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 lipos omes alone were nearly neutral, whereas the DC-chol liposome/DNA complexes were largely negatively charged which changed only slightly at all [liposom e]/[DNA] ratios (zeta = -27.1 to -21.8 mV). Three other commercial transfec tion reagents, Lipofectin((R)), LipofectAMINE((TM)) 2000, and SuperFect((TM )), were also largely negatively charged when complexed with DNA. The aggre gation of liposomes in medium was prevented by the addition of DNA. Incubat ion of the complexes in medium did not change their size, charge or lipofec tion activity for 30 min. These results suggest that, in medium, the liposo me/DNA complexes were formed at the time of mixing with negative charges. ( C) 2000 Elsevier Science B.V. All rights reserved.