Microscopic investigations into PEG-cationic polymer-induced DNA condensation

The morphology of condensates formed between plasmid DNA and a PEG-poly(ami doamine) triblock copolymer were investigated using TEM and AFM in aqueous conditions. Both techniques revealed the presence of ring and extended line ar structures; however, a greater proportion gf linear structures was obtai ned with the PEG copolymer than was seen with the poly(amidoamine) homopoly mer, indicating that the PEG was interfering with condensation. Differences in the ring diameters between the PEG copolymer and the homopolymer were m ore marked with TEM than AFM, suggesting that compact toroidal structures a re largely due to constriction on dehydration and that this is prevented fr om occurring in the presence of PEG. AFM revealed an increase in condensate size with increasing polymer/DNA ratio, which can be attributed to the ass ociation of more polymer with the complex. With TEM, the proportion of line ar structures increased with increasing polymer/DNA ratio, whereas it remai ned relatively constant for AFM, across all ratios. It is suggested that an increased amount of PEG associated with the complexes promotes disruption of the ring structure upon dehydration. This supports the hypothesis that r ings are formed via the bending of linear structures, with an equilibrium e xisting between the two, that is altered in favor of the linear structure b y PEG.