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

To the best of our knowledge, for the first time the stabilities of sextern ary complexes are determined by potentiometric pH titrations in aqueous sol ution 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 qu aternary cis-(NH3)(2)Pt(dGuo)(dGMP) complex. It is shown by stability const ant comparisons and spectrophotometric measurements (observation of charge- transfer bands for the Phen system) that the [cis-(NH3)(2)Pt(dGuo)(dGMP). C u(Arm)](2+) complexes can fold in such a way that aromatic ring stacking be tween the aromatic rings of Bpy or Phen and a guanine residue (most probabl y 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(B py)](2+) and [cis-(NH3)(2)Pt(dGuo)(dGMP). Cu(Phen)](2+) species, respective ly. 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 sexternar y complexes inhibits stacking but does not prevent it. This result is of ge neral importance because it demonstrates that in aqueous solution purine re sidues of nucleotides or nucleic acids that carry a metal ion at N7 can sti ll undergo stacking interactions with other suitable aromatic ring systems.