Kolesov Boris A, Minkov Vasil S, Boldyreva Elena V, Drebushchak Tatyana N
Institute of Inorganic Chemistry, SB RAS, Novosibirsk, Russia, REC-008 Novosibirsk State University, Novosibirsk, Russia.
J Phys Chem B. 2008 Oct 9;112(40):12827-39. doi: 10.1021/jp804142c. Epub 2008 Sep 13.
The role of the distortion of the hydrogen bond network and of the motions of the -CH 2SH side chains in the phase transition in the orthorhombic L-cysteine ( (+)NH 3-CH(CH 2SH)-COO (-)) on cooling and the reverse transformation on heating is discussed. The extended character of the phase transition, which was recently discovered by adiabatic calorimetry [ J. Phys. Chem. B 2007, 111, 9186 ], and its very high sensitivity to the thermal prehistory of the sample could be interpreted based on the changes in the polarized Raman spectra measured for the single-crystals in several orientations in the temperature range 3-300 K and precise diffraction data on the changes in intramolecular conformations and intermolecular hydrogen bonding. In the low-temperature phase the SH...S hydrogen bonds dominate as compared to the weaker SH...O contacts, and at ambient temperature the situation is inverse. The transition from one phase to another goes via a series of states differing in conformations of the cysteine zwitterions and the intermolecular contacts of the thiol-group. Motions of different molecular fragments (NH 3 (+), CH 2, CH, SH) are activated at different temperatures. Structural strain on cooling involves several dynamic processes, such as a rigid rotation of the molecule in the lattice, a rigid rotation of the NH 3 group with respect to NH 3-CH bond, and the rotation of the thiol side chain resulting in the switching of S-H hydrogen bonding from one type to another. Different NH...O hydrogen bonds forming the framework in the L-cysteine crystal structure are distorted to a different extent, and this provokes the rotation of the -CH 2SH side chains within the cavities of this framework resulting in a change in the coordination from SH...O to SH...S at low temperatures. The results are interesting for understanding the polymorphism of molecular crystals and the factors determining their dynamics and structural instability, and also for biophysical chemistry, since the properties of the hydrogen bonded thiole-groups in biomolecules can be mimicked using L-cysteine in the crystalline state, variations in temperature and pressure serving as powerful tools, to modify the intramolecular conformations and the intermolecular hydrogen bonding.
讨论了氢键网络畸变以及 -CH₂SH 侧链运动在正交晶系 L-半胱氨酸（(+)NH₃-CH(CH₂SH)-COO⁻）冷却时的相变以及加热时的逆转变中的作用。最近通过绝热量热法 [J. Phys. Chem. B 2007, 111, 9186] 发现的相变的扩展特性及其对样品热历史的极高敏感性，可以根据在 3 - 300 K 温度范围内对多个取向的单晶测量的偏振拉曼光谱变化以及关于分子内构象和分子间氢键变化的精确衍射数据来解释。在低温相中，与较弱的 SH...O 接触相比，SH...S 氢键占主导，而在室温下情况则相反。从一个相到另一个相的转变通过一系列状态进行，这些状态在半胱氨酸两性离子的构象和硫醇基团的分子间接触方面有所不同。不同分子片段（NH₃⁺、CH₂、CH、SH）的运动在不同温度下被激活。冷却时的结构应变涉及几个动态过程，例如分子在晶格中的刚性旋转、NH₃ 基团相对于 NH₃-CH 键的刚性旋转以及硫醇侧链的旋转，导致 S-H 氢键从一种类型切换到另一种类型。构成 L-半胱氨酸晶体结构框架的不同 NH...O 氢键被不同程度地扭曲，这促使 -CH₂SH 侧链在该框架的腔内旋转，从而导致在低温下配位从 SH...O 变为 SH...S。这些结果对于理解分子晶体的多晶型性以及决定其动力学和结构不稳定性的因素很有意义，对于生物物理化学也很有意义，因为生物分子中氢键硫醇基团的性质可以使用结晶态的 L-半胱氨酸来模拟，温度和压力的变化作为强大的工具来改变分子内构象和分子间氢键。
