Phase behavior of DNA in the presence of cetyltrimethylammonium bromide/alkyl polyglucoside surfactant mixture

Interaction of DNA with a CTAB/APG mixture was studied by determining its phase behavior. Results showed that, at low DNA concentration, the addition of APG could lead to earlier turbidity, indicating that APG can strengthen the interaction between CTAB and DNA. At high DNA concentration, however, APG had little influence on the turbidity boundary. We found that addition of salt could also lead to such an asymmetry of the phase map. Two mechanisms were presented to account for the asymmetry of the phase behavior. At the lower DNA concentration, it was assumed that the surfactant was bound to DNA in a way similar to that of micelle formation. That is, the binding is dominated by hydrophobic association processes. APG can facilitate this process by forming a surfactant mixture with CTAB, and salt facilitates this process by "salting out" effects. At high DNA concentrations, surfactants bind to DNA mainly through random electrostatic interaction. Addition of salt screens this interaction and therefore delays the turbidity. APG, however, exerts little influence on this process. Viscosity measurements at low DNA concentration showed that the complex with APG is more compact than that of CTAB alone with DNA.