compositions of the present invention will generally be provided as solutions.
Further Aspects of the Invention
As discussed in the Introduction, the methods may have numerous applications in genomics, whereby they can be used analogously to existing methods which employ a step in which nucleic acid sequence is analysed (see e.g. “Principles of Genome Analysis” by S B Primrose, Pub. Blackwell Science, Oxford, UK, 1995).
Molecular Beacons have been used in a wide number of applications to date, mainly involving PCR (Piatek, A. S.; Tyagi, S.; Pol, A. C.; Telenti, A.; Miller, L. P.; Kramer, F. R.; Alland, D. Nat. Biotechnol. 199
8, 16, 359-363; Park, S.; Wong, M.; Marras, S. A. E.; Cross, E. W.; Kiehn, T. E.; Chaturvedi, V.; Tyagi, S.; Perlin, D. S. Journal of Clinical Microbiology 2000, 38, 2829-2836) but have also recently been used as intracellular probes for mRNA (Bratu, D. P.; Cha, B. J.; Mhlanga, M. M.; Kramer, F. R.; Tyagi, S. Proc. Natl. Acad. Sci. USA 2003, 100, 13308-13313).
Some specific applications are as follows. Generally speaking all of these can be carried out using the single target sequence, or multiplexing approach. In the latter case, the combination of various results may be used to make a determination:
(i) Detection of the presence of an organism (e.g. virus, provirus, virion, prokaryote (such as bacterium), eucaryote (such as protozoan)) in a sample wherein the presence of the target sequence is associated with the presence of the organism, for instance because the sequence is unique to that organism.
Even in cases where the sequence probed may not actually be unique to the organism, its presence (in conjunction with other diagnostic information e.g. immunological, behavioural etc.) may be used to increase the certainty of a determination of its presence of absence. The detection may be confirmed where still further certainty is required by full sequencing.
The sample in this case can be anything suspected of containing the organism e.g. a sample taken from a different organism, a foodstuff, an environmental sample (e.g. soil, water etc.) The organism may be pathogenic, or may simply be associated with some other quality of interest.
(ii) Diagnosis of a disease associated with a pathogenic organism, by carrying out a determination as described above. The sample may be in vitro or in vivo. The test may be carried out in conjunction with other diagnostic techniques, or an assessment of symptoms etc.
(iii) Diagnosis of a disease associated with a DNA variation, by detecting the presence of the DNA variant comprising use of a method as discussed above wherein the target sequence corresponds to the sequence in which the variation occurs. The test may be carried out in conjunction with other diagnostic techniques, or an assessment of symptoms etc.
(iv) A method of selecting an organism having a particular phenotypic trait whereby the target sequence corresponds to a sequence associated with that trait.
(v) A method of isolating a nucleic acid encoding a specific gene whereby the target sequence corresponds to a sequence associated with, or within, that gene.
(vi) A method of phylogenetic classification, wherein the target sequence is associated with a particular individual, population, species, genus etc.
(vii) A method of identifying an individual wherein the target sequence is associated with that individual. Generally speaking this may entail scoring a number of discrete polymorphisms (see e.g. WO 96/01687 of Tully et al for sequences used in forensic typing and matching).
(viii) A method of expression profiling a cell or tissue. In this case the sample nucleic acid is mRNA, or is derived from it (e.g. cDNA).
In a further aspect there is disclosed a kit comprising the agents or compositions of the present invention, plus one or more additional materials for practising the methods of the present invention e.g. target nucleic acid for control experiments.
In a further aspect there is disclosed a system comprising an agent or composition described above plus a nucleic acid sample, which is preferably a sample of DNA or RNA, most preferably extracted from a cell taken from or constituting an organism.
Such a system may particularly comprise:
(i) a reaction vessel,
(ii) an agent as described above,
(iii) a nucleic acid.
preferably in a homogenous format.
In a further aspect there is disclosed an apparatus comprising a SERRS analyser plus an agent, composition or system as described above, and methods of use of such an apparatus, comprising (for instance) the steps of preparing and monitoring (e.g. at between 500 and 600 nm) a homogenous system in order to detect a SERS signal.
The invention will now be further explained with reference to the following non-limiting Figures and Examples. Other embodiments falling within the scope of the present invention will occur to those skilled in the art in the light of these.
The SERRS Beacon requires a label at each terminus and this has affected the design of the probes. There are a number of commercially available fluorophores for 5′ labelling but we had to add our specifically designed surface complexing dye, which is easier at the 5′-terminus. So far we have developed methods for achieving this at the 5′-terminus via a number of methods18,20 and now favour Diels Alder cycloaddition.19 However, this meant that a 3′-fluorophore was required.
