l-Alanine hydrochloride monohydrate

Colorless crystals of l-alanine hydrochloride monohydrate, C3H8NO2 +·Cl−·H2O, were obtained from a powder sample that had been left standing in a refrigerator for a few years. The structure displays several intermolecular hydrogen bonds: the hydroxyl O atom is involved in a single hydrogen bond to the chloride anion, while the ammonium group forms one hydrogen bond to the chloride anion and two hydrogen bonds to water molecules. An intermolecular bond between the carbonyl O atom and the ammonium group [2.8459 (15) Å] is also found.

KY appreciates support from the Nanotechnology Support Project of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. SY and TY appreciate support from the RIKEN Structural Genomics/Proteomics Initiative (RSGI), the National Project on Protein Structural and Functional Analyses, and MEXT. Comment L-Alanine is one of the 20 proteinogenic amino acids and has been currently recognized as one of the most abundant amino acids in natural proteins. In general, amino acids very often have polymorphs. The crystal structures of L-alanine (Simpson & Marsh, 1966;Dunitz & Ryan, 1966;Lehmann et al., 1972;Destro et al., 1988), DL-alanine (Donohue, 1950;Subha Nandhini et al., 2001;Trotter, 1962), and D-alanine hydrochloride (di Blasio et al., 1977) have been reported. In the present study, a single-crystal structure determination of L-alanine hydrochloride monohydrate, (I), is reported.
The distances and angles of the present L-alanine molecule are consistent in the typical values reported in the literature of L-alanine molecules (See Table 1 and Figure 1). The atoms N, C2, C3, O1, and O2 are found to be nearly coplanar.
The single-crystal diffraction analysis exhibits that the titled compound contains several intermolecular hydrogen bonds.
O2 is involved to a single hydrogen bond to a chloride ion with the hydrogen bond distance of 3.0446 (11) Å, while O1 is not involved to hydrogen bonds. Instead, a short contact is observed between O1 and N with the intermolecular bond length of 2.8450 (15) Å. A water molecule donates four hydrogen bonds to two chlorides and two ammonium groups, while a chloride ion accepts four hydrogen bonds from two water molecules, and ammonium and hydroxyl groups (See Table 2 and Figure 2).
It is interesting to compare the present structure with that of D-alanine hydrochloride (di Blasio et al., 1977). In the anhydrous D-alanine hydrochloride crystal, O1 (carbonyl oxygen) exhibits a single hydrogen bond to an ammonium group, and O2 (hydroxyl oxygen) also forms a single hydrogen bond to a chloride ion. The chloride anion, on the other hand, forms three hydrogen bonds, two of which with ammonium groups and one of which with a hydroxyl group. These intermolecular environments are different from the present observations.

Experimental
In the title compound, oxygen-17 isotope enrichments were carried out to the carboxyl group with the aim to perform solid-state 17 O NMR experiments. L-alanine hydrochloride was obtained by acid-catalyzed exchange (Steinschneider et al., 1981)  All the H atoms except for H4 and H5 were treated as riding atoms with the following distances: for the methyl C-H distance, C-H = 0.96 Å and U iso (H) = 1.5U eq (C); for the methyne C-H distance, C-H = 0.98Å and U iso (H) = 1.2U eq (C); for the hydroxyl O-H distance, O-H = 0.82Å and U iso (H) = 1.2U eq (O); for the ammonium N-H distance, N-H = 0.89Å and U iso (H) = 1.5U eq (N). The H4 and H5 atoms were found in difference density Fourier maps, and their positions and isotropic displacement parameters were freely refined.
It might be possible that some degree of racemization occurred since the titled compound had been placed in the refrigerator for a few years. The present diffraction measurements, however, exhibited negligible recemization (the Flack parameter = 0.02 (6)).
It should be noted that "Alert Level B (detecting a pseudo center of symmetry)" was generated by checkCIF/PLATON REPORT during the course of the publication check. The error may come from the fact that the chloride anions are close to a center of symmetry. The present experimental data certainly suggest non-centrosymmetric. Fig. 1. A view of the molecular structure of (I), showing the atom labeling scheme. Displacement ellipsoids are drawn at the 50% probability level.