DNA conformational changes induced by cationic gemini surfactants: the key to switching DNA compact structures into elongated forms

The DNA conformational changes induced by different members of the N,N0-bis(dimethyldodecyl)-a-ualkanediammonium dibromide series (m-s-m, m ¼ 12, s ¼ 3 and 6) and the analogous series of hexadecyl gemini surfactants (m ¼ 16, s ¼ 3 and 6) were investigated in aqueous media by means of circular dichroism (CD), zeta potential, dynamic light scattering (DLS), viscometry, and atomic force microscopy (AFM) methods. The measurements were carried out by varying the gemini surfactant–DNA molar ratio, R ¼ Cm-s-m/CDNA. For the conditions investigated two significantly different conformational changes were observed, the second of them being worth noting. At low molar ratios, all methods concurred by showing that gemini surfactants were able to form ordered aggregates which precedes DNA compaction. The second effect observed, at high molar ratios, corresponds to the transition from the compact state to a new more extended conformation. The degree of decompaction and the morphologies of the visualized structures are different not only depending on the surfactant tail's length, but also on the spacer's length. The results obtained for the 16-3-16/DNA and 16-6-16/DNA systems point out that the compaction/decompaction processes are somewhat different to those previously visualized for the analogous monoquaternary chain surfactant CTAB.