Review of methods to assign the nuclear magnetic resonance peaks of reductively methylated proteins

Reductive methylation of lysyl side-chain amines has been a successful tool in the advancement of high-resolution structural biology. The utility of this method has continuously gained ground as a protein chemical modification, first as a tool to aid protein crystallization and later as a probe in protein nuclear magnetic resonance (NMR) spectroscopy. As an isotope-labeling strategy for NMR studies, reductive methylation has contributed to the study of protein-protein interactions and global conformational changes. Although more detailed structural studies using this labeling strategy are possible, the hurdle of assigning the NMR peaks to the corresponding reductively methylated amine hinders its use. In this review, we discuss and compare strategies used to assign the NMR peaks of reductively methylated protein amines.