Hydrogen in N-methylacetamide Positions and dynamics of the hydrogen atoms using neutron scattering

Amer Chemical Soc
Publication Type:
Journal Article
Journal Of Physical Chemistry B, 2007, 111 (27), pp. 7725 - 7734
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This work reports neutron diffraction and incoherent neutron scattering experiments on N-methylacetamide (NMA), which can be considered the model building block for the peptide linkage of polypeptides and proteins. Using the neutron data, we have been able to associate the onset of a striking negative thermal expansion (NTE) along the a-axis with a dynamincal transition around 230K, consistent with our calorimetric experiments. Observation of the NTE raises the questions of possible proton transfer in NMA, which from our data alone wtill cannot be settled. We can only speculate that intermolecular repulsive forces increases as the O...H distance decreases upon cooling, and that around 230K the lattice relaxes wothout observation of an actual proton transfer. However, the existence ofa nonharmonic potential reflected by the behaviour of the phonon vibrations together with the observation of NTE, could be justified by the vibrational polaron theory in which a dynamic localization of the vibrational energy is created by coupling an internal molecular mode to a lattice phonon. More generally this work shows that neutron powder diffraction techniques can be very powerful for investigating structural deformations in small peptide systems.
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