Thermodynamics of DNA binding and condensation: Isothermal titration calorimetry and electrostatic mechanism

The thermodynamics of binding of the trivalent cations cobalt hexammine and spermidine to plasmid DNA was studied by isothermal titration calorimetry. Two stages were observed in the course of titration, the first attributed to cation binding and the second to DNA condensation. A standard calorimetr ic data analysis was extended by applying an electrostatic binding model, w hich accounted for most of the observed data. Both the binding and condensa tion reactions were entropically driven (T Delta S similar to + 10 kcal/mol cation) and enthalpically opposed (Delta H similar to + 1 kcal/mol cation) . As predicted from their relative sizes, the binding constants of the cati ons were indistinguishable, but cobalt hexammine had a much greater DNA con densing capacity because it is more compact than spermidine. The dependence of both the free energy of cobalt hexammine binding and the critical. coba lt hexammine concentration for DNA condensation on temperature and monovale nt cation concentration followed the electrostatic model quite precisely. T he heat capacity changes of both stages were positive, perhaps reflecting b oth the temperature dependence of the dielectric constant of water and the burial of polar surfaces. DNA condensation occurred when about 67% of the D NA phosphate charge was neutralized by cobalt hexammine and 87% by spermidi ne. During condensation, the remaining DNA charge was neutralized. (C) 2000 Academic Press.