The frequency of the so-called "amide I" band (amide C=O stretching vibration, vC=O) of proteins is discussed in terms of the dihedral angles of the various secondary structures present within proteins. We propose that in the case of intra- or intermolecular hydrogen-bonded amide carbonyl groups the frequency of this absorption can be directly related to the Φ, Ψ angles of the amide moieties for the major secondary structures. Amide 1 bands at frequencies above those found for non-hydrogen bonded amide carbonyl groups are rationalized in terms of a change in the third dihedral angle, ω. Rotation around the arnide C-N bond in sterically demanding structures, such as turns where ω deviates from 180°, is expected to cause an increase in the electron density of the amide carbonyl groups and so increase vC=O to frequencies greater than that seen for unperturbed carbonyl groups.
Canadian Journal of Chemistry69, no. 11: 1639–1642.