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, H₂(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°; I=0.1M , NaNO₃). The results showed that in H₂O one of the two protons of H₂(GDP)^? is located mainly at the N(7) site and the other one at the terminal β‐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 vs. pK (where R represents a non‐interacting residue in simple diphosphate monoesters ROP(O^?)(?O)? O? P(?O)(O^?)₂, R? DP^3?), 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 Cu²⁺ 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 Cu²⁺ 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.