pH-controlled DNA condensation in the presence of dodecyldimethylamine oxide

Conformational behavior of linear DNA in the presence of dodecyldimethylamine oxide (DDAO), an amphiphile that exists either in a neutral or cationic protonated form depending on the pH of aqueous solution, was examined. DNA phase behavior in the presence of free DDAO and DDAO/dioleoyl phosphatidylethanolamine (DOPE) mixed vesicles was found to be strongly pH-dependent. Using the fluorescence microscopy technique for the real-time visualization of DNA compaction process at the single-molecule level, a dramatic increase in the surfactant concentration needed for the collapsing coil-globule transition in individual DNAs was noticed as amphiphilic DDAO molecules are deprotonated by increasing the pH of an aqueous solution. A similar, but less pronounced, effect was detected for DNA condensation in the presence of DDAO-containing pH-sensitive mixed vesicles. In general, positively charged DDAO ions in vesicular form behave as a more efficient DNA-condensing agent than those in the micellar form. The structures of corresponding DNA-lipid complexes were studied by small-angle X-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryo-TEM) techniques and found to be hexagonal for the DNA-DDAO system and lamellar for the DNA-DDAO/DOPE mixed-vesicle system. When DNA was introduced to vesicular DDAO/DOPE solutions under acidic conditions, it induced the disintegration of vesicles and formation of multilamellar structures. Under alkaline conditions DNA showed no interaction with the neutral vesicles but induced their aggregation and flocculation.