CYTOSINE-CARBOXYLATE INTERACTIONS - CRYSTAL-STRUCTURE OF CYTOSINIUM HYDROGEN MALEATE

Interactions of carboxylate groups of proteins with nucleobases are so me of the important stereochemical patterns involved in protein-nuclei c acid recognition. The title compound serves as a model for such inte ractions, involving constrained carboxyl groups. Space group: <P(1)ove r 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. Th e structure was solved by direct methods. Cytosine is protonated at N( 3). The usual fork-like interaction of the carboxylate groups with cyt osine is absent and a network of hydrogen bonding involving the cytosi ne moiety and hydrogen maleate is present. The interaction of carboxyl ate 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 (o f maleate) interaction.