ADENOSINE-5'-CARBOXYLIC ACID

The two molecules in the asymmetric unit of adenosine-5'-carboxylic ac id, C10H11N5O5, exist as zwitterions with N1 protonated and the carbox yl groups ionized. Both molecules are in an anti conformation with gly cosyl torsion angles of -161.4(3) and -155.5(3)degrees. The ribose moi eties adopt a C3-endo-C2-exo twist conformation. The pseudo-rotation p arameters are P = 0.01(1) and 6.58(1)degrees, and tau(m) = 36.2(2) and 34.6(2)degrees, for molecules A and B, respectively. The carboxyl gro ups of A and B are not in the standard g(+), g(-) or t conformations. Both Watson-Crick sites, N1 and N6, of the adenine bases are involved in a pair of hydrogen bonds with the dissociated carboxyl groups, form ing a cyclic tetramer. The adenine base of molecule A stacks on the ri bose O4' atom of a symmetry-related B molecule at a distance of 2.88 A ngstrom; the adenine base of B stacks in an analogous way at a distanc e of 2.91 Angstrom.