Adenosine-5'-carboxylic Acid

The two molecules in the asymmetric unit of adenosine 5′-carboxylic acid, C₁₀H₁₁N₅O₅, exist as zwitterions with N1 protonated and the carboxyl groups ionized. Both molecules are in an anti conformation with glycosyl torsion angles of −161.4 (3) and −155.5 (3)°. The ribose moieties adopt a C3-endo–C2-exo twist conformation. The pseudorotation parameters are P = 0.01 (1) and 6.58 (1)°, and τ_m = 36.2 (2) and 34.6 (2)° for molecules A and B, respectively. The carboxyl groups 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 forming a cyclic tetramer. The adenine base of molecule A stacks on the ribose-O4′ of a symmetry-related B molecule at a distance of 2.88 Å; the adenine base of B stacks in an analogous way at a distance of 2.91 Å.

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