A 3′-FAM solid support was used to synthesise the molecular beacon sequence and a special 5′-butadiene monomer added at the 5′-terminus as the final addition during solid phase synthesis (see Scheme 1). The butadiene residue allows Diels Alder cycloaddition to be used to conjugate molecules such as labels to the end of the oligonucleotides in a convenient and high yielding manner.
We have a wide range of dyes that can be added to oligonucleotides in this way and chose the dye reported here (pABT) as in quenching experiments we found it to have a higher quenching constant than DABCYL when used with FAM. (97% versus 88%—see Example 4 below). The additional important property of this dye is the presence of the benzotriazole group. Previously we have demonstrated that benzotriazole azo dyes are excellent analytes for SERRS due to their ability to complex to the silver surface used to provide the surface enhancement.16,17
The use of Diels Alder allows the synthesis of the basic beacon but the option of varying the dye used for surface immobilisation.
In other embodiments, a solid support such as a diene solid support may be used to attach the benzotriazole dyes to the 3′-terminus, permitting use of the more widely available fluorophores for 5′-labelling.
As discussed above, in the SERRS Beacon a strong SERRS signal is generated when the beacon is closed that shows the presence of both the FAM and the pABT dye. This occurs as the pABT dye immobilises the SERRS Beacon on the nanoparticle and forces the FAM to be close to the surface and produce SERRS. In the absence of the surface seeking dye, a FAM labelled oligonucleotide would give poor SERRS as the FAM would not adsorb well onto the silver. In the SERRS Beacon the FAM signals actually dominate as FAM gives stronger signals than the pABT dye by approximately two orders of magnitude.
Under these conditions (2×10−7 M) we could not detect any fluorescence using a Cary Ecllipse fluorimeter, a Stratagene MX4000 fluorescence plate reader and a Renishaw Raman spectrometer.
A complementary sequence with overhanging bases was added to the SERRS Beacon and the SERRS recorded. The spectra clearly show a considerable change in the SERRS spectrum with the loss of the FAM signals allowing the pABT signals to be observed. In addition, fluorescence can now be observed with a maximum of 550 cm−1, which corresponds to 514.6 nm as expected for FAM when using excitation at 514.5 nm.
To investigate the sequence specificity two-control sequences were used. One was a nonsense sequence that was not expected to hybridise at all and the other was the complementary sequence as used to open the SERRS Beacon but with a single base mismatch.
The spectra shown in
To investigate different formats that could be used with the SERRS Beacon a silver/PVA surface was used as the substrate to give the additional quenching and generate the SERRS. The surface used was one that has been used in several SERRS experiments. For example use by Vo-Dinh et al.10 was as a surface for a SERRS Biochip and we have subsequently showed its use for the obtaining SERRS from benzotriazole dye labelled oligonucleotides.19 The silver/PVA film is robust, easily handled and allows accurate spotting of the SERRS Beacon due to the hydrophilic nature of the PVA surface preventing uncontrollable spreading of the spot. In this application, the surface was modified to contain the SERRS Beacon, which was added to the surface in a buffered solution and left for 20 minutes prior to washing with water several times. The same experiments as used with the nanoparticles were repeated on the silver film and the spectra acquired (see
In SERRS beacons incorporating fluorophores, the surface seeking SERRS label preferably has the effect of quenching the fluorescence from the fluorophore when both are present on the enhancing surface. In order to investigate and optimise SERRS labels, a range were prepared, and their quenching properties were compared with TAMRA and DABSYL against two fluorophores —FAM and Cy5.
Two different concentrations of fluorophore dye solution (1×10−7M and 5×10−8M) and six solutions of differing quencher dye concentration (1×10−7M to 1×10−8M) were prepared in 15% DMF, 15% acetonitrile, 70% water. Fluorophore-quencher mixtures were prepared using 1.5 ml of 1×10−7M fluorophore so as to give a final fluorophore concentration equal to 5×10−8M and desired quencher concentration. The fluorescence emission spectra of the low concentration fluorophore solution and that of the fluorophore-quencher mixtures were measured three times. An integral of an area under the curve was taken for each measurement and then averaged. This data was then used in the following equation to obtain the quantum yield of the quencher,
φu=(As/Au)*(Fu/Fs)*(nu/ns)2*φs
where:
φ=the quantum yield;
A=absorbance of the dye at the excitation wavelength;
F=area under the spectrum;
n=refractive index of the solution;
and the subscript s and u refer to the standard and sample solutions respectively.
