Owing to its distinct chemico-biological properties, chitosan, a cationic biopolymer, offers a great potential in multifarious bioapplications. One such application is as a dietary antilipidemic supplement to be used to reduce obesity/overweight and to lower cholesterol. The lipid-binding efficiency of chitosan, however, remains debatable. Accordingly, in this study we investigated the interactions of chitosan with selected lipids, cholesterol and fatty acids, the latter including saturated (stearic acid) and unsaturated (oleic, linoleic, alpha-linolenic) acids. The experiments were performed with the Langmuir monolayer technique, in which surface pressure-area isotherms were recorded for the lipid monolayers spread on the acetate buffer pH 4.0 subphase in the absence and presence of chitosan. We found that the presence of chitosan in the subphase strongly influenced the shape and location of the isotherms, proving that there existed attractions between chitosan and lipid molecules. The attractions were revealed by changes of the molecular organization of the monolayers. The common feature of these changes was that all the monolayers studied underwent expansion, in each case reaching saturation with increasing chitosan concentration. In agreement with the lipid molecular structures, the highest expansions were observed for the most unsaturated fatty acids, linoleic and alpha-linolenic, the lowest for stearic acid, with oleic acid and cholesterol being the intermediate cases. By contrast, the main distinguishing feature of these changes was that, although none of the monolayers studied changed its state when completely saturated with chitosan, compared to the parent ones the compactness of the monolayers was modified. The solid monolayers of stearic acid and cholesterol were loosened, whereas those of all the unsaturated acids, liquid in nature, were tightened. On the basis of these results we tentatively propose a mechanism of the chitosan action that includes both electrostatic and hydrophobic lipid-chitosan interactions as well as hydrogen bonding between them.

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The aim of this work was to get insight into cholesterol distribution between two leaflets of a phospholipids bilayer. In this order, the thermodynamic analysis of the interactions between membrane lipids in binary (cholesterol/phospholipid) and ternary (phospholipid/ phospholipid/cholesterol) mixed Langmuir monolayers has been performed. For our investigation, phosphatidylcholine and phosphatidylethanolamine, which are the main types of phospholipids determining the distribution of cholesterol in membrane leaflets, were chosen and mixed in proportions corresponding to their molar ratios in the inner and outer layers of the natural human erythrocyte membrane. Into these mixed systems, various amount of cholesterol were incorporated. It has been found that despite strong differences in the phospholipid composition of both investigated ternary mixed systems, the influence of cholesterol is very similar, which indicates that cholesterol is symmetrically distributed between the inner and outer leaflets of the human erythrocytes membrane.

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