Rapid and selective modification of phosphoserine residues catalysed by Ba2+ ions for their detection during peptide microsequencing.

The beta-elimination of phosphoserine residues by dilute alkali is catalysed by the presence of group II metal ions. The use of 0.1 M-Ba (OH)2 catalysed the rate of beta-elimination of phosphoserine by more than two orders of magnitude compared with the use of NaOH at the same OH-ion concentration. Serine and threonine residues are unaffected by this treatment. Free thiol groups and disulphide bonds are labile to these conditions, but carboxymethylcysteine is stable. The rate of beta-elimination of O-glycosidically linked moieties is not catalysed under these conditions, and the rate of reaction is thus two orders of magnitude slower than for phosphoserine. This specific catalysis was readily exploited in the rapid and selective modification of phosphoserine residues under mildly alkaline conditions with the nucleophile methylamine via the alpha beta-desaturated dehydroalanine intermediate to yield the beta-methylaminoalanine residue. This modified residue could be easily detected on sequence analysis and in amino acid compositions.