Improving the understanding of DNA–propanediyl-1,3-bis(dodecyldimethylammonium) dibromide interaction using thermodynamic, structural and kinetic approaches

Abstract

A kinetic, thermodynamic and structural study of the interaction of the gemini surfactant propanediyl-1,3-bis(dimethyldodecylammonium dibromide) (12-3-12.2Br) with calf thymus DNA was carried out at several ionic strengths (NaCl) in aqueous solutions. A new 12-3-12²⁺-selective membrane was prepared in order to gain insight into the factors that control the binding of 12-3-12.2Br to DNA. We used ethidium bromide (EB) as a fluorescence probe to follow the kinetics of the interaction by using the stopped-flow fluorescence technique. The results can be explained in terms of a reaction mechanism involving two consecutive reversible (fast and slow) steps. The fast step was attributed to the union/separation of the surfactant with/from the DNA polynucleotide. Changes in the kinetic constants in the forward and backward directions were discussed in terms of the Brönsted–Pitzer equation and of the increase in hydrophobic interactions of the surfactant tails as a consequence of salting-out effects, respectively. The slow step corresponds to a conformational change of the surfactant–DNA complex to a more compacted form. The equilibrium constant, calculated from the forward and reverse rate constants of these steps, agrees with the results obtained from potentiometric titration using a 12-3-12-²⁺ selective electrode.