Meyer et al. [(1986) FEBS Lett 204, 61–66] have shown that the phosperine can be converted to S-ethylcysteine by β-elimination and addition of ethanethiol. I have utilised this modification to develop a rapid method for the selective purification of phosphoserine-containing peptides from complex mixtures. Changing phosphoserine to S-ethylcysteine increases the hydrophobicity of a peptide, altering its mobility during reverse-phase chromatography. The number of S-ethylcysteine residues in a peptide can be qualified at the picomolar level, following acid hydrolysis and conversion to the phenylthiocarbamyl derivative. The procedure may be particularly powerful for the analysis of peptides that are phosphorylated at multiple sites in vivo.
