A cocrystal of pyridine-2,4-dicarboxylic acid and serine

The title compound, pyridine-2,4-dicarboxylic acid–S-serine (1/1), C7H5NO4·C3H7NO3, has serine in its zwitterionic form. The crystal structure is stabilized by an extensive series of intermolecular O—H⋯O, N—H⋯N and N—H⋯O hydrogen bonds, forming a three-dimensional network.


Experimental
Crystal data

Comment
The interactions of organic acids with amino acids in the solid state have been widely investigated due to their interesting hydrogen-bonding interactions (Coupar et al., 1997;Sobczyk et al., 2000;Pandiarajan et al., 2001;Srinivasan et al., 2002).
We report here the structure of the title co-crystal (I), Fig.1, formed from pyridine-2,4-dicarboxylic acid and serine in its zwitterionic form.
The crystal is stabilized by an extensive array of intermolecular O-H···O, N-H···N, and N-H···O hydrogen bonds (Table   1), forming a three-dimensional network (Fig. 2).

Experimental
The compound was crystallized by slow evaporation of an equimolar solution of pyridine-2,4-dicarboxylic acid and serine in a solution of ethanol/water (1:1, v/v).

Refinement
In the absence of significant anomalous dispersion effects, Friedel pairs were averaged. The H2A, H2B, and H2C atoms of the serine NH 3 group were located from a difference Fourier map and refined isotropically, with N-H distances restrained to 0.90 (1) Å, H···H distances restrained to 1.43 (2) Å, and with U iso (H) values fixed at 0.08 Å 2 . The other H atoms were placed in idealized positions and constrained to ride on their parent atoms with C-H distances in the range 0.93-0.98 Å, O-H distances of 0.82 Å, and with U iso (H) set at 1.2U eq (C) and 1.5U eq (O). Fig. 1. The asymmetric unit of the compound, showing 30% probability displacement and the atom-numbering scheme.

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 > 2sigma(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.