Solid-phase synthesis of phosphopeptides.

We report the solid-phase synthesis of peptides containing O-phosphoserine. Coupling was with commercially available Fmoc-amino acid pentafluorophenyl esters, with base used at each cycle to cleave Fmoc. Phosphorylation of those serine residues left unprotected on the peptide-resin was achieved with dibenzylphosphochloridate, and finally trifluoroacetic acid was used to remove side-chain protecting groups (including the benzyl groups used for the phosphate), and to cleave the peptide from the resin in the same step. This synthetic strategy enables the preparation of peptides with individual, selectively phosphorylated residues. Alternative approaches to introduce protected phosphate and continue with coupling of further amino acids were less advantageous due to the lability of the phosphate group to base and to steric hindrance.