Department of Chemistry, University of Oslo, N-0315 Oslo, Norway.
J Phys Chem B. 2012 Sep 6;116(35):10715-21. doi: 10.1021/jp305016k. Epub 2012 Aug 22.
Four polymorphs are known for the amino acid racemate DL-aminobutyric acid: one tetragonal form B and three monoclinic forms A, C, and D for which two solid-solid phase transitions take place between 209 and 337 K. The intermediate form A, in space group P2(1)/c, is known to display disorder in the crystalline state with the ethyl side chain occupying three different conformations. The high temperature form D and the low temperature form C have been assumed to display a similar type of disorder in the space group C2/c. The present investigation, on the basis of high quality crystal structures at 100 K (C form), 220 K (A form), 295 K (A form), and 295 K (D form), provides updated results for all three forms and in particular demonstrates that the A → C and A → D transitions in fact involve only a partial space group conversion, so that two monoclinic phases are simultaneously present at high and low temperatures. To the best of our knowledge, such a phenomenon has not been observed previously for an organic molecule. The nonroutine refinement of the crystallographic data was performed with a specially adapted version of the program JANA2006. The observed polymorphs are compared with those found for DL-norvaline, DL-norleucine, and DL-methionine.
已知氨基酸外消旋体 DL-氨基丁酸有四种多晶型物：一种四方晶型 B 和三种单斜晶型 A、C 和 D，其中两种固-固相变发生在 209 和 337 K 之间。中间相 A，属于空间群 P2(1)/c，已知在晶体状态下存在无序，其中乙基侧链占据三种不同的构象。高温相 D 和低温相 C 被认为在空间群 C2/c 中具有类似的无序类型。本研究基于高质量的晶体结构（100 K 时为 C 型、220 K 时为 A 型、295 K 时为 A 型和 295 K 时为 D 型），提供了所有三种形式的更新结果，特别是表明 A → C 和 A → D 转变实际上仅涉及部分空间群转换，因此在高温和低温下同时存在两种单斜相。据我们所知，这种现象以前在有机分子中尚未观察到。晶体数据的非常规精修是使用专门改编的 JANA2006 程序完成的。观察到的多晶型物与 DL-正缬氨酸、DL-正亮氨酸和 DL-蛋氨酸的多晶型物进行了比较。
