Isomeric equilibria in aqueous solution involving aromatic ring stacking in the sexternary complexes formed by the quaternary cis-(NH3)2Pt(2′-deoxyguanosine-N7)(dGMP-N7) complex and the binary Cu(2,2′-bipyridine)2+ or Cu(1,10-phenanthroline)2+ complexes (dGMP2-=2′-deoxyguanosine 5′-monophosphate)

To the best of our knowledge, for the first time the stabilities of sexternary complexes are determined by potentiometric pH titrations in aqueous solution at 25 degrees C and I = 0.1 M (NaNO3). The sexternary complexes form by binding of the binary Cu(Arm)(2+) complexes, where Arm = 2,2'-bipyridine (Bpy) or 1,10-phenanthroline (Phen), to the -PO32- group present in the quaternary cis-(NH3)(2)Pt(dGuo)(dGMP) complex. It is shown by stability constant comparisons and spectrophotometric measurements (observation of charge-transfer bands for the Phen system) that the [cis-(NH3)(2)Pt(dGuo)(dGMP). Cu(Arm)](2+) complexes can fold in such a way that aromatic ring stacking between the aromatic rings of Bpy or Phen and a guanine residue (most probably the one of dGMP(2-)) becomes possible. The formation degree of the stacks reaches approximately 25 and 50% for the [cis-(NH3)(2)Pt(dGuo)(dGMP). Cu(Bpy)](2+) and [cis-(NH3)(2)Pt(dGuo)(dGMP). Cu(Phen)](2+) species, respectively. By comparisons with Cu(Arm)(dGMP) complexes, it is shown that the cis-(NH3)(2)Pt2+ unit coordinated to N7 of the guanine residues in the sexternary complexes inhibits stacking but does not prevent it. This result is of general importance because it demonstrates that in aqueous solution purine residues of nucleotides or nucleic acids that carry a metal ion at N7 can still undergo stacking interactions with other suitable aromatic ring systems.