Methods to Identify the NMR Resonances of the ¹³C-Dimethyl N-terminal Amine on Reductively Methylated Proteins

Two methods for assigning the α- and ε-dimethylamine nuclear magnetic resonance signals of a reductively ¹³C-methylated N-terminal lysine are described. One method utilizes the pH-induced selectivity of the reductive methylation reaction, and the other uses aminopeptidase to selectively remove the N-terminal lysine.