Bifurcated Hydrogen Bonds in a Peptide Crystal Unveiled by X‑ray Diffraction and Polarized Raman Spectroscopy

The strength of hydrogen bonds in molecular crystals has been investigated through X-ray structural analysis and vibrational spectroscopy, and the interatomic distances and the stretching vibrational frequency involving hydrogen bonds have been known to exhibit a linear relationship. However, these studies are limited to relatively small molecular compounds, such as amino acids. In this study, we employed tetrapeptide as a model system to investigate the hydrogen bond network in the peptide crystal. Single crystal X-ray diffraction revealed that the peptide crystal structure had bifurcated hydrogen bonds. Raman spectroscopy exhibited that the vibrational frequency of amide bonds correlated linearly to their interatomic distance, but the rate of the change was significantly low compared with previous works. Fragment molecular orbital calculations also revealed that bifurcated hydrogen bonds affect the strength of the hydrogen bonds. This study provides a valuable model system for discussing the effects of bifurcated hydrogen bonds in various crystals of peptides.