Ratiometric Interactions of Anionic Surfactants with Calf Thymus DNA Bound Cationic Surfactants: Study II

Cationic surfactant cetyltrimethylammonium bromide (CTAB) and cetylpyridinium bromide (CPB) bind to the calf thymus DNA (ct-DNA) like anionic biopolymers electrostatically, and establish equilibrium in aqueous medium at pH 7. At low concentration, ct-DNA does not interact with anionic surfactants, sodium dodecylsulfate (SDS) and sodium dodecylbenzylsulfonate (SDBS). However, in the ground state, anionic surfactant is found to clearly establish equilibrium with ct-DNA-bound cationic surfactant whereby the same surfactant-DNA isosbestic point reappears. We herein report a detail ratiometric binding of CPB with ct-DNA, and interaction of anionic SDBS with DNA-bound CPB in comparison with the combined ct-DNA-CTAB-SDS system. Compaction of ct-DNA in presence of CPB and its decompaction using anionic SDBS is also studied in comparison with CTAB-SDS combination. The techniques used are tensiometry, spectrophotometry, viscometry, cyclic voltammetry, circular dichroism, isothermal titration calorimetry, and density functional theory (DFT)-based computational calculations. The size and surface charge density of the surfactant headgroups and the phosphate group in DNA have a contributing role in the DNA compaction-decompaction phenomenon.