Transition of nanostructure in DNA-cationic surfactant complexes with the added salt

Nanostructures of complexes of DNA with single-chain surfactant of octadecyltrimethylammonium (OTA) and double-chain surfactant of didodecyldimethylammonium (DDA) in aqueous NaCl solution at concentration, Cs, from 0 to 500 mM were studied using small-angle-scattering techniques (SAXS). SAXS profiles of the DNA-OTA complex show two SAXS peaks with a spacing ratio of 1:3(1/2) in the solution at Cs below 150 mM and three peaks with a spacing ratio of 1:3(1/2):4(1/2) at Cs above 250 mM. Contents of Na+ and Cl- ions in the complexes evaluated from the atomic absorbance for Na+ and the potentiometry for Cl- revealed charge molar ratios of OTA/DNA = 1 and DDA/DNA = 1.25. Contents of Na+ and Cl- ions per ionic unit of DNA molecule in the DNA-OTA complex equilibrating with the solution at Cs below 100 mM were much less than 0.1, while they increased with NaCl concentration at Cs above 200 mM. The DNA-OTA complex in the solution at Cs above 260 mM exhibited an endothermic peak in the DSC measurements, and the others did not. On the basis of the experimental results, the salt concentration dependent nanostructures are discussed.