DNA–chitosan complexation: A dynamic light scattering study

The interactions of DNA in mixed solutions with cationic macromolecular compounds have attracted great interest of biomedical sciences, in particular for application in gene therapy. One such compound is chitosan that in addition to being positive in charge has demonstrated to be non-toxic in animal and human trials.In this work the formation of complexes between chitosan and calf thymus DNA was studied by dynamic light scattering. The study was done in an attempt to characterize the effect of pH of the media and of molecular weight of chitosan, Mw, on (i) the hydrodynamic radii, RH, of the complexes, (ii) the stability of the complexes and (iii) the critical ratio of chitosan to DNA mass concentrations, rc, at which the complexation is completely achieved. Three chitosans of different viscosities (low, middle and high) were used. The viscosity average molecular weight of each chitosan was determined by means of the Mark-Houwink equation. Before determinations, the quality of all autocorrelation functions was confirmed by the calculation of the coherence factor f of the instrument. We demonstrate that the DNA–chitosan complexes obtained are stable, and that pH and Mw influence rc and RH, respectively. We found that the higher was the pH, the larger was the quantity of chitosan needed to completely complex DNA, which was an especially pronounced effect for the highest molecular weight chitosans (middle and highly viscous). By contrast, with increasing Mw, the size of the complexes grew. In general, compacted DNA–chitosan complexes were detectable and reproducibly measured to have average hydrodynamic radii RH around 190–250 nm only at ratios of chitosan to DNA mass concentrations higher than rc ≈ 3.