Journal of Colloid and Interface Science 311 (2007) 551–555 www.elsevier.com/locate/jcis Thermodynamic studies of bovine lung surfactant extract mixing with cholesterol and its palmitate derivative A.K. Panda a,b,1 , K. Nag b , R.R. Harbottle b , F. Possmayer c , N.O. Petersen b,* a Department of Chemistry, Behala College, Kolkata 700 060, West-Bengal, India b Department of Chemistry, The University of Western Ontario, London, ON N6A 5B7, Canada c Department of Obstetrics and Gynecology, The University of Western Ontario, London, ON N6A 5C1, Canada Received 18 November 2002; accepted 12 April 2004 Available online 16 April 2007 Abstract Langmuir film behavior of bovine lipid extract surfactant (BLES), mixed with cholesterol (CHOL) and cholesterol palmitate (CHOLP), has been studied by surface pressure (π )–area (A) measurements. Associative interactions, observed for both systems, were less favored at lower BLES content. The presence of unsaturated phospholipids and surfactant proteins in BLES favored the association. Miscibility of BLES was better with CHOLP than with CHOL at all compositions, indicating more compact packing of the BLES–CHOLP than of the BLES–CHOL system. The most stable mixtures were found at 30–40 mol% CHOL and at low π and at 20–25 mol% CHOLP but at higher π . These results suggest that BLES–CHOL miscibility is better at low π and low CHOL concentrations, while BLES–CHOLP miscibility is better at high π and high CHOLP concentrations.  2004 Elsevier Inc. All rights reserved. Keywords: BLES; Cholesterol; Cholesterol palmitate; Excess area; Miscibility; Surface pressure 1. Introduction Monomolecular films are important in several biological systems such as membranes [1,2] and lung surfactants [3–7]. The functionality of a mixed monolayer is usually better than that of the individual components [2] and several studies on mixed monolayer systems have demonstrated that one can obtain information on the molecular interactions between components [2,3,8–10]. Cholesterol, an important constituent of all cell mem- branes [11,12], regulates rigidity and fluidity in monolayers and membranes [2]. It has drastic effects on the physico- chemical properties of lipid systems, the mechanism of which has yet to be fully understood [8,13]. Thus, under- standing cholesterol activity in cell membranes has been a * Corresponding author. Fax: +1 (780) 641 1601. E-mail address: nils.petersen@nrc-cnrc.gc.ca (N.O. Petersen). 1 On leave from Behala College on a Department of Science & Technol- ogy, Government of India-sponsored BOYSCAST Fellowship program. challenging problem for many years. Studies on cholesterol- containing lipid monolayers at the air–water interface have the potential to shed light on this problem, as the lateral inter- actions between molecules in monolayers, lipid bilayers, and biological membranes are likely to be similar at comparable molecular densities. The monolayer system has the special advantage of simplicity and the ease with which intermole- cular interactions can be changed by changes of molecular density through changes in surface pressure [14,15]. Cholesterol and its esters also play major roles in the function of pulmonary surfactant, a lipid–protein com- plex which lines the air–water interface with a putative monomolecular film. The surfactant prevents alveolar col- lapse at low lung volume and hence is essential for normal lung function [5] because of complex interactions of the sur- factant phospholipids and the four surfactant-associated pro- teins (SP-A, B, C, and D). Dipalmitoylphosphatidylcholine, DPPC, is the key to reduced surface tension during alveo- lar/monolayer film compression, but it is a poor surfactant alone, since it cannot spread easily across the surface on its 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.04.010