Influence of a Decreasing Solvent Polarity on the Stability and Structure of Mixed-Ligand Complexes Formed by Copper(II), 2,2'-Bipyridine or 1,10-Phenanthroline and Guanosine 5'-Diphosphate

Abstract

The stability consts. of the 1:1 complexes formed between Cu(Arm) 2+, where Arm = 2,2'-bipyridine or 1,10-phenanthroline, and (GDP)3- or its monoprotonated form H(GDP)2- were detd. by potentiometric pH titrns. in water and in water contg. 30 or 50 vol.% of 1,4-dioxane (25°; I = 0.1 M, NaNO3). The stability of the binary Cu(GDP)- complex is enhanced due to macro-chelate formation of the diphosphate-coordinated Cu2+ with N7 of the guanine residue as previously shown. In Cu(Arm)(GDP)- the N7 is released from Cu2+ and the stability enhancement of more than one log unit in aq. soln. is clearly attributable to intramol. stack formation between the arom. rings of Arm and the guanine moiety. Indeed, stacked isomers occur to more than 90% in equil. with open unstacked forms. Surprisingly, the same formation degrees of the stacks have previously been obsd. for Cu(Arm)(dGMP) complexes, where dGMP2- = 2'-deoxyguanosine 5'-monophosphate, despite the fact that the overall stability of the latter species is by about 2.7 log units lower. In 1,4-dioxane-water mixts. stack formation is drastically reduced, probably due to hydrophobic solvation of the arom. rings by the ethylene bridges of 1,4-dioxane. The relevance of these results regarding biol. systems is indicated