DNA/chitosan electrostatic complex

•This original paper for the first time established clearly the stoichiometric charge complex formation for DNA/chitosan.•Well defined and soluble samples of chitosan were used for this work.•For the first time also a large number of techniques are combined to study the complex formation.•The complex is established as being purely electrostatic in dependence of pH controlling the net charge of chitosan.

Up to now, chitosan and DNA have been investigated for gene delivery due to chitosan advantages. It is recognized that chitosan is a biocompatible and biodegradable non-viral vector that does not produce immunological reactions, contrary to viral vectors. Chitosan has also been used and studied for its ability to protect DNA against nuclease degradation and to transfect DNA into several kinds of cells. In this work, high molecular weight DNA is compacted with chitosan. DNA-chitosan complex stoichiometry, net charge, dimensions, conformation and thermal stability are determined and discussed. The influence of external salt and chitosan molecular weight on the stoichiometry is also discussed. The isoelectric point of the complexes was found to be directly related to the protonation degree of chitosan. It is clearly demonstrated that the net charge of DNA-chitosan complex can be expressed in terms of the ratio [NH3+]/[P−], showing that the electrostatic interactions between DNA and chitosan are the main phenomena taking place in the solution. Compaction of DNA long chain complexed with low molar mass chitosan gives nanoparticles with an average radius around 150 nm. Stable nanoparticles are obtained for a partial neutralization of phosphate ionic sites (i.e.: [NH3+]/[P−] fraction between 0.35 and 0.80).