This information for each quencher concentration was then used to determine the quenching constant, Ks, for each dye. This was achieved using the equation below to plot a graph for each quencher dye so that the slope of the graph would be equal to the value of Ks,
φf°/φf=KsCq+1
where:
φf°=quantum yield of fluorophore solution;
φf=quantum yield of fluorophore-quencher mixture solution;
Cq=concentration of quencher in solution;
Ks=quenching constant.
The value of Ks was the major tool used for comparison of quenching ability in this study.
The quantum yield data for the quencher concentration of 5×10−8M was also used to calculate a percentage quantum yield for the quencher. This was done to give a more easily understood value for the quenching efficiency of the dye. It was calculated at an equal concentration to that of the fluorophore to give a representative value. The equation used was,
% eff=[(φf°−φf)/φf°]*100.
A number of different dyes were studied in order to determine if structural differences influence the quenching ability. Mono-substituted and di-substituted dyes with methoxy, nitro and nitrile substitutions were investigated as was the substitution of a maleimide group for an amine and the position of the azo link.
The solutions of dyes were made in water with an addition of DMF and acetonitrile.
The structures of the dyes are shown below. “MI” denotes the maleimide derivative.
The following Table shows the calculated Ks and percentage efficiencies for quencher dyes with FAM.
It can be concluded from the results that for mono-substituted methoxy groups, the maleimide substitution gives the best quenching, the cycloadduct less so and the amine group is the least efficient. The opposite is true for the naphthyl substituted dyes, BTAN & BTAN MI, where the amine substituted dye is more efficient than the maleimide. This could be due to the size of the dye molecule.
For di-methoxy substituted dyes the quenching efficiency of the amine is higher than for the maleimide substitution, this could again be due to the size of the dye molecule.
It can be seen for the di-methoxy substituted dyes that substitution at the 3′,5′-positions is more effective than substitutions at the 2′,4′-positions. For mono-methoxy substituted dyes it can be seen that quenching is far more efficient in the para position than when substituted in the meta position.
It is apparent for all dyes that di-substitution makes quenching more efficient than mono-substitution. This can be observed for both the methoxy and nitro group substitutions.
While it has been observed that the greatest quenching efficiency is obtained using di-methoxy substituted dyes only a small difference is observed between di-methoxy and di-nitro dyes substituted with the same isomerisation. The same is found for the difference between mono-substituted dyes, substituted at identical positions.
Methoxy functionalised dyes are better quenchers than nitro but there is only a small difference in Ks values.
The following Table shows the calculated Ks and percentage efficiencies for the quencher dyes with Cy5:
The trend showing that maleimide substitution is more efficient than the cycloadduct and that more than the amine substituted dyes was observed again with the BTAN and BTAN MI also behaving the opposite way again.
This study confirmed that di-substituted dyes are more efficient quenchers than their mono-substituted equivalents for both methoxy and nitro functionalised dyes and that greatest quenching ability, within the dyes studied, is achieved using di-methoxy substituted dyes. However the efficiency of the di-nitro substituted dye is very much less than that of the di-methoxy equivalent.
With Cy5 as the fluorophore it was determined that the presence of a naphthyl group reduces the quenching efficiency of the dye. The dyes with the azo link at the “6 position” (that is, meta to the NH group of the benzotriazole; see Formula A5-3) again proved to be less effective than the dyes with the azo link at the “7 position” (that is, ortho to the NH group of the benzotriazole; see Formula A5-4).
The same overall conclusion can be drawn for quenchers with Cy5 as for quenchers with fluorescein, di-methoxy substituted dyes with maleimide groups make the most efficient quenchers and using a quencher with the azo link at the “6 position” causes a large drop in efficiency. For the top performing dyes, the substitution of an amine group for a maleimide can increase the efficiency slightly.
A separate series of experiments are shown in
In summary therefore, the results showed that:
It was also observed that the overlap between fluorophore and quencher absorption is not always a good indicator of quenching efficiency.
Another conclusion reached was on the influence of molar absorptivity on quenching. In general it was observed that the higher the molar absorptivity, the better the quenching. Although there was no direct correlation, when considered alongside the other factors it is clearly an important consideration.
