Cytosine-carboxylate interactions: Crystal structure of cytosinium hydrogen maleate

Interactions of carboxylate groups of proteins with nucleobases are some of the important stereochemical patterns involved in protein-nucleic acid recognition. The title compound serves as a model for such interactions, involving constrained carboxyl groups. Space group: <P(1)over bar>, a = 7.424(1), b = 7.952(2), c = 8.248(2) Angstrom, alpha = 106.61(3)degrees, beta = 94.97(3)degrees, and gamma = 91.26(3)degrees. The structure was solved by direct methods. Cytosine is protonated at N(3). The usual fork-like interaction of the carboxylate groups with cytosine is absent and a network of hydrogen bonding involving the cytosine moiety and hydrogen maleate is present. The interaction of carboxylate group and the amino group of cytosine observed here is reminiscent of carboxylate-GC pair interaction observed in a protein-DNA complex (DNA repressor of phage 434 complex). There is also a C(6)-H .... O (of maleate) interaction.