Protein Substrates for Reaction Discovery: Site-Selective Modification with Boronic Acid Reagents

Chemical modification of natural proteins must navigate difficult selectivity questions in a complex polyfunctional aqueous environment, within a narrow window of acceptable conditions. Limits on solvent mixtures, pH, and temperature create challenges for most synthetic methods. While a protein's complex polyfunctional environment undoubtedly creates challenges for traditional reactions, we wondered if it also might create opportunities for pursuing new bioconjugation reactivity directly on protein substrates. This Account describes our efforts to date to discover and develop new and useful reactivity for protein modification by starting from an open-ended screen of potential transition-metal catalysts for boronic acid reactivity with a model protein substrate. By starting from a broad screen, we were hoping to take advantage of the very many potential reactive sites on even a small model protein. And perhaps more importantly, whole proteins as reaction screening substrates might exhibit uniquely reactive local environments, the results of a dense combination of functional groups that would be nearly impossible to mimic in a small-molecule context. This effort has resulted in the discovery of four new protein modification reactions with boronic acid reagents, including a remarkable modification of specific backbone N-H bonds. This histidine-directed Chan-Lam coupling, based on specific proximity of an imidazole and two amide groups, is one important example of powerful reactivity that depends on a combination of functional groups that proteins make possible. Other bioconjugation reactions uncovered include a three-component tyrosine metalation with rhodium(III), a nickel-catalyzed cysteine arylation, and an unusual ascorbate-mediated oxidative process for N-terminal modification. The remarkably broad scope of reactivity types encountered in this work is a testament to the breadth of boronic acid reactivity. It is also a demonstration of the diverse reactivities that are possible by the combined alteration of boronic acid structure and metal promoter. The discovery of specific backbone modification chemistry has been a broadly empowering reactivity. Pyroglutamate, a naturally occurring posttranslational modification, exhibits remarkably high reactivity in histidine-directed backbone modification, which allows us to treat pyroglutamate as a reactive bioorthogonal handle that is readily incorporated into proteins of interest by natural machinery. In another research direction, the development of a vinylogous photocleavage system has allowed us to view backbone modification as a photocaging modification which is released by exposure to light.