title: Structures of N-acetyl-DL-isoleucine, N-acetyl-DL-alloisoleucine and their ammonium salts; role of ammonium ions in crystal structure formation.
abstract: 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.
authors.affiliation: Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan.
authors.forename: Tatsuo
authors.initials: T
authors.lastname: Yajima
authors.affiliation: Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan.
authors.forename: Makiko
authors.initials: M
authors.lastname: Kimura
authors.affiliation: Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan.
authors.forename: Yoshihiro
authors.initials: Y
authors.lastname: Hori
authors.affiliation: Faculty of Chemistry, Materials and Bioengineering, Kansai University, 3-3-35 Yamate-cho, Suita, Osaka 564-8680, Japan.
authors.forename: Tadashi
authors.initials: T
authors.lastname: Shiraiwa
mesh_terms: D000107:Acetylation; D064751:Ammonium Compounds / Q000737:chemistry*; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D007477:Ions / Q000737:chemistry; D007532:Isoleucine / Q000031:analogs & derivatives*; D008958:Models, Molecular; D008968:Molecular Conformation; D012492:Salts / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D064751:Ammonium Compounds; D007477:Ions; D012492:Salts; D007532:Isoleucine
keywords: N-protected amino acid; alloisoleucine; ammonium salt; hydrogen-bond network; isoleucine