Chitosan-induced perturbation of dipalmitoyl-sn-glycero-3-phosphocholine membrane bilayer

Recently, chitosan, a positively charged polysaccharide in slightly acidic condition, has been used as a membrane perturbant in a novel gene delivery assay. In this study, the fundamental interactions between chitosan and DPPC membrane bilayers were investigated with cross-polarization microscopy, differential scanning calorimetry and Fourier transform (FT) Raman spectroscopy. The cross-polarized images showed that chitosan induced fusions of multilamellar vesicles. It was determined that the mixing of chitosan with dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and subsequent hydration of the mixture at 60 °C significantly suppressed the enthalpy of the gel-liquid crystalline transition in a concentration-dependent manner. Chitosan also affected the thermotropic behavior of DPPC bilayer during the cooling cycle. However, chitosan addition to DPPC had no effect on the main phase transition temperature (T_m) of DPPC bilayers. When DPPC and chitosan were mixed in chloroform before hydration, the initial rate of enthalpy reduction against chitosan concentration was significantly increased. Furthermore, the dependence of the cooperative unit of the DPPCs main transition on the chitosan mole fraction showed that chitosan tuned the intermolecular interactions between neighboring lipid molecules. FT-Raman spectroscopy provided solid evidence that the attractive interchain and intermolecular forces of the hydrophobic core (acyl chains) in the DPPC bilayer were significantly reduced by the chitosan-membrane interactions. The addition of chitosan also reduced the order in the two-dimensional packing of the acyl chains and increased the fluidity of the DPPC bilayer. This study provided new insights into the physicochemical interactions between model membrane and chitosan that might aid the development of a novel membrane perturbant for gene delivery.