Long-term kinetics of DNA interacting with polycations

DNA complex has been widely used as non-viral vectors for the delivery of genes or siRNA. Owing to the strong and long-ranged electrostatic interaction, the structure and property of the DNA complex should evolve with time in a long-term. To test this hypothesis, we choose 2000 bp double-stranded DNA and 21 bp oligonucleotide as the model molecules, and comparatively studied their complexation with narrowly-distributed poly-L-lysine (PLL150) by time-resolved laser light scattering. In the time range of about one week, the complexation of both DNA samples underwent three stages: formation of preliminary complex, further aggregation, followed by precipitation. The aggregation and precipitation rate of the complex formed by oligonucleotide was much faster than that of the complex formed by 2000 bp dsDNA. After precipitation, the amount of the longer chain polyelectrolyte, as determined by UV and fluorescence, was about twice that of the short chain polyelectrolyte in the sediment. The precipitates were far from being fully neutralized. Moreover, the component ratio in the sediment was independent of the mixing charge molar ratio. A rational complex mechanism was proposed on the basis of these findings. During complexation, the relaxation of polyelectrolyte inside the complex and the exchange of polyelectrolyte between complex determined the aggregation and precipitation rate. The competition of the two kinetic processes governed the structure and property of the complex in the solution and in the