Strength of NH...S Hydrogen Bonds in Methionine Residues Revealed by Gas-Phase IR/UV Spectroscopy

Despite of being ubiquitous in proteins, (NHbackboneS)-S-... hydrogen bonds linking the sulfur atom of methionine or cysteine to backbone NH groups remain poorly documented. Here, we report vibrationally resolved IR NH stretch spectra of two methionine-containing dipeptides (Ac-Phe-Met-NH2 and Ac-Met-Phe-NH2). The conformations observed for both molecules, assigned with the help of DFT-D quantum chemistry, provide spectroscopic evidence for the formation of (NHbackboneS)-S-... H-bonds, surprisingly strong enough to challenge the classical intrabackbone (NHO)-O-...=C H-bonds. The methionine side chain is found to fold locally, forming a H-bond with the neighboring amide groups (NH(i) or NH(i+1)). Comparison with protein data bank structural information shows that such a local folding is also common in proteins where it concerns 24% of the methionine residues that have a sulfur atom linked to a backbone NH group. This convergence between the strength of these (NHS)-S-... H-bonds and protein structural data illustrates their contribution to the stability of protein chains.