Egg Lecithin Bilayers Research Articles

The influence of cholesterol on molecular motion in egg lecithin bilayers—A variable-frequency electron spin resonance study of a cholestane spin probe

Electron spin resonance spectra at 9.5, 24. and 35 GHz were obtained for a cholestane spin probe in oriented multibilayers of egg lecithin of varying cholesterol content. In agreement with earlier studies, cholesterol induced a higher degree of spectral anisotropy in the multibilayers—the variation of the hyperfine separations with cholesterol content was in agreement with the model of Lapper et al. ( Can. J. Biochem. 50, 969 (1972)) where the amplitude of anisotropic probe motion decreased with increasing cholesterol content. Analysis of the electron spin resonance line shapes was done using the relatively simple modified Bloch equation approach, and correlation times for anisotropic probe motion were extracted from the spectra at three frequencies. The data demonstrate that increasing cholesterol content results in a decreased rate of anisotropic motion of the probe, providing further insight into the molecular mechanism of the condensing effect of cholesterol.

Differential interaction of cholesterol with phosphatidylcholine on the inner and outer surfaces of lipid bilayer vesicles.

The separate identification of the -N(+)-(CH(3))(3) groups located on the outside and inside of phospholipid bilayers observed in proton magnetic resonance spectroscopy upon addition of low concentrations of praseodymium ion was exploited to investigate the effects of incorporated cholesterol. Only about 7% of the phosphate groups on the outside of the bilayer belong to a class of strong binding or shifting sites. Upon addition of up to about 30% cholesterol to egg lecithin bilayers, no changes in chemical shift or ratio of areas of peaks due to outer and inner -N(+)(CH(3))(3) groups appear. At about 30% incorporated cholesterol, an abrupt decrease occurs in the chemical-shift difference between -N(+)(CH(3))(3) groups located on the outer and inner bilayer surfaces, and an abrupt increase occurs in the ratio of the areas of the two peaks. For L-alpha-dipalmitoyl lecithin bilayers, an abrupt change in chemical-shift difference occurring between 10 and 20% cholesterol is not accompanied by a change in the relative number of -N(+)(CH(3))(3) groups located on the outer and inner surfaces. These results are interpreted as due to the homogeneous distribution of up to 30% cholesterol in egg lecithin bilayers. Above 30%, cholesterol is asymmetrically distributed in favor of the inner layer. In egg lecithin with a variety of polyunsaturated side chains, the side chains with the greater number of double bonds are preferentially displaced by high concentrations of cholesterol, which accounts for the increase in the ratio of outer to inner -N(+)(CH(3))(3) groups. Such preferential displacement by cholesterol cannot occur with the saturated L-alpha-dipalmitoyl lecithin. It is suggested that modified phospholipid vesicles of low radii of curvature may provide high concentrations of "active sites" present in membranes.

High resolution deuterium magnetic resonance - An approach to the study of molecular organization In biological membranes and model systems

Abstract : Reported are the results obtained from the high resolution deuterium resonance (2H NMR) spectra of selectively deuterated lauric acid intercalated in sonicated egg lecithin bilayers. Deuterium resonance has the advantage that selective deuteration can be easily effected and that the linewidths are principally determined by quadrupole relaxation. Intermolecular effects (dipole-dipole interaction) which complicate proton relaxation mechanisms are eliminated in 2H NMR.

An interacting spin label study of the fluidizing and condensing effects of cholesterol on lecithin bilayers

The molecular origin of the fluidizing and condensing effects of cholesterol has been investigated using interacting spin label pairs in multibilayer films of various lecithins. The spin label pair method is a probe of the lateral separation of molecules within the bilayer. The cholestane spin probe separation is found to increase with increasing cholesterol composition in dipalmitoyllecithin bilayers and to decrease with increasing cholesterol composition in both egg and dioleoyllecithin bilayers. These changes in close-packing of the molecules within the lecithin bilayers correspond respectively to the fluidizing and condensing (and rigidifying) effects of cholesterol. The measured decrease in lateral separation corresponding to fluidization of the dipalmitoyllecithin—cholesterol bilayers correlates reasonably well with the latent heat and change in volume at the liquid crystal transition of pure dipalmitoyllecithin. The size of the decrease in lateral separation in egg lecithin bilayers indicates that the condensing effect of cholesterol arises from both molecular interaction with the lecithin chains and the existence of molecular cavities within the lecithin chain region of the bilayer.

The effect of cholesterol on the optical properties of lecithin bilayers

Measurements have been made of the optical reflectance of egg lecithin-cholesterol bilayers and a value of 64 ± 4 Å obtained for the bilayer thickness. There is some evidence that Ca ++ binds to the bilayer when the ion concentration in the aqueous phase is greater than 2M. These results are compared with those obtained for egg lecithin bilayers in the absence of cholesterol.

Semiconductive properties of lipids and their possible relationship to lipid bilayer conductivity

The de semiconductive behavior of lipid films (cholesterol palmitate, egg lecithin and synthetic lecithin) has been studied. Although these lipids are insulators in the dry state, they exhibit conductivities greater than 10 8 times larger in the fully hydrated condition. The semiconduction activation energies for the “dry” and fully hydrated states of these films have been measured: for synthetic lecithin, the “dry” activation energy has values of 4.8 to 6.3 eV depending upon the sample, while for cholesterol palmitate the values range from 2.9 to 3.5 eV. The fully hydrated values for the lecithins range from 1.7 to 2.8 eV, whereas those for the cholesterol palmitate are 1.2 to 1.9 eV. For egg lecithin, an effect paralleling that for hydration is found upon exposure of the film to iodine vapor. Lipid bilayers, formed from egg lecithin, exhibit resistance values of 10 8Ω·cm 2 when formed in distilled water. Addition of iodine to the water drops the resistance to less than 10 4Ω·cm 2. Spectroscopic evidence indicates that iodine forms a donor-acceptor complex both with egg lecithin films and egg lecithin bilayers. The increase in electrical conductivity in both the lipid films and lipid bilayers is taken to be due to the formation of this donor—acceptor complex. Since all presently known donor—acceptor complexes are electronic conductors this is taken as suggestive of an electronic conduction mechanism in the bilayer.