State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Wuhan, PR China.
J Phys Chem B. 2011 Mar 31;115(12):2814-23. doi: 10.1021/jp110224b. Epub 2011 Mar 10.
DL-Norvaline (NVA) is an important pharmaceutical intermediate and undergoes two polymorphic transitions between 140 and 300 K. To understand molecular dynamics of NVA accompanied with these transitions, we conducted a comprehensive study on its molecular motions at multiple time scales (10(-9)-1 s) with various solid-state NMR methods. (13)C CPMAS NMR spectra revealed four sets of clearly resolved signals for NVA carbons corresponding to at least three crystal modifications with two polymorphic transitions. Proton and (13)C relaxation results showed that, apart from the reorientations of methyl and amino groups, NVA had another relaxation process following the second transition with the activation energy of 16-21 kJ/mol corresponding to the side-chain motions. This was further confirmed with the data from dipolar and chemical shift experiments. No motions were detected at CODEX time scale (ms-s). These results provide essential information for understanding the mechanistic aspects of the polymorphic transitions in aliphatic α-amino acids with linear side-chains.
DL-正缬氨酸（NVA）是一种重要的药物中间体，在 140 到 300K 之间经历两次多晶型转变。为了了解 NVA 伴随这些转变的分子动力学，我们使用各种固态 NMR 方法在多个时间尺度（10(-9)-1s）上对其分子运动进行了全面研究。(13)C CPMAS NMR 谱表明，NVA 碳的四个明显分辨信号对应于至少三种晶体修饰形式，存在两个多晶型转变。质子和(13)C 弛豫结果表明，除了甲基和氨基的旋转外，NVA 在第二个转变后还有另一个弛豫过程，其活化能为 16-21kJ/mol，对应于侧链运动。这一结果通过偶极和化学位移实验得到了进一步证实。在 CODEX 时间尺度（ms-s）上没有检测到运动。这些结果为理解具有线性侧链的脂肪族α-氨基酸的多晶型转变的机制方面提供了重要信息。
