(504b) Polyplex Disassembly and DNA Release Observed by Real-Time AFM

This paper describes AFM studies of plasmid DNA release from individual polyplex nanoparticles. Three triggering mechanisms were used to cause polyplex disassembly: temperature gradient, redox reaction, and interpolyelectrolyte exchange reaction.   First, AFM captured morphological change of the polyplex made by poly(ethylenimine)-graft-poly(N-isopropylacrylamide-co-N-dimethylacrylamide) (PEI-g-P(NIPA-co-DMA)) when temperature was moved across the lower critical solution temperature of the polycation. Second, in presence of glutathione or dithiothreitol, DNA release is triggered by a depolymerization of high-molecular-weight bioreducible polycations into low-molecular-weight oligocations via thiol and disulfide exchange reaction. The AFM images revealed a three-stage pathway beginning with a morphological change from metastable nanostructures into the more favorable toroid structure. Then toroids interact with each other by aggregation and fusion. Finally, DNA wormlike chains gradually unravel from the polyplex resulting in loose loops/tails that are held by a central compact core. Third, in DNA release induced by interpolyelectrolyte exchange, AFM images showed distinct intermediate polyplex structures including core-shell, toroid, nanoparticle decorated toroid, and loose chains held by a compact core structure. In all these studies, AFM is capable of capturing intermediate states of DNA release from polyplexes that point to some common features. For example, toroids and bundles tend to develop during early stages of polyplex disassembly. Loose DNA chains held by a central core tend to develop during late stages of polyplex disassembly. We were able to correlate some of the AFM results to in vitro observations.