Interaction between covalent DNA gels and a cationic surfactant

The interaction of covalently cross-linked double-stranded (ds) DNA gels and cetyltrimethylammonium bromide (CTAB) is investigated. The volume transition of the gels that follows the absorption of the oppositely charged surfactant from aqueous solution is studied. As do other polyelectrolyte networks, DNA networks form complexes with oppositely charged surfactant micelles at surfactant concentrations far below the critical micelle concentration (cmc) of the polymer-free solution. The size of the absorbed surfactant aggregates is determined from time-resolved fluorescence quenching (TRFQ). At low surfactant concentrations, small discrete micelles (160 < N < 210) are found, whereas large micelles (N > 500) form at surfactant concentrations of 1 mM. When the DNA is in excess of the surfactant, the surfactant binding is essentially quantitative. The gel volume decreases by 90% when the surfactant to DNA charge ratio, beta, increases from 0 to 1.