Determination of structural parameters and hydration of unilamellar POPC/C12E4 vesicles at high water excess from neutron scattering curves using a novel method of evaluation

A novel method of evaluating SANS curves is presented which for the first t ime allows the simultaneous determination of the internal geometry and hydr ation of single bilayers in unilamellar vesicles at high water excess. A mu ltistrip model unambiguously defines the water-free hydrophobic core of the membrane as a region where the neutron scattering length density equals th at of a typical hydrocarbon chain. Further, the outer water-accessible part s of the membrane are defined as regions where the neutron scattering lengt h density differs from those of pure hydrocarbons and from water. Three ind ependent structural parameters (thickness c of the water-free core, thickne ss h of the water-accessible coat of the bilayer membrane, and the relation ship between the scattering length densities of these membrane regions) are directly obtained by fitting the scattering curve in the q range from 0.06 nm(-1) to 3.82 nm(-1). The surface requirements of the amphiphiles and the number of water molecules located in the membrane can be calculated from t hese parameters. If there are multilamellar vesicles, one obtains additiona lly the repeat distance and the percentage of these vesicles. The potential of the method is demonstrated by elucidating structural and hydration para meters of mixed unilamellar POPC/C12E4 vesicles.