Görbitz Carl Henrik, Karen Pavel
Department of Chemistry, University of Oslo, N-0315 Oslo, Norway.
J Phys Chem B. 2015 Apr 16;119(15):4975-84. doi: 10.1021/acs.jpcb.5b01483. Epub 2015 Apr 1.
Three quasiracemates, L-norleucine:D-methionine, L-norvaline:D-norleucine, and L-norvaline:D-methionine, were crystallized to see how they differ from regular racemates in terms of crystal structure (studied by single-crystal X-ray diffraction) and of thermally induced phase transitions (studied by differential scanning calorimetry). Two types of transitions are detected between 100 and 450 K and structurally characterized: (1) displacive transitions of the molecular bilayers that form the crystal and (2) continuous or discontinuous disordering transitions in the amino acid side chains. Uniquely for the quasiracemates, the displacive transition proceeds in two close steps as only one surface of each molecular bilayer slides at first, upon forming an intermediate phase, while the other surface follows at a slightly higher temperature. Altogether, 18 new single-crystal structure-refinement data sets are reported for these three quasiracemates.
三种准外消旋体，即L-正亮氨酸：D-甲硫氨酸、L-正缬氨酸：D-正亮氨酸和L-正缬氨酸：D-甲硫氨酸，通过结晶来观察它们在晶体结构（通过单晶X射线衍射研究）和热致相变（通过差示扫描量热法研究）方面与常规外消旋体有何不同。在100至450K之间检测到两种类型的转变，并对其进行了结构表征：（1）形成晶体的分子双层的位移转变，以及（2）氨基酸侧链中的连续或不连续无序转变。对于准外消旋体而言，独特之处在于位移转变分两个紧密步骤进行，因为每个分子双层的仅一个表面首先滑动，形成中间相，而另一个表面在略高的温度下随后滑动。总共报告了这三种准外消旋体的18个新的单晶结构精修数据集。
