Sum frequency generation spectroscopic study of the condensation effect of cholesterol on a lipid monolayer

Sum frequency generation (SFG) spectra and surface pressure-molecular area (π-A) isotherms have been obtained for mixed cholesterol-DPPC monolayers with cholesterol mole fractions, x(chol.), from 0 to 1.0, at the air-water interface, under same conditions, at 22 °C. Analysis of the spectra indicated that incorporation of cholesterol into the monolayers at 3 mN m-¹ greatly increases the conformational and orientational order of the alkyl chains of DPPC, maximizing these properties at x(chol.)=0.4. Analysis also indicated that order in the mixed monolayers at 15 and 35 mN m-¹ is not affected by incorporation of cholesterol. The π-A isotherms measured at 3 mN m-¹ for the mixed monolayer with x(chol.)=0.4 have the largest negative deviation of the molecular area relative to those of ideal mixtures (the so-called “condensation effect” of cholesterol), indicating the most thermodynamically stable state. Comparison of results from SFG spectra and π-A isotherms explicitly proved that the condensation effect can be interpreted in terms of conformational and orientational ordering of the alkyl chains of DPPC.