Information for proteomics: ESI-MS titration by sodium ions gives the number of carboxylate groups in peptides

We studied the electrospray ionization (ESI) of various peptides containing amino acids with acidic side chains to test whether the pattern of molecular ion peaks provides information on the number of acidic side chains. When we increased the concentration of sodium salt in the ESI solution containing a peptide, a characteristic pattern arose, and it represents the various sodium salts of carboxylic acid and an additional sodium ion to add charge to the species. Both C-terminal and side chain carboxylic acid groups readily form sodium carboxylates. After reaching full substitution of all acidic hydrogens for sodium ions, further increases in the concentration of sodium cations do not lead to the attachment of additional sodium atoms. This effect allows counting of acid residues in a peptide. This information is easily acquired and may be an appropriate supplement to exact mass measurements and partial sequence data that are currently used in proteomic database searches.