Intramolecular stacking interactions in mixed ligand complexes formed by copper(II), 2,2′-bipyridine or 1,10-phenanthroline, and monoprotonated or deprotonated adenosine 5′-diphosphate (ADP3−). Evaluation of isomeric equilibria

The stability constants of the 1:1 complexes formed between Cu2+ or Cu(Arm)2+, where Arm=2,2′-bipyridine (Bpy) or 1,10-phenanthroline (Phen), and adenosine 5′-diphosphate (ADP3−) or its monoprotonated form H(ADP)2− were determined by potentiometric pH titrations in aqueous solution (25°C; I=0.1 M, NaNO3). It is shown that the stability of the binary Cu(ADP)− complex is enhanced due to macrochelate formation of the diphosphate-coordinated metal ion with N7 of the adenine residue. Such a macrochelate is also formed in the monoprotonated Cu(H;ADP) complex in which the proton is at the terminal β-phosphate group. The latter is also true for the ternary Cu(Arm)(H;ADP) species, but here intramolecular stacks are formed between the aromatic rings of Arm and the adenine moiety. The isomeric equilibria of both protonated complexes are evaluated. The enhanced stability of about 0.7–0.8 log units of the Cu(Arm)(ADP) complexes is clearly attributable to intramolecular stack formation; indeed, the corresponding isomer occurs to about 80% being in equilibrium with the open, unstacked form. Comparison of the stacking tendencies observed for a series of Cu(Arm)(N) complexes, where N=AMP2−, ADP3− and ATP4− (adenosine 5′-mono-, di-, or triphosphate) or UMP2−, UDP3− or UTP4− (uridine 5′-mono-, di-, or triphosphate), reveals that the extent of intramolecular stack formation in the complexes does not depend significantly on the length of the phosphate residue but rather on the size of the nucleobase, i.e. one ring (uracil) versus two rings (adenine). Roughly speaking, the formation degree of the intramolecular stacks in the ternary complexes containing the uracil residue amounts to about 50% (corresponding to a stability increase of about 0.3 log units) whereas in the corresponding adenine complexes about 80–90% (corresponding to a stability enhancement of approximately 0.8–1 log units) are reached; the relevance of this kind of adduct formation for recognition reactions in nature is evident.