PROTEIN SECONDARY STRUCTURE FROM FT-IR SPECTROSCOPY - CORRELATION WITH DIHEDRAL ANGLES FROM 3-DIMENSIONAL RAMACHANDRAN PLOTS

The frequency of the so-called ''amide I'' band (amide C = O stretching vibration, nu-C = 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 phi, psi-angles of the amide moieties for the major secondary structures. Amide I 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, omega. Rotation around the amide C-N bond in sterically demanding structures, such as turns where omega deviates from 180-degrees, is expected to cause an increase in the electron density of the amide carbonyl groups and so increase nu-C = O to frequencies greater than that seen for unperturbed carbonyl groups.