Yajima Tatsuo, Kimura Makiko, Hori Yoshihiro, Shiraiwa Tadashi
Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan.
Acta Crystallogr B Struct Sci Cryst Eng Mater. 2016 Aug 1;72(Pt 4):650-7. doi: 10.1107/S2052520616007319.
The crystal structures of N-acetyl-DL-isoleucine, N-acetyl-DL-alloisoleucine and their ammonium salts show that these four compounds exist as racemic compounds around room temperature. The two ammonium salts are arranged around a 21 screw axis, forming a helical column which consists of ammonium ions and single enantiomeric anions similar to the crystals of the ammonium salts of optically active N-acetyl-L-isoleucine and N-acetyl-D-alloisoleucine. The ammonium ion and the carboxylate ion in the helix are connected by three hydrogen bonds, the fourth hydrogen bond being formed between the ammonium ion and an external acetyl amino group of the neighboring helical column. The fourth hydrogen bond is formed between the ammonium ion and an external acetyl amino group of the neighboring 21 column. Ammonium N-acetyl-DL-alloisoleucinate was revealed to exist as an unstable racemic compound due to conformational similarity between the racemic and optically active compounds in the solid state and was optically resolved by fractional crystallization at 293 K.
N-乙酰-DL-异亮氨酸、N-乙酰-DL-别异亮氨酸及其铵盐的晶体结构表明，这四种化合物在室温左右以外消旋化合物形式存在。两种铵盐围绕一个21螺旋轴排列，形成一个螺旋柱，该螺旋柱由铵离子和单一对映体阴离子组成，类似于光学活性的N-乙酰-L-异亮氨酸和N-乙酰-D-别异亮氨酸铵盐的晶体。螺旋中的铵离子和羧酸根离子通过三个氢键相连，第四个氢键在铵离子与相邻螺旋柱的外部乙酰氨基之间形成。第四个氢键在铵离子与相邻21柱的外部乙酰氨基之间形成。由于固态外消旋化合物与光学活性化合物之间的构象相似性，N-乙酰-DL-别异亮氨酸铵被发现以不稳定的外消旋化合物形式存在，并在293 K下通过分步结晶进行光学拆分。
