Solid-phase synthesis of difficult peptide sequences at elevated temperatures: a critical comparison of microwave and conventional heating technologies.

The Fmoc/t-Bu solid-phase synthesis of three difficult peptide sequences (a 9-mer, 15-mer, and 24-mer) was performed using N,N'-diisopropylcarbodiimide/1-hydroxybenzotriazole as coupling reagent on polystyrene, Tentagel, and ChemMatrix resins. In order to obtain an insight into the specific role of the elevated temperature and/or the electromagnetic field for peptide syntheses carried out using microwave irradiation, peptide couplings and Fmoc-deprotection steps were studied under microwave and conventionally heated conditions at the same temperature. While room temperature couplings/deprotections generally produced the difficult peptides in rather poor quality, excellent peptide purities were obtained using microwave heating at a temperature of 86 degrees C for both the coupling and deprotection steps in only 10 and 2.5 min reaction time, respectively. While for most amino acids no significant racemization was observed, the high coupling temperatures led to considerable levels of racemization for the sensitive amino acids His and Cys. It was demonstrated for all three peptide sequences that when performing the coupling/deprotection steps at the same reaction temperature using conventional heating, nearly identical results in terms of both peptide purity and racemization levels were obtained. It therefore appears that the main effect of microwave irradiation applied to solid-phase peptide synthesis is a purely thermal effect not related to the electromagnetic field.