Colloids and Surfaces A: Physicochem. Eng. Aspects 434 (2013) 349–358 Contents lists available at SciVerse ScienceDirect Colloids and Surfaces A: Physicochemical and Engineering Aspects jo ur nal ho me page: www.elsevier.com/locate/colsurfa Chitosan as a subphase disturbant of membrane lipid monolayers. The effect of temperature at varying pH: I. DPPG Barbara Krajewska ∗ , Paweł Wydro, Agnieszka Kyzioł Jagiellonian University, Faculty of Chemistry, 30-060 Kraków, Ingardena 3, Poland h i g h l i g h t s • Models of bacterial cell membranes, DPPG monolayers were disturbed by chitosan, the more strongly, the higher was temperature and the lower was pH. • Chitosan inserted into DPPG mono- layers using both electrostatic, and importantly, non-electrostatic forces. • Chitosan antibacterial activity may thus at least in part consist of distur- bing cell membrane structures. g r a p h i c a l a b s t r a c t a r t i c l e i n f o Article history: Received 14 November 2012 Received in revised form 12 February 2013 Accepted 1 March 2013 Available online 1 April 2013 Keywords: Chitosan Antibacterial activity DPPG Langmuir films Compression isotherms Structural and thermodynamic parameters a b s t r a c t 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 Lang- muir film technique was employed, where the compression surface pressure–area (–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 –A isotherms relative to pure DPPG films. The characteristics revealed that chitosan inter- acted 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. © 2013 Elsevier B.V. All rights reserved. ∗ Corresponding author. Tel.: +48 12 6632235; fax: +48 12 6340515. E-mail addresses: krajewsk@chemia.uj.edu.pl (B. Krajewska), wydro@chemia.uj.edu.pl (P. Wydro), kyziol@chemia.uj.edu.pl (A. Kyzioł). 1. Introduction Made of water-insoluble lipids at the air–aqueous interface, the monolayers mimic half a cellular membrane [1,2]. Simple as they are, they serve as useful models for probing the behavior of bio- logical membranes. Their usefulness consists in that they allow the molecular organization of the film to be appraised, along with the intermolecular forces within the film and between the film and 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.03.015