Polypeptide synthesis using an expressed peptide as a building block for condensation with a peptide thioester: Application to the synthesis of phosphorylated p21Max protein(1-101)

An expressed peptide proved to be useful as a building block for the synthesis of a polypeptide via the thioester method. A partially protected peptide segment, for use as a C-terminal building block, could be prepared from a recombinant protein; its N-terminal amino acid residue was transaminated to an α-oxoacyl group, the side-chain amino groups were then protected with t-butoxycarbonyl (Boc) groups, and, finally, the α-oxoacyl group was removed. On the other hand, an O-phosphoserine-containing peptide thioester was synthesized via a solid-phase method using Boc chemistry. These building blocks were then condensed in the presence of silver ions and an active ester component. During the condensation, epimerization at the condensation site could be suppressed by the use of N,N-dimthylformamide (DMF) as a solvent. Using this strategy, a phosphorylated partial peptide of the p21Max protein, [Ser(PO₃H₂)^2,11]-p21Max(1-101), was successfully synthesized.