Methods
Target Sequences
SERRS Beacon Synthesis
The SERRS Beacons were produced by synthesising the desired sequence on a FAM CPG solid support (Transgenomic, Glasgow, UK) using an Expedite 8909 DNA synthesiser with phosphoramidite chemistry and adding the butadiene monomer as the final 5′-residue. Cleavage and deprotection in concentrated ammonia at 50° C. for 16 hours gave the 5′-diene 3′-FAM modified sequence which was purified by reverse phase HPLC. The benzotriazole azo dye maleimide (3 eq, sodium phosphate buffer pH 5.5 with 30% MeCN) was added to the diene oligonucleotide and left for 16 hours at 40° C. in the dark. The SERRS Beacon was purified by reverse phase HPLC on a C18 Supelco column with buffer A=0.1 M triethylammonium acetate pH 6.5, buffer B=0.1 M triethylammonium acetate with 75% acetonitrile running from 20 to 70% B over 25 minutes at 1 mlmin−1.
Hybridisation Conditions
To the SERRS Beacon (150 μl, 2×10−6 M) in buffer (1 mM MgCl2, 20 mM Tris.HCL, pH 8.0) was added the desired sequence as above (5 equivalents) and then left for 40 minutes at room temperature prior to analysis.
Nanoparticle SERRS
Citrate reduced silver nanoparticles were produced as previously reported.9 For the analysis the SERRS Beacon mixture from above (20 μl) was added to silver nanoparticles (200 μl) with spermine tetrahydrochloride (10 μl, 1×10−5 M) to give a final concentration of the SERRS Beacon of 1.2×10−7 M. SERRS was accumulated on a Renishaw Ramascope 200 with 514.5 nm excitation using two scans of ten seconds.
The spermine acts as an excellent neutraliser of the phosphate backbone of the DNA21 but also aggregates the nanoparticles to provide maximum surface enhancement. It was found that, generally speaking, SERRS Beacons require less spermine than other samples (Refs 11-13) since an excess could cause over-aggregation of the nanoparticles—for example a ratio of about two to one spermine: SERRS Beacon was found to be effective in examples described herein.
Silver/PVA SERRS
The silver/PVA films were prepared as previously reported.10 The SERRS Beacon mixture from above (10 μl) was added to the film and left to sit for 20 minutes. The film was washed well with water and then examined using the Renishaw Ramascope as before but using 25% of the laser power.
Other Analysis, Spectroscopy & Chromatography
1H NMR spectra were recorded on a Bruker DPX 400 MHz spectrometer. Mass spectroscopy was performed as a university service on a JEOL AX505 spectrometer. UV-visible spectra were recorded on a Varian Carey 300 Bio spectrophotometer in a solvent mixture of water, DMF, acetonitrile (70:15:15). Fluorescence emission spectra were recorded on a Varian Cary Eclipse fluorescence spectrophotometer.
Thin layer chromatography (TLC) was carried out on aluminium sheets, silica gel 60 F254 0.2 mm layer (Merck) using a dichloromethane: methanol 9:1 v/v solvent system.
5-Aminobenzotriazole was visualised using iodine vapours giving a brown colour.
Syntheses
PABT was prepared as described in the Analyst (2003) 128, 6, 692-699.
7-(3′,5′-Dimethoxyazophenyl)-6-aminobenzotriazole (3,5-DMABT) was prepared as follows:
3,5-Dimethoxyaniline was dissolved in 50% v/v HCl (1-15 volume) and diazotised by dropwise addition of sodium nitrite (1.2 equiv. in <1 volume H2O) at 0° C. An excess of sodium nitrite was detected using starch iodide paper. A dark blue colour indicated excess nitrous acid which inferred the formation of the diazonium salt. 5-Aminobenzotriazole (1 equiv.) was dissolved in sodium acetate buffer (1.0 M, pH 6, 6-60 volumes) and minimum amount of methanol. Diazotised amine (1.1 equiv.) was added dropwise and stirred at room temperature for 1 h. The precipitate formed was isolated by filtration and washed with water. The dye was obtained in 62% yield, RF (B) 0.45, δH (acetone-d6) 3.84 (6H, s, 2×CH3), 6.57 (1H, m, Ar—H), 6.80 (2H, br s, NH2), 7.03 (1H, d, J8.9, H-4), 7.41 (2H, s, Ar—H), 7.84 (1H, d, J9.0, H-5), 14.53 (1H, br s, NH), λmax (MeOH) 443 nm, CI MS m/z 298.11872 [C14H15N6O2 (M+H)+<0.1 ppm]
| Number | Date | Country | Kind |
|---|---|---|---|
| 03199494 | Aug 2003 | GB | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/GB04/03671 | 8/26/2004 | WO | 5/25/2006 |
