Peptide thioester has been used as a key intermediate for the peptide condensation methods, such as the thioester method [1] and the native chemical ligation method [2]. Its preparation by the Fmoc method usually requires indirect multi-step reaction, as the thioester linkage is labile to the piperidine used for the Fmoc group removal.
In 2006, Bröhmer et al. succeeded in the synthesis of peptide methyl ester by the solvolysis of peptide N,N-dipicolyl (DP) amide in methanol using the copper (II) ion activation [3]. A peptide with DP amide was easily synthesized by the conventional Fmoc method. Therefore, in our initial thought, if the direct aminolysis of peptide DP amide by the other segment proceeds by the activation using copper (II) ions, an efficient peptide ligation method would be realized. However, the direct aminolysis did not proceed. We then examined the peptide ligation using peptide DP amide via the 1,1,1,3,3,3-hexafluoropronan-2-ol (HFIP) ester intermediate. In this study, the method was successfully applied to the synthesis of atrial natriuretic peptide by the condensation between two segments. It was further applied to the synthesis of RNase T1 of 104 amino acid resides, by the sequential condensation of three segments. The detail of the synthetic procedure and the results will be presented.