Project Summary/Abstract Biocompatible chemical transformations that are promoted by light have become powerful tools in chemical biology by virtue of enabling spatiotemporal control over activity. Whilst genetically encoded photoactivatable tools have become mainstays in the bio-orthogonal toolbox, light driven conjugation methods that effectively interface with native biomolecular structures (no genetic encoding) under biologically relevant conditions, are comparatively limited. In this project, we will evolve a method recently developed by our group for the photobioconjugation of Tryptophan (Trp) residues using redox-active N-carbamoylpyridinium salts that engage Trp in photo-induced electron transfer. We will show that, by carefully modulating the optical and electrochemical properties of these reagents, that we will be able to both (1) dramatically enhance the kinetic capabilities of this labelling reaction and (2) enable the discovery of new mechanistic paradigms that promote this labelling chemistry. Moreover, we will demonstrate that, through careful manipulation of optical and electrochemical properties of the N-carbamoyl pyridinum salt reagent, that we will be able to invoke mechanistic control over Trp labelling in a wavelength-dependent fashion (i.e. we can control reaction mechanism with a given wavelength of light). This, in turn, will allow us to design new application-based experiments that can both command precise reaction outcomes and markedly expand the capabilities of photobioconjugation chemistry. Specifically, we will harness this optical and mechanistic control for the design of new activity-based sensing applications as well as through the design of proximity labelling approaches that we apply to the study of poorly understood processes in mitochondrial dynamics.