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

Random and sequential copolypeptides containing O-phospho-L-threonine and L-aspartic acid; roles in CaCO3 biomineralization

The present study describes the synthesis of novel polypeptides containing O-phospho-L-threonine [Thr(PO(3)H(2))] and L-aspartic acid. Random copolypeptides copoly[Thr(PO(3)H(2))(X)Asp(Y)] (X:Y = 25:75, 50:50, 75:25), were conventionally prepared by copolymerization of Thr(PO(3)Ph(2)) N-carboxyanhydride (NCA) and Asp(OBzl) NCA followed by deprotection of the phenyl and benzyl groups by catalytic hydrogenolysis over PtO(2). Polycondensation of the protected peptide p-nitrophenyl esters [Thr(PO(3)Ph(2))](Z)-Asp(OBzl)-ONp and subsequent deprotection yielded the sequential polypeptides poly[Thr(PO(3)H(2))(Z)-Asp] (Z = 1-4). By using the synthetic polypeptides, their effects on the growth of CaCO(3) crystals were examined. In the poly[Thr(PO(3)H(2))(Z)-Asp]/CaCO(3) systems, brushlike calcite and spherical vaterite were formed, with the former being found at [Ca(2+)]/[Res] ratios of > or =180, > or =140, > or =120, and > or =100 for Z = 1, 2, 3, and 4, respectively. These results indicate that an increase of Thr(PO(3)H(2)) residues in the repetitive unit induces the characteristic brushlike calcite, a fact indicating that Thr(PO(3)H(2)) residues can modify the CaCO(3) crystal morphology.

Further studies into the Boc/solid-phase synthesis of Ser(P)- and Thr(P)-containing peptides

The Ser(P)-containing peptide corresponding to phospholamban 11-19, Ac-Ala-Ile-Arg-Arg-Ala-Ser(P)-Thr-Ile-Glu-NH2, was prepared by the use of Boc-Ser(PO3Ph2)-OH in Boc/solid-phase peptide synthesis followed by HF cleavage of the peptide from the polystyrene resin and subsequent platinum-mediated hydrogenolytic cleavage of the phenyl phosphate groups. A study of the HF deprotection step showed that extensive dephosphorylation of the Ser(PO3Ph2)-residue occurred using three commonly used HF conditions and gave rise to large quantities of the Ser-containing peptide. The subsequent study of model peptide systems under standard HF conditions established firstly that the extent of dephosphorylation was dependent on the HF-contact time, and secondly that the Ser(PO3Ph2) residue underwent dephosphorylation at a slightly higher rate than the Thr(PO3Ph2) residue.

Solid-phase syntheses of phosphorylated and thiophosphorylated peptides related to an EGFR sequence

The 13 amino acid EGFR-sequence AENAEYLRVAPQS-NH2 containing the in vivo autophosphorylated Tyr 1171, was synthesized by Fmoc continuous-flow SPPS with and without N-terminal Boc protection. In addition to the native sequence, peptides in which tyrosine was exchanged by serine and threonine were prepared. Global phosphorylation of the unprotected hydroxyl amino acids on the resin with di-tert-butyl-N,N-diethylphosphoramidite and 1H-tetrazole followed by in situ oxidation of the resulting phosphites with tert-butyl hydroperoxide or with dibenzoyl tetrasulfide resulted in the tyrosine-, serine- and threonine-phosphorylated and -thiophosphorylated sequences, respectively. The quality of the products after phosphorylation with N-terminal protection was better than without. Whereas the serine- and threonine-thiophosphate group was stable, tyrosine-thiophosphate turned out to be hydrolytically labile under acidic conditions. The rate of hydrolysis was determined with the tyrosine-thiophosphorylated model dipeptide Ac-Tyr-Gly-OH between pH 0.1 and 8. Hydrolysis was fastest at pH 3, with a half-time of 12.5 h at room temperature. The tyrosine-thiophosphate group was completely stable at pH 8.