Structural Modifications in the Liquid Crystalline L_α Phase of Distearoylphosphatidylcholine Induced by Variable Cholesterol Concentration

Abstract

The melted chain liquid crystal phase of distearoyllecithin liposomes in water was investigated at various cholesterol content. X-ray diffraction and differential scanning calorimetry techniques were used. A fluidifying effect with respect to the chain melting temperature and a stiffening effect, deduced from the progressive increase of the lamellar repeat spacing in the melted chain phase, were deduced for low cholesterol concentrations, in agreement with the previous results in other phospholipids. An increase of the in plane lattice parameter and a decrease of the lamellar repeat spacing, corresponding to an overall non monotonic behaviour, were observed at higher concentrations, below the saturation value. The positional order inside the layers was considered for the first time in a phospholipid in a quantitative way by evaluating the correlation length ξ. That in plane correlation length remains constant for cholesterol concentrations lower than 35% and then decreases, indicating opposite effects concerning the in plane translational order as compared to the orientational order: the cholesterol increses the last kind of order but decreases the first one.