Stacking interactions between aromatic amino acids and adenine ring of ATP in zinc mediated ternary complexes

Spectrophotometric studies have provided evidence for zinc-mediated ternary complexes between ATP and aromatic amino acids. The hypochromicity observed in the 260 nm band of ATP increased in the order phenylalanine less than tyrosine less than tryptophan. Adding alanine did not produce any change of the ATP spectrum. The association constant was four fold higher for the ATP-Zinc-Tryptophan complex than for that of the ATP-Zinc-Alanine. The increased stability of the former complex was ascribed to the stacking interaction between indole and adenine rings. The maximum concentration of the ATP-Zinc-Tryptophan complex occurred at about pH 8.0. For these ternary complexes several possible stacked structures involving or not involving N(7) of adenine are discussed.