Sum frequency generation spectroscopic study of the condensation effect of cholesterol on a lipid monolayer |
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Authors: | C Ohe T Sasaki M Noi Y Goto K Itoh |
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Institution: | (1) Department of Chemistry, School of Science and Engineering, Waseda University, Shinjuku-ku, Tokyo 169-8555, Japan |
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Abstract: | 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. |
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Keywords: | Sum frequency generation spectroscopy Cholesterol Lipids DPPC Monolayers Air-water interface |
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