The association of DNA and stable catanionic amino acid-based vesicles

Cationic surfactants associate strongly to DNA and compact but are often toxic. The interaction of some novel cationic amino acid-based surfactants, which may enhance transfection and appear to be nontoxic, is described. A cationic arginine-based surfactant, ALA, gives in combination with anionic surfactants spontaneously stable vesicles, and special attention is given to the association of these catanionic vesicles, with a net positive charge, to DNA. The ability of this surfactant alone to compact DNA is compared in fluorescence microscopy studies to classical cationic surfactants. Addition of DNA to a solution of the catanionic vesicles results in associative phase separation at very low vesicle concentrations; there is a separation into a precipitate and a supernatant solution, which is first bluish but becomes clearer as more DNA is added. From studies using cryogenic transmission electron microscopy (cryo-TEM) and small angle X-ray scattering it is demonstrated that there is a lamellar structure with DNA arranged within the surfactant bilayers. Analysis of the supernatant by means of proton nuclear magnetic resonance (1H NMR) showed that above the isoelectric point between ALA, anionic surfactant (sodium octyl sulfate, SOS) and DNA, anionic surfactant starts to be expelled from the bilayers on further incorporation of DNA. There appears to be a transition from a lamellar to a hexagonal liquid crystal structure when most of SOS has been expelled from the aggregate bilayers; at higher DNA-to-surfactant ratios, self-assembled SOS micelles and the excess of DNA added seem to coexist in solution. Regarding the phase-separating DNA–surfactant particles, cryo-TEM demonstrates a large and nonmonotonic variation of particle size as the DNA–surfactant ratio is varied, with the largest particles obtained in the vicinity of overall charge neutrality.