Amide I Vibrational Properties Affected by Hydrogen Bonding Out-of-Plane of the Peptide Group.

The amide I vibrational properties of a peptide-water complex in various intermolecular configurations are analyzed theoretically to see whether a water molecule with a weak out-of-plane hydrogen bond really induces a large low-frequency shift. It is shown that the frequency shift strongly depends on the C═O···H angle, with a larger low-frequency shift as the C═O···H becomes more bent, suggesting that the so-called hydrated helix with a rather low amide I frequency has an additional water molecule located out-of-plane of the peptide group as compared with a typical α-helix. The infrared intensity also depends on the angular position of water. A new model parameter set (that can be combined with molecular dynamics) is developed for a more correct representation of the hydration-induced frequency shift. The question regarding the scalar and vectorial nature of the molecular properties related to the frequency shift is also discussed.