Counterion of Chitosan Influences Thermodynamics of Association of siRNA with a Chitosan-Based siRNA Carrier.

Purpose: The work aimed to compare quality of a siRNA carrier prepared with chitosan of two different sources having similar degree of deacetylation and molecular weights. Differences were analyzed from thermodynamic characteristics of interactions with siRNA.

Methods: The siRNA carrier (chitosan-coated poly(isobutylcyanoacrylate) nanoparticles) was prepared with home-prepared, CS, and commercial, CS, chitosans. Chitosan counterion was identified and chitosans CSmod1 and CSmod2 were obtained from CS exchanging counterion with that found on CS. Carrier quality was checked considering the size, zeta potential and siRNA association capacity by gel electrophoresis. Thermodynamic parameters of interactions between siRNA and chitosans in solution or immobilized at the carrier surface were determined by isothermal titration calorimetry (ITC).

Results: CS and CSmod2 having a high content of acetate counterion associated better siRNA than CS and CSmod1 which counterion included mainly chloride. ITC measurements indicated that siRNA interactions with chitosan and the siRNA carrier were driven by entropic phenomena including dehydration, but thermodynamic parameters of interactions clearly differed according to the nature of the counterion of chitosan. The influence of chitosan counterions was interpreted considering their different lyotropic character.

Conclusion: Association of siRNA with our siRNA carrier was influenced by the nature of counterions associated with chitosan. Driven by entropic phenomena including dehydration, interactions were favored by acetate counterion. Although more work would be needed to decipher the influence of the counterion of chitosan during association with siRNA, it was pointed out as a new critical attribute of chitosan to consider while formulating siRNA carrier with this polysaccharide.