Sum-frequency vibrational spectroscopy (SFVS) is used to measure the intrinsic rate of lipid flip-flop for 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) in planar-supported lipid bilayers (PSs). Asymmetric PSLBs were prepared using the Langmuir-Blodgett/Langmuir-Schaefer method by placing a perdeuterated lipid analog in one leaflet of the PSLB. SFVS was used to directly measure the asymmetric distribution of the native lipid within the membrane by measuring the decay in the CH3 v(s) intensity at 2875 cm(-1) with time and as a function of temperature. An average activation energy of 220 kJ/mol for the translocation of DMPC, DPPC, and DSPC was determined. A decrease in alkyl chain length resulted in a substantial increase in the rate of flip-flop manifested as an increase in the Arrhenius preexponential factor. The effect of lipid labeling was investigated by measuring the exchange of 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-n,n-Dimethyl-n-(2',2',6',6'-tetramethyl-4'-piperidyl) (TEMPO-DPPC). The rate of TEMPO-DPPC flip-flop was an order-of-magnitude slower compared to DPPC. An activation energy of 79 kJ/mol was measured which is comparable to that previously measured by electron spin resonance. The results of this study illustrate how SFVS can be used to directly measure lipid flip-flop without the need for a fluorescent or spin-labeled lipid probe, which can significantly alter the rate of lipid translocation.

The conformational order of sodium dodecyl sulfate (SDS) adsorbed at the D2O-CCl4 interface has been examined by total internal reflection sum-frequency vibrational spectroscopy. A change in conformation of the alkyl chain with increased surface coverage at the liquid−liquid interface is observed. A series of aqueous surfactant concentrations have been examined in order to determine the effect of surface coverage on the conformation of the alkyl chains at the interface. Polarization studies indicate that, for the concentration range examined, the symmetry axis of the terminal methyl group on the alkyl chain is oriented primarily along the surface normal. Identification of spectral features in the C−H region of the infrared region is facilitated by examination of the sum-frequency spectrum from an analogous deuterated compound.

The direct measurement of the transbilayer movement of 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) in a planar supported lipid bilayer (PSLB) at the fused silica/D2O interface was obtained with sum-frequency generation (SFG) vibrational spectroscopy. The intrinsic sensitivity of SFG to the symmetry of an interface was used to measure the asymmetric distribution of DSPC and perdeuterated DSPC (DSPC-d83) lipids in asymmetrically prepared DSPC/DSPC-d83 PSLBs. Changes in the membrane lipid composition due to exchange between leaflets was monitored by measuring the decay in the CH3 symmetric stretch intensity at 2875 cm-1 with time. The activation energy for transverse motion was determined directly from spectral relaxation measurements at several temperatures and was determined to be 206 +/- 18 kJ/mol. At room temperature (25 degrees C) the half-time of lipid flip-flop was calculated to be approximately 25 days. At 51 degrees C, only 7 degrees C below the main phase-transition temperature of DSPC, the half-time decreases to 25 min. These results have important implications for understanding the transbilayer movement of lipids in biological membranes.

Amphiphilic molecules are known to spontaneously order at liquid-solid' and liquid-air interfaces,2 forming well-ordered layers, but little is known about the structure of these molecules at the liquid-liquid interface due to experimental inaccessibility. A number of linear and nonlinear spectroscopic studies of surfactants at this interface have been p e r f~r m e d ,~-~ but these studies could not probe alkyl chain ordering. The goal of our work has been to measure the vibrational spectrum of a simple surfactant at an interface between two immiscible liquids in order to understand how such surfactants orient and assemble at this interface. In this communication, we report the first measurement of this type where the vibrational spectrum of sodium dodecyl sulfate (SDS) at the D20-CCb interface is obtained. The results provide spectroscopic evidence for interfacial ordering. The success of these studies has come from performing sum frequency generation (SFG) measurements in a total intemal reflection geometry (TIR).Surface SFG is gaining in use as a probe of surfaces and buried interfaces and is thoroughly described in the literatu1e.7-I~ SFG is more powerful than conventional spectroscopies such as FTIR because of its inherent surface selectivity, and for this reason, SFG is an excellent probe of interfaces. In TIR-SFG, selective probing of the interface between the two phases is obtained with much greater sensitivity than normal SFG much in the way that attenuated total reflectance FTIR gains its sensitivity. Using resonant TIR-SFG, the vibrational spectrum of the surfactant SDS at the D2O-CCL interface has been obtained, shown in Figure 1, This spectrum of a full monolayer at the interface was obtained using a bulk concentration of 10 mM SDS.To ascertain the order of SDS as a function of surface concentration, the intensities of the methyl and methylene symmetric stretch peaks in the sum frequency spectra are examined at 10 mM SDS and at several successive lower concentrations. The data in Figure 2 show the symmetric stretch region for methyl (2866 cm-') and methylene (2840 cm-I) groups as the concentration of SDS is changed from 0.1 to 5mM.The surface coverage of SDS is determined from the corre-(1) Bigelow, W. C.; Pickett, D. L.; Zisman, W. A. J. Colloid Interfie (2) Pockels, A. Nature 1891, 43, 437. ( 3 ) Higgins, D. A.; Com, R. M.

In the study presented here, the structure of the RTIL/SiO2 interface has been examined by sum-frequency vibrational spectroscopy (SFVS). A series of hydrophobic RTILs composed of 1-alkyl-3-methylimidazolium (C n mim, n = 6, 8 and 10) and bis(perfluoromethylsulfonyl)imide (BMSI) and bis(perfluoroethylsulfonyl)imide (BETI) anions have been examined. SFVS was used to determine the orientation of the cation as well as the structure of water at the RTIL/SiO2 interface. The alkyl chain of the imidazolium cation was determined to be nearly normal to the surface for all the RTILs examined. The conformational order of the alkyl chain on the cation was also determined and was found to be dependent on the length of the alkyl chain, with longer chains being more ordered with few gauche defects. The tilt of the imidazolium ring with respect to the surface normal was also determined and was found to be ∼28° for the BMSI salts with a reduction in the tilt angle to ∼18° for the BETI RTILs. The water at the surface associated with the RTIL was determined to be hydrogen-bonded either singly or doubly to the anions at the SiO2 interface. In addition, the nature of the hydrogen-bonding was found to be dependent on the amount of bulk water contained in the RTIL.