Cholesterol-Induced Structural Changes in Saturated Phospholipid Model Membranes Revealed through X-ray Scattering Technique

Lateral and out-of-plane organization of cholesterol and its effect on regulating the physicochemical properties of zwitterioraic phospholipid model membranes-have been investigated by-a pressure area-isotherm study from the Langmuir monolayer, atomic force microscopy (AFM), and X-ray reflectivity (XRR) measurements from supported binary monolayer films. The systematic isotherm studies on the Langmuir monolayer of phospholipids and the subsequent extraction of excess Gibbs free energy (A Gexc) revealed the mechanism of cholesterol interaction and the molecular cooperativeness-for different arrangements in the phospholipid Model membranes. We have found a critical cholesterol molar concentration (chi(c)) up to which the lipid cholesterol miscibility gradually increases and then further increase in, the concentration leads to an inhomogeneous structure formation similar to raft structures. The thickening in the lipid acyl chain,and the subsequent lowering of the lipid head group thickness-up to xc are also evident from the XRR study. Beyond chi(c), large-sized domains ate observed in the AFM images from the deposited monolayer. chi(c) has als0 been observed to, depend on the phase of the monolayer, in particular, similar to 25 molar % in the gel phase and similar to 40 molar % in the fluid phase, wherein a regular distribution has been found with the highest separation between the cholesterol molecules. The extracted isothermal compressibility coefficient (Cs) and OGexc from the monolayer isotherms indicate that the molecular arrangement at chi(c) are the most stable configurations of the monolayer. Our study provides direct evidence into cholesterol-induced 'evolution in phase behavior-and the,consequent model on the structure at different phases in the phospholipid Langmuir monolayers.