Competitive binding of Mg2+, Ca2+, Na+, and K+ ions to DNA in oriented DNAfibers: Experimental and Monte Carlo simulation results

Competitive binding of the most common cations of the cytoplasm (K+, Na+, C a2+, and Mg2+) with DNA was studied by equilibrating oriented DNA fibers wi th ethanol/water solutions (65 and 52% v/v EtOH) containing different combi nations and concentrations of the counterions, The affinity of DNA for the cations decreases in the order Ca > Mg much greater than Na approximate to K, The degree of Ca2+ and/or Mg2+ binding to DNA displays maximum changes j ust at physiological concentrations of salts (60-200 mM) and does not depen d significantly on the ethanol concentration or on the kind of univalent ca tion (Na+ or K+). Ca2+ is more tightly bound to DNA and is replaced by the monovalent cations to a lesser extent than is Mg2+. Similarly, Ca2+ is a be tter competitor for binding to DNA than Mg2+ the ion exchange equilibrium c onstant for a 1:1 mixture of Ca2+ and Mg2+ ions, K-c(Mg)Ca , changes from K -c(Mg)Ca approximate to 2 in 65% EtOH (in 3-30 mM NaCl and/or KCl) to K-c(M g)Ca approximate to 1.2-1.4 in 52% EtOH (in 300 mM NaCl and/or KCl). DNA do es not exhibit selectivity for Na+ or K+ in ethanol/water solutions either in the absence or in the presence of Ca2+ and/or Mg2+. The ion exchange exp erimental data are compared with results of grand canonical Monte Carlo (GC MC) simulations of systems of parallel and hexagonally ordered, uniformly a nd discretely charged polyions with the density and spatial distribution of the charged groups modeling B DNA, A quantitative agreement with experimen tal data on divalent-monovalent competition has been obtained for discretel y charged models of the DNA polyion (for the uniformly charged cylinder mod el, coincidence with experiment is qualitative). The GCMC method gives also a qualitative description of experimental results for DNA binding competit ions of counterions of the same charge (Ca2+ with Mg2+ or K+ with Na+).