A Tale of Two Bioconjugations: pH Controlled Divergent Reactivity of Protein alpha-oxo-Aldehydes in Competing alpha-oxo-Mannich and Catalyst-Free Aldol Ligations

Site-selective chemical methods for protein bioconjugation have revolutionized the fields of cell and chemical biology through the development of novel protein/enzyme probes bearing fluorescent, spectroscopic, or even toxic cargos. Herein, we report two new methods for the bio-conjugation of alpha-oxo aldehyde handles within proteins using small molecule aniline and/or phenol probes. The "alpha-oxo-Mannich" and "catalyst-free aldol" ligations both compete for the electrophilic alpha-oxo aldehyde, which displays pH divergent reactivity proceeding through the "Mannich" pathway at acidic pH to afford bifunctionalized bioconjugates, and the "catalyst-free aldol" pathway at neutral pH to afford monofunctionalized bioconjugates. We explore the substrate scope and utility of both of these bioconjugations in the construction of neoglycoproteins, in the process formulating a mechanistic rationale for how both pathways intersect with each other at different reaction pH's.