Elsevier

Biophysical Chemistry

Volume 53, Issues 1–2, December 1994, Pages 155-168
Biophysical Chemistry

Inelastic neutron-scattering study of the proton transfer dynamics in polyglycine I at 20 K

https://doi.org/10.1016/0301-4622(94)00086-7Get rights and content

Abstract

Inelastic neutron-scattering (INS) spectra of three isotopic derivatives of polyglycine I (COCH2NH)n, (COCD2NH)n, and (COCH2ND)n at 20 K are presented from 30 to 4000 cm−1. The band frequencies are compared to those observed in the infrared and Raman. Assignments in terms of group vibrations are proposed. These mostly resemble previous assignment schemes, except for the amide bands. The INS intensities reveal that the proton dynamics for the (N)H proton are totally different from those proposed previously. They are independent of the molecular frame and the valence bond approach is not consistent with observation. A phenomenological approach is proposed in terms of localized modes. The calculated intensities reveal that the (N)H stretching mode has two components at ∼ 1377 and 1553 cm−1. This is a dramatic change compared to all former assignments at ∼ 3280 cm−1 based on infrared and Raman data. These proton-dynamics are associated with a weakening of the NH bond due to the ionic character of the hydrogen bond (Nδ−…H+…Oδ′−) and proton transfer. The infrared and Raman spectra are re-examined and a new assignment scheme is proposed for the amide bands; the amide A and B bands are re-assigned to the overtones of the stretching modes. A symmetric double-minimum potential for the proton is consistent with all the observations.

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