Cytosinium–hydrogen maleate–cytosine (1/1/1)

The title organic salt, C4H6N3O+·C4H3O4 −·C4H5N3O, was synthesized from cytosine base and maleic acid. An intramolecular O—H⋯O hydrogen bond occurs in the hydrogen maleate anion. The crystal packing is stabilized by intermolecular N—H⋯O, N—H⋯N and C—H⋯O hydrogen bonds, giving rise to a nearly planar two-dimensional network parallel to (101).


Comment
The pyrimidine base, Cytosine, leads to the nucleoside cytidine and its corresponding nucleotide: cytidine 5'-monophosphate. It may be found in very small quantities as a post-modified form, 5-methylcytosine, in certain nucleic acids (Devlin, 1986) such as in tuberculinic acid (Johnson & Coghill, 1925). More recently, 5-fluoro-cytosine (5-FC) has been used as a prodrug in suicide gene therapy of cancer with the crystal structure of bacterial cytosine deaminase (bCD) (Mahan et al., 2004).
We report here the molecular structure of a novel compound (I) formed from the reaction of cytosine with maleic acid, namely cytosine cytosinium hydrogen maleate. It was prepared in order to extend our study on D-H···A hydrogen bonding in organic systems.
The asymmetric unit in (I) contains a hydrogen maleate anion, a cytosinium cation and a cytosine molecule which are held together by N-H···O and N-H···N hydrogen bonds ( Fig. 1; Table 1). As observed in other hydrogen maleate anion, the H atom is roughly in between O1 and O3 (Madsen & Larsen, 1998).

Experimental
The title compound was prepared by the reaction between cytosine and maleic acid. A colorless prismatic single-cristals were grown after few days of evaporation at room temperature.

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.
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.