Phase diagram, stability, and overcharging of lamellar cationic lipid-DNA self-assembled complexes

Cationic lipid-DNA (CL-DNA) complexes comprise a promising new class of syn thetic nonviral gene delivery systems. When positively charged, they attach to the anionic cell surface and transfer DNA into the cell cytoplasm. We r eport a comprehensive x-ray diffraction study of the lamellar CL-DNA self-a ssemblies as a function of lipid composition and lipid/DNA ratio, aimed at elucidating the interactions determining their structure, charge, and therm odynamic stability. The driving force for the formation of charge-neutral c omplexes is the release of DNA and lipid counterions. Negatively charged co mplexes have a higher DNA packing density than isoelectric complexes, where as positively charged ones have a lower packing density. This indicates tha t the overcharging of the complex away from its isoelectric point is caused by changes of the bulk structure with absorption of excess DNA or cationic lipid. The degree of overcharging is dependent on the membrane charge dens ity, which is controlled by the ratio of neutral to cationic lipid in the b ilayers. Importantly, overcharged complexes are observed to move toward the ir isoelectric charge-neutral point at higher concentration of salt cc-ions , with positively overcharged complexes expelling cationic lipid and negati vely overcharged complexes expelling DNA. Our observations should apply uni versally to the formation and structure of self-assemblies between opposite ly charged macromolecules.