Hydration effects of Ni2+ binding to synthetic polynucleotides with regularly alternating purine-pyrimidine sequences

High precision ultrasonic and densimetric techniques have been used to study the interaction of Ni2+ ions with right-handed poly[d(G-C)].poly[d(G-C)], poly- [d(A-C)].poly[d(G-T)] and poly[d(A-T)].poly[d(A-T)] in 5 mM CsCl, 0.2 mM HEPES, pH 7.5 at 20 degrees C, From these measurements the changes in the apparent molar volume and the apparent molar adiabatic compressibility due to the interaction have been obtained. The volume effects of the binding, calculated per mole of Ni2+ ions, range from 11,7 to 23.9 cm(3) mol(-1) and the compressibility effects range from 19.3 x 10(-4) to 43.1 x 10(-4) cm3 mol(-1) bar'. These data are interpreted in terms of dehydration of the polynucleotides and Ni2+ ions, i,e, the release of water molecules from the hydration shells of the molecules. An increase in G + C content gives an increase in volume and compressibility effects, indicating a rise in the extent of dehydration. The dehydration effects of Ni2+ binding to poly[d(GC)].poly[d(G-C)] are approximately twice those of poly[d(A-T)].poly[d(A-T)]. The volume and compressibility effects of Ni2+-EDTA complex formation have also been measured and used as a model system for quantitative estimation. These values revealed that Ni2+ ions can coordinate two atomic groups of poly[d(G-C)].poly[d(c-C)], while in the case of the Ni2+ -poly[d(A-T)] poly[d(A-T)] complex volume and compressibility effects correspond to one direct or two indirect (through water) contacts.