Influence of decreasing solvent polarity (dioxane–water mixtures) on the stability and structure of binary and ternary complexes of adenosine 5′-triphosphate and uridine 5′-triphosphate

Abstract

The influence of dioxane on the complex equilibria involving the reactants adenosine 5′-triphosphate (ATP^4–), uridine 5′-triphosphate (UTP^4–), Cu²⁺, 2,2′-bipyridyl (bipy), and 1,10-phenanthroline (phen) has been determined. The concentration dependence of the chemical shifts of the protons of bipy and phen has been measured and the self-association property of these aromatic ligands (arm) was quantified with the isodesmic model of indefinite non-co-operative stacking. The stacking tendency is considerably diminished by dioxane: e.g., in D₂O K_phen^self= 31.1 ± 3.4 l mol^–1 and in 50%(v/v)[²H₈]dioxane–D₂O k_phen^self= 0.63 ± 0.13 l mol^–1. Similarly, the formation of the binary stacks (arm)(ATP)^4– is also inhibited by dioxane: the stability of these adducts decreases by factors of Ca. 1/20 (or more) by changing the solvent from water to 50%(v/v) dioxane–water. The acidity constants of the mentioned (two-fold protonated) nucleoside 5′-triphosphates (NTP) and the stability constants of their binary and ternary complexes have been determined by potentiometric pH titrations in water, 30 and 50%(v/v) dioxane–water. By using the results obtained for the UTP systems mainly for comparisons, the following three intramolecular equilibria have been evaluated. (i) The proton in Cu(H-ATP)^–may be located at N-1 or at the terminal γ-phosphate group: in water the isomer with the proton at N-1 occurs in significant amounts (∼50%), while in the dioxane–water mixtures the phosphate-protonated isomer strongly dominates. (ii) Cu(ATP)^2– exists in two forms: one isomer has a phosphate co-ordination only, while the other is a macrochelate involving in addition N-7; the macrochelated isomer decreases from 68% in water to about 24% in 50% dioxane–water. (iii) While there is evidence that intramolecular stacks may also be formed in Cu(arm)(H-NTP)^– and Cu(arm)(UTP)^2–(and as far as possible their formation was quantified), the extent of stacking in the Cu(arm)(ATP)^2– systems could be well characterized: e.g., with Cu(phen)(ATP)^2– in water, ca. 92% exists in the stacked form and in 50%(v/v) dioxane–water ca. 49% of the ternary complex still remains stacked. This means, by going from water to 50% dioxane–water the stability of the metal-bridged Cu(arm)(NTP)^2– stacks decreases only by a factor of Ca. 1/2, while the stability of unbridged binary (arm)(NTP)^4– stacks decreases by Ca. 1/20 (or more). Similar trends are expected for the corresponding equilibria with other metal ions; the related search for selectivity regarding biological systems is discussed.