Hydrophobic interactions of DNA with long-chain amines.

The binding ratio, Γ_a, for several long-chain amines to calf-thymus DNA was measured as function of the ligand concentration, C, using the equilibrium dialysis method. The different amines used in the binding experiments at constant temperature were dodecyl trimethyl ammonium bromide (DTAB), myristyl trimethyl ammonium bromide (MTAB), cetyl trimethyl ammonium bromide (CTAB), and cetyl pyridinium chloride (CPCL). The formation and dissociation of the saturated DNA–amine complex were reversible. The initial slope of the binding isotherm decreased sharply with the reduction of the electrostatic effect as a result of the increase of the ionic strength of the medium. A sharp inflexion region was noted in the binding isotherm where the ligands bound in significant numbers may undergo hydrophobic interactions with each other. Γ_a increased with C until a maximum value, Γ_a^m, was reached, beyond which binding slowly decreased with an increase of concentration. Both Γ_a^m and Γ_a increased significantly with the increase of the hydrocarbon chain length of a ligand. The free energy change ΔG^m for each saturated DNA–amine complex was evaluated on the basis of a thermodynamic relation and the standard state for binding was defined. The average free energy change for the binding per CH₂ group of the amine was found to be ?1550 cal/mol. The difference between ΔG^m for CTAB and CPCL was examined on the basis of the structural difference of their head groups. The binding isotherms for MTAB and CPCL were obtained from the binding data at 15, 30, and 45°C. The binding increased with increasing temperature. From the plot of ΔG^m/T vs 1/T, the changes in enthalpy and entropy due to the binding were evaluated for MTAB and CPCL. The binding reactions in these two cases were driven primarily by the entropy change due to the hydrophobic interaction. Standard free energy changes ΔG₀^m for the unsaturated complexes were close to ΔG^m for the saturated complexes. The binding isotherms also depended on the nature of the neutral salt of the medium. At a given salt concentration, the order of the binding of the inorganic salts was as follows: KCl > NaCl > LiCl > Na₂SO₄ > MgCl₂. The effect of pH on binding was also examined. The importance of these results on the formation of the reconstituted and natural nucleohistone complexes is discussed.