Studies on surfactant-biopolymer interaction. II. Interaction of cetyl trimethyl ammonium-, cetyl ethanolyl dimethyl ammonium-, cetyl diethanolyl methyl ammonium- and cetyl triphenyl phosphonium bromides and cetyl pyridinium chloride with calf thymus DNA.

The interaction of the surfactants cetyl trimethyl ammonium-, cetyl ethanolyl dimethyl ammonium-, cetyl diethanolyl methyl ammonium-, and cetyl triphenyl phosphonium bromides and cetyl pyridinium chloride with calf thymus DNA was studied at 303 K in phosphate buffer (pH 7.0) at 10 mM NaCl using spectrophotometric, viscometric, tensiometric, dynamic light scattering, circular dichroism, fluorescence microscopic and microcalorimetric techniques. All the surfactants interacted fairly with DNA, making the biopolymer condensed, even to the aggregated globular configuration at higher [surfactant]/[DNA] mole ratio (R), with direct evidence from fluorescence microscopy. Melting temperature and light scattering intensity of the DNA increased, whereas viscosity decreased in the presence of the surfactants. Tensiometry evidenced effective interaction at [surfactant] as low as 7.6 microM. Isothermal titration calorimetric measurements supported low enthalpy of binding, induced aggregation of the surfactants, increased critical micellar concentration and association of aggregates with the biopolymer at higher R, evidencing distinctions in thermal behaviour.