Ternary complexes in solution. Intramolecular stacking interactions in mixed ligand complexes formed by copper(II), 2,2′-bipyridyl or 1,10-phenanthroline and a pyrimidine-nucleoside 5′-diphosphate (CDP 3−, UDP 3−, dTDP 3−)

Abstract

The stability constants of the 1:1 complexes formed between Cu 2+ or Cu(Arm) 2+, where Arm = 2,2′-bipyridyl (Bpy) or 1,10-phenanthroline (Phen), and pyrimidine-nucleoside 5′-diphosphates (NDP 3−, i.e. CDP 3−, UDP 3− or dTDP 3−), were determined by potentiometric pH titrations in aqueous solution (25°C; I = 0.1 M, NaNO 3). It is shown that the stability of the binary Cu(NDP) − complexes is determined solely by the basicity of the diphosphate group; this is different for the ternary Cu(Arm)(NDP) − complexes. It is demonstrated that the equilibrium, Cu(Arm) 2+ + Cu(NDP) −Cu(Arm)(NDP) − + Cu 2+, is displaced considerably to the right-hand side. Part of this displacement is due to the well known experience that mixed ligand complexes formed by a divalent 3d ion, a heteroaromatic N base and an O donor ligand possess increased stability. This increased stability is now accounted for by the results obtained recently for the Cu 2+/Arm/methylphosphonylphosphate (CH 3-P(O) 2 −-O-PO 3 2−) systems (B. Song et al., Inorg. Chim. Acta, 273 (1998) 101). The other part of this displacement, which amounts on average to an increased stability of the mixed ligand Cu(Arm)(NDP) − complexes of about 0.35 log unit, is due to an intramolecular ligand—ligand stacking between the nucleobase residues and the aromatic rings of Bpy or Phen. The formation degree of these intramolecular stacks in the Cu(Arm)(NDP) − complexes corresponds to approximately 55%, thus leaving about 45% of the complexes in an unstacked or open form in this intramolecular isomeric equilibrium. The relevance of the results regarding biological systems is indicated briefly.