Orbital Interactions in Native Chemical Ligation Reaction of Proline Thioesters

Abstract

A systematic theoretical study was carried out to investigate the origin of the relatively low reactivity of peptide-prolyl-thioesters in the native chemical ligation(NCL) reaction. Mechanistic calculations were performed on the two NCL reactions of peptide-prolyl-thioester(Path-Pro) and peptidealanyl-thioester(Path-Ala). The results show that both include three steps: intermolecular thiol-thioester exchange, transthioesterification, and a final intramolecular S→N acyl migration. The calculations indicate that the first step is the rate determining step of both pathways. Path-Pro is kinetically disfavored, so the peptide-prolyl-thioester is found to be less reactive in NCL reaction. This conclusion is consistent with the experimental observations. Further examination of the rate determining steps of these two pathways shows that the n→π* interaction of prolineαN carbonyl increases the LUMO orbital energy of peptidyl- prolylthioester, decreases the interaction energy between proline carbonyl and the sulphur atom in aryl thiol, and finally increases the total energy barrier.