Synthesis of a phosphorylated polypeptide by a thioester method

A method for the synthesis of phosphorylated polypeptides was investigated through the synthesis of the phosphorylated cAMP response element binding protein 1 (19-106) amide, [Thr(PO3H2)(69,71)]-CRE BP1(19-106)-NH2, as a model compound. Three partially protected peptide segments: Boc-[Lys(BoC)(23,46,48,51), CyS(ACm)(27,32)]-CRE BP1(19-56)-SCH2CH2CO-beta-Ala-NH2, Fmoc-[Thr(PO3H2)(69,71), Lys(Boc)(77), Cys(Acm)(79)]-CRE BP1(57-83)-SCH2CH2CO-beta-Ala-NH2, and [Lys(Boc)(97,98,105,106)]-CRE BP1(84-106)-NH2, were prepared by thr Boc solid-phase method and were used as building blocks. The segment condensations were carried out sequentially by a thioester method in the presence of a silver compound, HOObt, and DIEA. The desired product was obtained in 19% yield, based on the C-terminal building block, after removal of all the protecting groups on the peptide. These results indicate that the thioester method is quite useful for the preparation of phosphorylated polypeptide even if it contains cysteine residues. One thing which remains to be improved is the efficiency of the method for the preparation of partially protected peptide thioesters, which contain phosphorylated amino acid residues.