Competitive substitution of hexammine cobalt(III) for Na+ and K+ ions in oriented DNA fibers

Competition of the trivalent cation, Co(NH3)(6)(3+), with K+ and Na+ ions i n binding to DNA was studied by equilibrating oriented DNA fibers with etha nol/water solutions (65 and 52% v/v EtOH), containing different combination s and concentrations of KCI and NaCl and constant concentration (0.8 mM) of Co(NH3)(6)Cl-3. The degree of Co(NH3)(6)(3+) binding to DNA does not depen d significantly on the ethanol concentration or on the kind of univalent ca tion (Na+ or K+). The ion exchange selectivity coefficient of monovalent-tr ivalent ion competition. D-c3(1), increases with the concentration of Me+, C-+(o), and the monotonic dependence of log D-c3(1) vs log C-+(0) has an in flection between 100 and 300 mM that is caused bg a structural transformati on of DNA from A- to B-form. The ion exchange experimental data are compare d with results of grand canonical Monte Carlo (GCMC) simulations of systems of parallel and hexagonally ordered, discretely charged polyions with dens ity and spatial distribution of the charged groups modeling B- and A-forms of DNA. The GCMC method for discretely charged models of the DNA polyion pr oduces a quantitative agreement with experimental data on trivalent-monoval ent ion competition in dependence on DNA structural stair and salt concentr ation. Based on this and previous studies it is concluded that the affinity of DNA for the cations decreases in the order Co(NH3)(6)(3+) much greater than Ca2+ > Mg2+ much greater than Na+ approximate to K+ > Li+. DNA does no t exhibit selectivity for Na+ or K+ in ethanol/water solutions either in th e absence or in the presence of Co(NH3)(6)(3+), Ca2+, and Mg2+. (C) 2001 Jo hn Wiley & Sons, Inc.