Institut des Sciences Moléculaires d'Orsay (ISMO), CNRS, University Paris-Sud, Université Paris-Saclay, Orsay, France.
Chirality. 2020 May;32(5):693-703. doi: 10.1002/chir.23195. Epub 2020 Feb 20.
The solid-state structure of LL/DD or LD/DL diphenylalanine diluted in KBr pellets is studied by infrared (IR) absorption and vibrational circular dichroism (VCD) spectroscopy. The structure depends on the absolute configuration of the residues. The natural LL diphenylalanine exists as a mixture of neutral and zwitterionic structures, depending on the humidity of the sample, while mostly the zwitterion is observed for LD diphenylalanine whatever the experimental conditions. The system undergoes spontaneous cyclization upon heating at 125°C, resulting to the formation of a diketopiperazine (DKP) dipeptide as the only product. The reaction is faster for LD than for LL diphenylalanine. As expected, LL and DD diphenylalanine react to form the LL and DD enantiomers of cyclo diphenylalanine. Interestingly, the DKP dipeptides formed from the LD or DL diphenylalanine show unexpected optical activity, with opposite VCD spectra for the products formed from the LD and DL reagents. This is explained in terms of chirality synchronization between the monomers within the crystal, which retain the symmetry of the reagent, resulting to the formation of a new chiral phase made from transiently chiral molecules.
固态结构的 LL/DD 或 LD/DL 二苯丙氨酸稀释在 KBr 颗粒进行了红外（IR）吸收和振动圆二色（VCD）光谱研究。结构取决于残基的绝对构型。天然 LL 二苯丙氨酸存在中性和两性离子结构的混合物，取决于样品的湿度，而对于 LD 二苯丙氨酸，无论实验条件如何，主要观察到两性离子。该系统在 125°C 加热时会自发环化，导致二酮哌嗪（DKP）二肽作为唯一产物形成。LD 比 LL 二苯丙氨酸的反应更快。正如预期的那样，LL 和 DD 二苯丙氨酸反应形成环二苯丙氨酸的 LL 和 DD 对映体。有趣的是，由 LD 或 DL 二苯丙氨酸形成的 DKP 二肽表现出意想不到的旋光性，产物的 VCD 光谱对于由 LD 和 DL 试剂形成的产物是相反的。这可以用晶体中单体之间的手性同步来解释，这种手性同步保留了试剂的对称性，从而形成了由瞬态手性分子组成的新手性相。
