Order?Disorder Phase Transition between High- and Low-<i>Z</i>′ Crystal Structures of the <i>P</i>1 Space Group
Order?Disorder Phase Transition between High- and Low-<i>Z</i>′ Crystal Structures of the <i>P</i>1 Space Group
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<i>P</i>1空间群高、低<i>Z</i>晶体结构之间的有序·无序相变
DOI：
10.1021/acs.cgd.1c01330
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发表时间：
2022
影响因子：
3.8
通讯作者：
Tsue Hirohito
中科院分区：
文献类型：
--
作者：
Oketani Ryusei;Takahashi Hiroki;Clevers Simon;Oyamada Akira;Hisaki Ichiro;Coquerel G?rard;Yamanaka Kazuaki;Tsue Hirohito
本文报道了Boc-1-甲硫氨酰甘氨酸甲酯（MGP）的晶体结构，其不对称单元中有8个分子（Z′ = 8），是二肽类化合物中数目最多的。有趣的是，在−113 °C时观察到它的有序-无序相变，形成Z ′ = 4的晶体结构。通过单晶X射线衍射和温度相关的二次谐波信号测量，揭示了相变的可逆有序-无序特征。高温相的蛋氨酸部分的无序反映了低温相的构象，这意味着这些晶体结构基本相同，并且由于热波动引起的原子位移触发了相变。虽然差示扫描量热（DSC）分析和热容（Cp）测量没有提供明确的信号来揭示和检测相变的特征，但SHG技术能够检测有序-无序结构的变化。理论计算阐明了MGP高Z ′晶体结构所涉及的分子间相互作用能和分子的构象自由度。通过晶格能与由观察到的分子构象推导出的虚拟生成的低Z '晶体结构的比较研究，为结晶领域的普遍问题，即分子为什么会以高Z'晶体结构结晶提供了定量分析？这项研究还揭示了熵项的贡献，它在理解不寻常的高Z ′晶体结构的起源方面起着关键作用。
We report a crystal structure of Boc-l-methionyl glycine methyl ester (MGP) with eight molecules in the asymmetric unit (Z′ = 8), which is the highest number in dipeptide compounds. Interestingly, its order–disorder phase transition to the crystal structure ofZ′ = 4 was observed at −113 °C. The reversible order–disorder character of the phase transition was revealed by single-crystal X-ray diffraction and temperature-dependent second harmonic generation (SHG) signal measurements. The disorder of the methionine moiety of the high-temperature phase reflected the conformation of the low-temperature phase, implying that these crystal structures were essentially identical and that atomic displacement due to thermal fluctuation triggered the phase transition. Although differential scanning calorimetry (DSC) analysis and heat capacity (Cp) measurements did not provide clear signals to reveal and detect the character of phase transition, the SHG technique was able to detect the order–disorder structural change. The theoretical calculation clarified the intermolecular interaction energy together with the conformational freedom of the molecule involved in the high-Z′ crystal structure of MGP. The comparison study of the lattice energy with the virtually generated low-Z′ crystal structures derived from the observed molecular conformations provided quantitative analysis for the general problem in the field of crystallization, namely, why does a molecule crystallize in a high-Z′ crystal structure? This study also revealed the contribution of the entropy term, which plays a critical role in understanding the origin of an unusual high-Z′ crystal structure.