Chitosan as a subphase disturbant of membrane lipid monolayers. The effect of temperature at varying pH: I. DPPG

Temperature-mediated effects of chitosan dissolved in acetate buffer at different pHs, on the structural and thermodynamic characteristics of dipalmitoylphosphatidylglycerol (DPPG) monolayers, employed here as a model of the bacterial cell membrane, were investigated. The investigation was done to improve the understanding if in its antibacterial activity chitosan disturbs the bacterial cell membrane. The Langmuir film technique was employed, where the compression surface pressure–area (π–A) isotherms of DPPG monolayer formed at the air-buffer interface were measured at five temperatures in the range 15–37 °C for three pHs in the range 3.5–6.0, with the chitosan concentration in the subphase varied between 0.002 and 1 mg/mL. Illustrating the extent of DPPG–chitosan interactions, the characteristics of the monolayers on the chitosan-containing solutions were assessed by monitoring differences in the course of the π–A isotherms relative to pure DPPG films. The characteristics revealed that chitosan interacted with the lipid film not only superficially but also inserted to a certain degree into the film. Increasing temperature enhanced these effects, the more strongly, the lower was the pH. Furthermore, the transition of the monolayer from the liquid-expanded to liquid-condensed phase was found to be an endothermic process accompanied by an increase in disorder. The effects were most pronounced at pH 3.5, and were markedly enhanced by chitosan. Most interestingly, the analysis of the critical temperatures provided evidence that the interaction of chitosan with DPPG monolayers is not only due to electrostatic but also to non-electrostatic contributions. The most effective disturbing effects of chitosan on DPPG monolayer were observed at the highest temperature 37 °C applied at pH 3.5.