PROJECT SUMMARY Post-translational changes in the redox state of cysteine residues can rapidly and reversibly alter protein function, modulating biological processes and drug pharmacology. During the last funding period, our lab reported several innovations in organosulfur chemistry, small-molecule tools, and computational methods for proteomic analysis that dramatically improved selectivity, cellular application, and site-specific quantitation of the cysteine redoxome (J. Am. Chem. Soc., 2017; Nat. Chem. Biol., 2018; Nat. Chem., 2021; Nat. Comm., 2022). Application of these chemoproteomic methods has contributed to meaningful discovery of new paradigms in redox biology (Nat. Cell Biol., 2019; Cell Metab., 2019; Blood Adv., 2020; Nat. Comm., 2021; Redox Biol., 2021 & 2022; Proc. Natl. Acad. Sci., 2022) and raise interesting new questions about the links between the cellular redox landscape, molecular thiol-based redox switches, and the emerging field of redox medicine. Herein, the following three Specific Aims are proposed: Development and application of 1) chemical methods to address spatiotemporal control in redox signaling; 2) genetic incorporation of oxidized cysteine in proteins to pinpoint molecular mechanisms underlying thiol-based redox regulation; and 3) nucleophile-fragment libraries to discover covalent ligands that target redox- regulated proteins in the cysteinome. Strong preliminary data demonstrate that studies proposed herein are both promising and feasible in our hands. Deliverables resulting from these studies include: 1) valuable new chemical biology approaches to understand fundamental mechanisms in thiol-based redox signaling, and 2) novel chem- ical matter that can be mined as a source of small-molecule probes and as starting points for drug discovery.