Crystal structure of (S)-2-amino-2-methylsuccinic acid

The title compound, C5H9NO4, crystallized as a zwitterion. There is an intramolecular N—H⋯O hydrogen bond involving the trans-succinic acid and the ammonium group, forming an S(6) ring motif. In the crystal, molecules are linked by O—H⋯O hydrogen bonds, forming C(7) chains along the c-axis direction. The chains are linked by N—H⋯O and C—H⋯O hydrogen bonds, forming sheets parallel to the bc plane. Further N—H⋯O hydrogen bonds link the sheets to form a three-dimensional framework.


S1. Comment
Solid-phase synthesis is now the accepted method to synthesis peptides, in which protected natural or non-natural amino acids are widely used; for example, 2-methylaspartic acid (MeASP) a non-natural amino acid. It has attracted attention as a substrate analog of aspartate aminotransferase (EC 2.6.1.1), and acts as a competitive inhibitor in the external aldimine (Delbaere et al., 1989;Nobe et al., 1998). Despite the biological and pharmaceutical interest, no crystal structures of MeASP derivatives have been reported except for the structure of DL-MeASP monohydrate (Brewer et al., 2013).
In the title compound, Fig. 1, the succinic acid group has a trans-conformation versus. a cis-conformation [48.8 (4) °] in DL-MeASP. The carboxy group and the amino group make a hydrogen bonded half-chair S(6) ring motif (Table 1 and Fig. 1). The S(6) ring half-chair conformation and the trans-succinic acid arrangement are similar to the situation found in for DL-ASP (DLASPA03: Flaig et al. 1998).
In the crystal, molecules are linked by O-H···O hydrogen bonds, involving the succinic acid groups, to form C (7) chains along the c axis direction (Table 1 and Fig. 2). This is in contrast to the N-H···O hydrogen bonded C(5) chains observed in the crystal structure of DL-MeASP. The chains are linked by N-H···O and C-H···O hydrogen bonds forming sheets parallel to the bc plane. Further N-H···O hydrogen bonds link the sheets to form a three-dimensional framework (Table 1 and Fig. 3). The methyl groups are surrounded by the hydrophilic planes and make a columnar structure (Fig. 3).

S2. Synthesis and crystallization
The title compound was purchased from Nagase-Sangyo Co. Ltd. The absolute configuration could not be established by anomalous-dispersion effects. The (S) enantiomer has been chosen by referring the sign of known polarity in the synthetic procedure (Terashima et al., 1966). Rod-like colourless crystals of the title compound were obtained by vapour-phase diffusion of an ethanol-chloroform mixture at room temperature.

S3. Refinement
Crystal data, data collection and structure refinement details are summarized in

Figure 1
A view of the molecular structure of the title compound, with atom labelling. Displacement ellipsoids are drawn at the 50% probability level. The dashed line indicates the intramolecular N-H···O hydrogen bond (see Table 1).

Figure 2
A partial view of the crystal packing of the title compound. Dashed lines indicate the O-H···O and N-H···O hydrogen bonds (see Table 1).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (