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

The strength of hydrogen bonds in molecular crystalshas been investigatedthrough X-ray structural analysis and vibrational spectroscopy, andthe interatomic distances and the stretching vibrational frequencyinvolving 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 amodel system to investigate the hydrogen bond network in the peptidecrystal. Single crystal X-ray diffraction revealed that the peptidecrystal structure had bifurcated hydrogen bonds. Raman spectroscopyexhibited that the vibrational frequency of amide bonds correlatedlinearly to their interatomic distance, but the rate of the changewas significantly low compared with previous works. Fragment molecularorbital calculations also revealed that bifurcated hydrogen bondsaffect the strength of the hydrogen bonds. This study provides a valuablemodel system for discussing the effects of bifurcated hydrogen bondsin various crystals of peptides. Thetetrapeptide crystal exhibited a hydrogen network extendingthrough the entire crystal. The strength of hydrogen bonds was characterizedby polarized Raman spectroscopy. An unexpectedly low rate of changein the vibrational frequency of the amide bond as increasing interatomicdistance of the hydrogen bond was found. A computational calculationrevealed the contribution of the branched hydrogen bonds to this phenomenon.