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 (ADP(3-)). 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 (ADP(3-)) or its monoprotonated form H(ADP)(2-) we re determined by potentiometric pH titrations in aqueous solution (25 degre es C; I = 0.1 M, NaNO3). It is shown that the stability of the binary Cu(AD P)(-) complex is enhanced due to macrochelate formation of the diphosphate- coordinated metal ion with N7 of the adenine residue. Such a macrochelate i s also formed in the monoprotonated Cu(H;ADP) complex in which the proton i s at the terminal P-phosphate group. The latter is also true for the ternar y 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 o f the stacking tendencies observed for a series of Cu(Arm)(N) complexes, wh ere N = AMP(2-), ADP(3-) and ATP(4-) (adenosine 5'-mono-, di-, or triphosph ate) or UMP2-, UDP3- or UTP4- (uridine 5'-mono-, di-, or triphosphate), rev eals that the extent of intramolecular stack formation in the complexes doe s not depend significantly on the length of the phosphate residue but rathe r on the size of the nucleobase, i.e. one ring (uracil) versus two rings (a denine). Roughly speaking, the formation degree of the intramolecular stack s in the ternary complexes containing the uracil residue amounts to about 5 0% (corresponding to a stability increase of about 0.3 log units) whereas i n the corresponding adenine complexes about 80-90% (corresponding to a stab ility enhancement of approximately 0.8-1 log units) are reached; the releva nce of this kind of adduct formation for recognition reactions in nature is evident. (C) 2000 Elsevier Science S.A. All rights reserved.