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 (CDP3-, UDP3-, DTDP(3-))

The stability constants of the 1.1 complexes formed between Cu2+ or Cu (Arm)(2+), where Arm = 2,2'-bipyridyl (Bpy) or 1,10-phenanthroline (Ph en), and pyrimidine-nucleoside 5'-diphosphates (NDP3-, i.e. CDP3-, UDP 3- or dTDP(3-)), were determined by potentiometric pH titrations in aq ueous solution (25 degrees C; I = 0.1 M, NaNO3). It is shown that the stability of the binary Cu(NDP)(-) complexes is determined solely by t he basicity of the diphosphate group; this is different for the ternar y Cu(Arm) (NDP)- complexes. It is demonstrated that the equilibrium, C u(Arm)(2+) + Cu(NDP)(-) reversible arrow Cu(Arm) (NDP)(-) + Cu2+, is d isplaced considerably to the right-hand side. Part of this displacemen t is due to the well known experience that mixed Ligand complexes form ed by a divalent 3d ion, a heteroaromatic N base and an O donor ligand possess increased stability. This increased stability is now accounte d for by the results obtained recently for the Cu2+/Arm/methylphosphon ylphosphate (CH3-P(O)(2)(-)-O-PO32-) systems (B. Song et al., Inorg. C him. Acta, 273 (1998) 101). The other part of this displacement, which amounts on average to an increased stability of the mixed ligand Cu(A rm) (NDP) - complexes of about 0.35 log unit, is due to an intramolecu lar ligand-ligand stacking between the nucleobase residues and the aro matic rings of Bpy or Phen. The formation degree of these intramolecul ar stacks in the Cu(Arm) (NDP) - complexes corresponds to approximatel y 55%, thus leaving about 45% of the complexes in an unstacked or open form in this intramolecular isomeric equilibrium. The relevance of th e results regarding biological systems is indicated briefly. (C) 1998 Elsevier Science S.A. All rights reserved.