Ulf Olsson

The co-assembly of DNA and lipids has broad implications, from the structure of the cell nucleus to gene delivery formulations. To progress in the fundamental understanding of DNA-lipid interactions the self-assembly behavior of a cationic surfactant (dodecyltrimethylammonium, DTA) with DNA as a counterion in mixtures of water and lecithin was investigated. The phase diagram was established and the different regions of the phase diagram characterized with respect to the microstructure by H-2 NMR, small-angle X-ray scattering (SAXS), and other techniques. Three liquid crystalline (LC) phases were identified: cubic, lamellar and hexagonal. All LC phases are stable in the temperature range 20-65 degrees C. The cubic phase is stable only in a narrow composition range. Upon increasing the lecithin content the phase transforms to the lamellar phase via an extended biphasic area. Drying of the lamellar phase leads to the formation of hexagonal phase. Adding salt to the ternary mixtures causes a slight increase in the unit cell dimension of the LC phases. Very high ionic strength may lead to DNA release from liquid crystals and the formation of DNA free liquid crystalline phases. In particular we observe a coexistence of two lamellar phases. One contains DNA and one being essentially free of DNA.