Intermolecular Interaction Effects in the Amide I Vibrations of β Polypeptides

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  • S. Krimm
    Harrison M. Randall Laboratory of Physics, University of Michigan, Ann Arbor, Mich. 48104
  • Yasuaki Abe
    Harrison M. Randall Laboratory of Physics, University of Michigan, Ann Arbor, Mich. 48104

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<jats:p> Previous perturbation treatments of the Amide I vibrations of β polypeptides are inconsistent with a detailed normal coordinate analysis of crystalline polyglycine I. This analysis indicates that the <jats:italic>D</jats:italic> <jats:sub>10</jats:sub> interaction constant is essentially zero, rather than the large value (about 20 cm <jats:sup>-1</jats:sup> ) required by the earlier application of the perturbation theory. It is suggested that the previously neglected <jats:italic>D</jats:italic> <jats:sub>11</jats:sub> term should be included in the perturbation expression, and it is shown that the physical origin of such a term can be accounted for by transition dipole coupling. This mechanism is shown to give a reasonable explanation of splittings of the C=O stretching vibrations in hydrogenbonded carboxylic acid dimers. Its application to β polypeptides provides a satisfactory interpretation of splittings in the Amide I modes. </jats:p>

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