Influence of decreasing solvent polarity (1,4-dioxane/water mixtures) on the acid-base and copper(II)-binding properties of guanosine 5′-diphosphate

The acidity constants of twofold protonated guanosine 5 '-diphosphate, H-2(GDP)(-), and the stability constants of the [Cu(H;GDP)] and [Cu(GDP)](-) complexes were determined in H2O as well as in 30 or 50% (v/v) 1,4-dioxane/H2O by potentiometric pH titrations (25 degrees; l =0.1 (M), NaNO3). The results showed that in H2O one of the two protons of H-2(GDP)(-) is located mainly at the N(7) site and the other one at the terminal beta-phosphate group. In contrast, for 50% 1,4-dioxane/H2O solutions, a micro acidity-constant evaluation evidenced that ca. 75% of the H2(GDP)- species have both protons phosphate-bound, because the basicity of pyridine-type N sites decreases with decreasing solvent polarity whereas the one of phosphate groups increases. In the [Cu(H;GDP)] complex, the proton and the metal ion are in all three solvents overwhelmingly phosphate-bound, and the release of this proton is inhibited by decreasing polarity of the solvent. Based on previously determined straight-line plots of log K-Cu(R-DP)(Cu) vs. pK(H(R-DP))(H) (where R represents a non-interacting residue in simple diphosphate monoesters ROP(O-)(=O)-O-P(=O)(O-)(2), R-DP3-), which were now extended to mixed solvents (based on analogies), it is concluded that, in all three solvents, the [Cu(GDP)]- complex is more stable than expected based on the basicity of the diphosphate residue. This increased stability is attributed to macrochelate formation of the phosphate-coordinated Cull with N(7) of the guanine residue. The formation degree of this macrochelate amounts in aqueous solution to ca. 75% (being thus higher than that of the Cull complex of adenosine 5 '-diphosphate) and in 50% (v/v) 1,4-dioxane/H2O to ca. 60%, i.e.. the formation degree of the macrochelate is only relatively little affected by the change in solvent, though it needs to be emphasized that the overall stability of the [Cu(GDP)](-) complex increases with decreasing solvent polarity. By including previously studied systems in the considerations, the biological implications are shortly discussed, and it is concluded that Nature has here a tool to alter the structure of complexes by shifting them on a protein surface from a polar to an apolar region and vice versa.