Quantitation of Cysteine Residues Alkylated with 3-Bromopropylamine by Amino Acid Analysis

A new versatile reagent, 3-bromopropylamine, for the quantitative analysis of cysteine residues in proteins and peptides is reported. When added to amino acid standards, the 3-bromopropylamine derivative of cysteine, S-3-aminopropylcysteine, elutes in a unique position on four different amino acid analysis systems without modification to their standard gradients. Optimized conditions for the complete alkylation of cysteines in proteins with 3-bromopropylamine are described. The S-3-aminopropylcysteine is stable to standard acid hydrolysis conditions used for amino acid analysis. Cysteine values are within 10% of the predicted value in the amino acid analysis of acid hydrolysates of known proteins based on quantitation with S-3-aminopropylcysteine. No evidence of alkylation of other amino acids by 3-bromopropylamine is apparent from the amino acid analysis of proteins alkylated under the optimal conditions. These results expand the application of 3-bromopropylamine to include quantitation of cysteine by amino acid analysis as well as the previously reported identification of cysteines by protein sequencing.