Isovaline monohydrate

The title compound, C5H11NO2·H2O, is an isomer of the α-amino acid valine that crystallizes from water in its zwitterion 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 no α-H atom. The compound exhibits hydrogen bonding between the water molecule and the carboxylate O atoms and an amine H atom. In addition, there are intermolecular hydrogen-bonding interactions between the carboxylate O atoms and amine H atoms. In the crystal, these extensive N—H⋯O and O—H⋯O hydrogen bonds lead to the formation of a three-dimensional network.

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 alpha 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 naturally occurring chiral amino acids. Glycine is achiral. However, it is known that there are over eighty amino acids that have been detected in meteorites or comets (Glavin & Dworkin 2009;Burton et al., 2012). One of these extraterrestrial non proteinogenic amino acids is isovaline. The majority of these amino acids show little or no enrichment of one enantiomer over the other. An intriguing question is the process that lead 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 & Klussmann, 2007). Only two of the twenty amino acids used biologically crystallize in a chiral space group, which allows for spontaneous separation of enantiomers, at the level of the crystal, from a racemic solution (Blackmond et al., 2008). Isovaline, a non proteinogenic amino acid also allows for this separation of enantiomers at the level of the crystal as it crystallizes in the chiral space group, P2 1 2 1 2 1 .
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 & Groy, 2011). The structure of isovaline given here can be used to examine the role that crystallization can play in the enrichment of L isovaline (Glavin & Dworkin 2009;Bada, 2009), and as a starting point in mechanistic studies of racemization mechanisms of isovaline (Bonner et al., 1979).
In the structure of the title compound the amino acid is in the usual zwitterionic form. While the structure of the title compound or its salts have not been reported there are reports of homoleptic coordination complexes of isovaline with zinc(II) (Strasdelt et al. (2001). However, there are several important differences between these structures and the title compound. The metal complexes all crystallized in non-chiral space groups and, of course, when coordinated to a metal, the isovaline will not be in a zwitterionic form but, apart from the COOgroup, 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.

Experimental
A sample of the title compound was obtained from Acros Organics. Crystals of the title compound were grown from the slow evaporation of a racemic solution of the amino acid in water.

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).  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.