G-Protein Coupled Receptor (GPCR) signaling is one of the most pharmacologically important pathways in biology. A critical reglatory step is played by the Regulator of G-protein Signaling (RGS) proteins, which terminate G-protein action by binding to the α subunit and accelerating hydrolysis of bound GTP. Multiple RGS isoforms exist, each with distinct tissue distributions. By targeting RGS proteins, better tissue specificity may be acheived than by global GPCR agonist application. In particular, targeting of RGS4 may be therapeutically beneficial in treatment of Parkinson’s disease. Thiadiazolidinones are a series of compounds that inhibit RGS4, RGS8, and RGS19 by covalent modification of cysteine residues. However, cysteines shared by inhibited RGS proteins are not exposed to the protein surface. To understand the action of these compounds, it will be essential to elucidate the role of protein dynamics in how buried cysteines are accessed (Aim 1). In addition, the role of covalent binding at other cysteines not shared between each protein will be assessed (Aim 2). It will also be necessary to understand the structural basis of protein flexibility (Aim 3). Taken together, this knowledge will assist in the rational design of non-covalent inhibitors (Aim 4).