Dynamic and structural scalings of the complexation between pDNA and bPEI in semidilute and low-salt solutions.

Using a combination of static and dynamic laser light scattering, we investigated the complexation of a supercoiled plasmid DNA (pDNA, 10(4) bp) and a branched polyethyleneimine (bPEI, M(w) = 25 kD) in semidilute and low-salt aqueous solutions. Our results unearth some scaling laws for dynamic and structural properties of the resultant complexes (polyplexes) with different bPEI:pDNA (N:P) molar ratios. Namely, the average scattering intensity () and the average linewidth of the Rayleigh peak () are scaled to the scattering vector (q) as proportional, variant q(alpha(s) ) or proportional, variant q(alpha(D) ), where alpha(S) and alpha(D) are two N:P dependent scaling exponents. The N:P ratio strongly affects the complexation. When N:P < 2.0, the motions of the negatively charged and extended pDNA chains and the polyplexes are highly correlated so that they behave like a transient network with a fractal dimension. As the N:P ratio increases, nearly all of pDNA chains condensed and the overall charge of the polyplexes reverses to slightly positive, resulting in a turbid dispersion of large loose aggregates made of smaller, but more compact, polyplexes. Further increase of N:P finally disrupts large loose aggregates, leading to a homogeneous transparent dispersion of the polyplexes.