(S)-2-(1H-Imidazol-1-yl)succinic acid

The title compound, C7H8N2O4, is a zwitterion, [formal name = (S)-3-carboxy-2-(imidazol-3-ium-1-yl)propanoate], in which the deprotonated negatively charged carboxylate end shows almost identical C—O bond distances [1.248 (4) and 1.251 (4) Å] due to resonance. The molecules are involved in intermolecular O—H⋯O and N—H⋯O hydrogen bonds, which define a tightly bound three-dimensional structure.


Experimental
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: BG2234).
(S)-2-(1H-Imidazol-1-yl)succinic acid J.-M. Xiao Comment Imidazol-1-ylalkanoic acids are used as new probes to determine the intracellular and extracellular pH and cell volume by 1 H NMR. (López et al., 1996). In this report we present the structure of (S)-2-(1H-imidazol-1-yl)succinic acid. As shown in Fig.   1, the title compound C 7 H 8 N 2 O 4 exists in the form of an inner salt where the unprotonated, negatively charged carboxylato end shows almost identical C-O bond distances (1.248 (4) and 1.251 (4)Å respectively) due to resonance.. The molecules are involved in intermolecular O-H···O and N-H···O hydrogen bonds (Table 1) which define a tightly bound 3D structure.

Experimental
The ligand was prepared according to a literature method (Bao et al., 2003). A formaldehyde water solution (36%, 1.67 g) and a glyoxal water solution (32%, 3.62 g) were mixed in a 50 ml, three-necked flask provided with a stirrer and a reflux condenser. While the mixture was heated at 50 °C with stirring, a mixture of L-2-aminosuccinic acid (2.66 g, 0.02 mol), ammonia solution (28%, 1.21 g) and sodium hydroxide solution (10%, 8 g) was added in small portions during 0.5 h. After the mixture was stirred for an additional 8 h at 50 °C, the cooled mixture was acidified to pH=3 with concentrated hydrochloric acid. After stirring for 30 min, the suspension was filtered. The resulting solid was washed with H 2 O and dried in vacuum over P2O5 at room temperature. Colourless crystals suitable for X-ray diffraction were obtained from a solution of 100 mg in 15 ml H 2 O by slow evaporation after one month.

Refinement
Positional parameters of all the H atoms except for H3C were calculated geometrically and the H atoms were set to ride on the C and N atoms to which they are bonded, with Uiso(H) = 1.2Ueq(C or N). The carboxyl H3C was initially refined and subsequently allowed to ride with Uiso(H) = 1.5Ueq(O). Due to the abscence of anomalous diffraction effects, Friedel pairs were merged. Fig. 1

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
supplementary materials sup-3 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 Rfactors(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.