Hydrogen in N-methylacetamide: positions and dynamics of the hydrogen atoms using neutron scattering.
Hydrogen in N-methylacetamide: positions and dynamics of the hydrogen atoms using neutron scattering.
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N-甲基乙酰胺中的氢：使用中子散射观察氢原子的位置和动力学。
DOI：
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发表时间：
2007
影响因子：
3.3
通讯作者：
T. Seydel
中科院分区：
文献类型：
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作者：
H. Bordallo;D. Argyriou;M. Barthès;W. Kalceff;S. Rols;K. Herwig;C. Fehr;F. Jurányi;T. Seydel
这项工作报告了 N-甲基乙酰胺 (NMA) 的中子衍射和非相干中子散射实验，NMA 可以被认为是多肽和蛋白质肽键的模型构建块。利用中子数据，我们已经能够将沿 a 轴的惊人负热膨胀 (NTE) 的开始与 230 K 左右的动态转变联系起来，这与我们的量热实验一致。对 NTE 的观测提出了 NMA 中可能存在质子转移的问题，仅从我们的数据来看，这个问题仍然无法解决。我们只能推测，冷却时分子间排斥力随着 O...H 距离的减小而增加，并且在 230 K 左右晶格松弛，而没有观察到实际的质子转移。然而，声子振动行为以及 NTE 观测所反映的非简谐势的存在可以通过“振动”极化子理论来证明，其中振动能量的动态局域化是通过将内部分子模式耦合到晶格声子来创建的。更一般地说，这项工作表明中子粉末衍射技术对于研究小肽系统的结构变形非常有效。
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 dynamical transition around 230 K, consistent with our calorimetric experiments. Observation of the NTE raises the question of possible proton transfer in NMA, which, from our data alone, still cannot be settled. We can only speculate that intermolecular repulsive forces increase as the O...H distance decreases upon cooling, and that around 230 K the lattice relaxes without observation of an actual proton transfer. However, the existence of a nonharmonic potential, reflected by the behavior 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.