Elementary Patterns in Protein-Nucleic Acid Interactions. III. Crystal Structure of Adenine: Phthalic Acid (3:1) Complex Hexahydrate

Crystal of adenine :phthalic acid (3:1) complex hexahydrate were obtained from an aqueous solution containing equimolar amounts of adenine and N,N-phthaloyl-Dl-glutamic acid. The space group is P1 with unitcell dimensions of a=14.232(2), b=16.730(2), c=7.336(1) Å, α=78.97(1), β=96.14(1), and γ=68.99(1)°. The structure was solved by the direct method and its parameters were refined by a block-diagonal least-squares technique. The two carboxyl groups of the phthalic acid are dissociated and they are hydrogen-bonded to the amino group and the protonated N(1) of the two adeninium monocations. The geometrical characteristics in adeninium monocations are explained with the electronic structures perturbed by the surroundings. A typical feature of the C–H···N hydrogen bond is observed between the adeninium cations. The third adenine molecule is neutral and disordered. A comparison of the dissociation constants of the related compounds suggests that the protonation of adenine occurs when the hydrogen bonds are formed between the carboxyl group of acidic amino acid residue in protein and the unpaired adenine base in nucleic acid.