Efficient solution phase synthesis and use of multiple O-phosphothreonyl-containing peptides for calcium phosphate binding studies

The protected phosphothreonine derivative Boc-Thr(PO₃Ph₂)-OH was prepared in high yield from Boc-Thr-OH by a simple three-step procedure which involved 4-nitrobenzylcarboxyl protection, either phosphorotriester (diphenyl phosphorochloridate) or “phosphite-triester” (diphenyl N.N-diethylphosphoramidite) phosphorylation of the thrconine hydroxyl group of Boc-Thr-ONb followed by hydrogenolytic carboxyl deprotection. The three Thr(P)-containing peptides, H-Thr(P)-Glu-Glu-NHMe.TFA, H-Thr(P)-Thr(P)-Glu-Glu-NHMe.TFA and H-Thr(P)-Thr(P)-Thr(P)-Glu-Glu-NHMe.TFA, were prepared in high yield by the use of Boc-Thr(PO₃Ph₂)-OH in the Boc mode of peptide synthesis (mixed anhydride method) followed by platinum-mediated hydrogenolytic deprotection of the Thr(PO₃3Ph₂)-containing peptides. The use of the phosphopeptides in calcium phosphate binding studies showed that the triple Thr(P)-cluster was a basic structural requirement, since only the pentapeptide was able to bind calcium phosphate efficiently.