Complexation of DNA with cationic surfactants as studied by small-angle X-ray scattering

The phase behaviors of the complex formed by didodecyldimethylammonium bromide (DDAB) and cetyltrimethylammonium bromide (CTAB) interacting with three different types of DNAs, salmon testes DNA (similar to 2000 bp), 21-bp double-stranded oligonucleotides (oligo-dsDNA), and 21-nt single-stranded oligonucleotides (oligo-ssDNA) were studied by synchrotron small-angle X-ray scattering. It was found that the DNA length and flexibility, together with the positive/negative charge ratio, determined the final structure. At higher charge ratios, the DNA length exhibited negligible effect. Both oligo-dsDNA and salmon DNA formed inverted hexagonal packing of cylinders with CTAB, as well as bilayered lamella with DDAB. However, at lower charge ratios, oligo-dsDNA formed a distorted hexagonal phase with CTAB and a new structure with DDAB, which was different from the behaviors of salmon DNA. The flexible oligo-ssDNA formed rich structures that were subject to environmental disturbance. Kinetic study also indicated that the structures of the complex formed by oligo-ssDNA took much longer to mature than the structures formed by oligo-dsDNA. We attributed this result to the conformational adjustment of oligo-ssDNA in the complex.