Cationic liposome and plasmid DNA complexes formed in serum-free medium under optimum transfection condition are negatively charged.

In medium where in vitro transfection is routinely performed, DC-chol liposomes alone were nearly neutral, whereas the DC-chol liposome/DNA complexes were largely negatively charged which changed only slightly at all [liposome]/[DNA] ratios (zeta=-27.1 to -21.8 mV). Three other commercial transfection reagents, Lipofectin(R), LipofectAMINE 2000, and SuperFect, were also largely negatively charged when complexed with DNA. The aggregation of liposomes in medium was prevented by the addition of DNA. Incubation of the complexes in medium did not change their size, charge or lipofection activity for 30 min. These results suggest that, in medium, the liposome/DNA complexes were formed at the time of mixing with negative charges.