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 (pi-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 degrees 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 pi-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 pi-A isotherms explicitly proved that the condensation effect can be interpreted in terms of conformational and orientational ordering of the alkyl chains of DPPC.
Ohe
C
C
Department of Chemistry, School of Science and Engineering, Waseda University, Shinjuku-ku, Tokyo, 169-8555, Japan.
Sasaki
T
T
Noi
M
M
Goto
Y
Y
Itoh
K
K
D002784:Cholesterol / Q000737:chemistry*; D008567:Membranes, Artificial; D010767:Phosphorylcholine / Q000737:chemistry*; D013057:Spectrum Analysis; D013499:Surface Properties
D008567:Membranes, Artificial; D010767:Phosphorylcholine; C038151:1,2-dipalmitoyl-sn-glycero-3-thiophosphocholine; D002784:Cholesterol