NANOSCOPIC STRUCTURE OF DNA CONDENSED FOR GENE DELIVERY

Scanning force microscopy was used to examine DNA condensates prepared with varying stoichiometries of lipospermine or polyethylenimine in p hysiological solution. For the first time, individual DNA strands were clearly visualized in incomplete condensates without drying. Using li pospermine at sub-saturating concentrations, discrete nuclei of conden sation were observed often surrounded by folded loops of DNA. Similar packing of DNA loops occurred for polyethylenimine-induced condensatio n. Increasing the amount of the condensing agent led to the progressiv e coalescence or aggregation of initial condensation nuclei through fo lding rather than winding the DNA. At over-saturating charge ratios of the cationic lipid or polymer to DNA, condensates had sizes smaller t han or equal to those measured previously in electron micrographs. Pol yethylenimine condensates were more compact than lipospermine condensa tes and both produced more homogeneously compacted plasmids when used in a 2-4-fold charge excess. The size and morphology of the condensate s may affect their efficiency in transfection.