Abstract
Stopped-flow circular dichroism and fluorescence spectroscopy are used to characterize the assembly of
complexes consisting of plasmid DNA bound to the cationic lipids dimethyldioctadecylammonium bromide and 1, 2-dioleoyl- 3-
trimethylammonium-propane and a series of polyamidoamine dendrimers. The kinetics of complexation determined from the
stopped-flow circular dichroism measurements suggests complexation occurs within 50 ms. Further analysis, however, was
precluded by the presence of mixing (shear) artifacts. Stopped-flow fluorescence employing the high-affinity DNA dyes Hoechst
33258 and YOYO-1 was able to resolve two sequential steps in the assembly of complexes that are assigned to binding/
dehydration and condensation events. The rates of each process were determined over the temperature range of 10–50 C and
activation energies were determined from the slope of Arrhenius plots. The behavior of polyamidoamine dendrimers can be
separated into two classes based on their differing binding modes: generation 2 and the larger generations (G4, G7, and G9). The
larger generations have activation energies for binding that follow the trend G4 . G7 . G9. The activation energies for
condensation (compaction) of complexes composed of these same dendrimers have the opposite trend G9 . G7 . G4. It is
postulated that a balance between a more energetically favorable condensation and less favorable binding may prove beneficial in
enhancing gene delivery.