Head group interaction in membrane - cholesterol association: Studies in Aerosol OT model membrane system by 13C chemical shifts, relaxation and nuclear Overhauser enhancement effects

Abstract

The 13C NMR chemical shifts, relaxation times and nuclear Overhauser effects of interfacial Aerosol OT molecules in reverse micellar systems in the presence of cholesterol were studied in order to observe the structure and dynamic properties of the amphiphilic assemblies. It was found that a hydrogen bonding takes place between the 3β-OH group of cholesterol and the oxygen atom of the carbonyl ester group of the Aerosol OT molecule which is close to the electronegative sulphonic ion. The negative nuclear Overhauser enhancement factors (NOEF) of this carbonyl carbon atom gradually increase to positive values with the increased addition of cholesterol to the micelles. The result can be explained by the reduced local segmental motion of the carbonyl group as a consequence of its complexation with a bulky molecule, such as cholesterol, so that the internal oscillatory motion contributes mainly to the relaxation mechanism. The NOEF of the other carbonyl carbon atom is reduced with increased cholesterol content, which indicates that the local motion of the carbonyl group is reduced by the condensation effect of the hydrocarbon chains. It is argued that the cholesterol interaction with the amphiphilic Aerosol OT molecule leads both the polar ester groups towards the hydrophobic part of the interface, leaving only the sulphonic group exposed to the hydrophilic aqueous core.