Project Summary/Abstract The generation of chemical space beyond that can be achieved by Nature is one of the main goals of synthetic chemistry and an important tenet in drug discovery research. The BN/CC isosterism (i.e., replacement of a carbon-carbon bond with a boron-nitrogen bond) has emerged as a viable strategy to increase the chemical space of compounds relevant to biomedical research. This proposal describes a medicinal chemistry program geared toward developing BN isosteres of a ubiquitous structural motif in pharmaceutical research, i.e., arenes. Potential benefits of research into these BN heterocycles (also known as azaborines) include discovery of novel azaborine-specific mechanisms of biological activity that are unattainable by conventional organic molecules and improved pharmacological profiles. While good progress has been made in the preparation of azaborines, their interactions with biomacromolecules remains virtually unexplored. Thus, this proposed research is focused on establishing 1,2-azaborine's proposed distinctive features (better aqueous solubility and NH hydrogen bonding) in a biological context. Specifically, we seek to: 1) establish the therapeutic potential of azaborines in collaboration with Novartis by directly comparing the pharmacological profiles and biological activity of known biologically active compounds (e.g., CDK-2, PPAR- ? inhibitors) to their corresponding BN isosteres. 2) develop a fundamental understanding of the hydrogen bonding interaction between the NH group of 1,2- azaborines and a protein by employing mutant T4 lysozyme as a well-defined and tunable protein model from which quantitative structural and energetic parameters can be extracted. 3) investigate how proteins accommodate incremental changes in ligand structure in discrete conformational states using a congeneric series of 1,2-azaborine ligands, taking advantage of azaborine's improved aqueous solubility. Completion of the proposed aims will yield new fundamental knowledge related to BN/CC isosterism in the context of biomedical research and at the same time promote the design and development of a new pharmacophore from which future drug candidates can be built upon.