Hydrophobic interactions of DNA with long-chain amines.

Abstract

AbstractThe binding ratio, Γa, 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. Γa increased with C until a maximum value, Γam, was reached, beyond which binding slowly decreased with an increase of concentration. Both Γam and Γa increased significantly with the increase of the hydrocarbon chain length of a ligand. The free energy change ΔGm 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 ΔGm 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 ΔGm/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 ΔG0m for the unsaturated complexes were close to ΔGm 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.