2-Methylaspartic acid monohydrate

The title compound, C5H9NO4·H2O, is an isomer of the α-amino acid glutamic acid that crystallizes from water in its zwitterionic form as a monohydrate. It is not one of the 20 proteinogenic α-amino acids that are used in living systems and differs from the natural amino acids in that it has an α-methyl group rather than an α-H atom. In the crystal, an O—H⋯O hydrogen bond is present between the acid and water molecules while extensive N—H⋯O and O—H⋯O hydrogen bonds link the components into a three-dimensional array.

The title compound, C 5 H 9 NO 4 ÁH 2 O, is an isomer of theamino acid glutamic acid that crystallizes from water in its zwitterionic form as a monohydrate. It is not one of the 20 proteinogenic -amino acids that are used in living systems and differs from the natural amino acids in that it has anmethyl group rather than an -H atom. In the crystal, an O-HÁ Á ÁO hydrogen bond is present between the acid and water molecules while extensive N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds link the components into a three-dimensional array.
RJB wishes to acknowledge the NSF-MRI program (grant CHE-0619278) for funds to purchase the diffractometer. GB wishes to acknowledge support of this work from NASA (NNX10AK71A).

Comment
The α-amino acids are essential for life as they are the building blocks of all proteins and enzymes. Nature uses almost exclusively the L form of the nineteen common chiral amino acids. However, there are over eighty amino acids that have been identified in meteorites (Pizzarello et al., 2006;Burton et al., 2012). One of these extraterrestrial non-proteinogenic amino acids is 2-methylaspartic acid. The majority of meteoritic amino acids show little or no enrichment of one enantiomer over the other. However, a number of alpha methyl amino acids have been observed with L-enantiomeric excesses up to 20% that are not believed to be the result of contamination (Pizzarello & Cronin, 2000;Glavin & Dworkin, 2009;Glavin et al., 2011;Glavin et al., 2012;Burton et al., 2013). An intriguing question is the process that leads to the separation and enrichment of the L enantiomer over the D. There are several possible explanations for this including the role that crystallization plays (Blackmond et al., 2007;Glavin et al., 2012). Only two of the twenty amino acids used biologically crystallize in a chiral space group from a racemic solution, which allows for spontaneous separation of enantiomers, at the level of the crystal (Blackmond et al., (2008).
Racemic 2-methylaspartic crystallizes from water in an achiral space group acid, forming a racemic compound, in which there are equal numbers of D and L enantiomers in the unit cell. Thus, crystallization under these conditions would not provide a mechanism for separation of enantiomers at the level of the crystal. Another important aspect in the prebiotic chemistry of the amino acids is the role of racemization. All of the nineteen naturally occurring chiral amino acids have a hydrogen atom on the alpha carbon atom, which enhances the rate of racemization (Yamada et al., 1983).
However, little is known about the mechanism of racemization of amino acids lacking an alpha hydrogen atom (Pizzarello et al., 2011). We recently reported the structure of another non-proteinogenic amino acid, isovaline, which crystallized as a racemic conglomerate from water in contrast to the present example which crystallizes in a centrosymmetric space group and is thus a racemate as indicated above (Butcher et al., 2013). Resolved 2-methylaspartic acid and the structure given here can be used as a starting point in mechanistic studies of racemization mechanisms of amino acids lacking an alpha hydrogen atom.
In the structure of the title compound the amino acid is in the usual zwitterionic form involving the α carboxylate group and all the the bond lengths and angles are in the normal range for such compounds (Orpen, 1993). There is extensive N -H···O and O-H···O hydrogen bonding linking the zwitterions into a 3-D array.

Computing details
Data collection: CrysAlis PRO (Agilent, 2012); cell refinement: CrysAlis PRO (Agilent, 2012); data reduction: CrysAlis PRO (Agilent, 2012); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).  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 (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.