NEUTRON REFLECTIVITY AND ATOMIC-FORCE MICROSCOPY STUDIES OF A LIPID BILAYER IN WATER ADSORBED TO THE SURFACE OF A SILICON SINGLE-CRYSTAL

Specular reflection of neutrons has been used to characterize the stru cture of single lipid bilayers adsorbed to a planar silicon surface fr om aqueous solution. We used a novel experimental setup which signific antly decreased the incoherent background scattering and allowed us to measure neutron reflectivities as low as 5 x 10(-7). Thicknesses and neutron scattering length densities were determined by a fitting proce dure using (i) randomly generated smooth functions represented by para metric B-splines and (ii) stepped functions based on the theoretical l ipid composition. The size of lipid domains at the surface and the deg ree of surface coverage were determined by atomic force microscopy. Ch ain-protonated and -deuterated dipalmitoylphosphatidylcholine (DPPC) b ilayers were investigated in (H2O)-H-2 and a mixture of (H2O)-H-2 and H2O which matches the scattering density of silicon. Also, one measure ment on a distearoylphosphatidylcholine bilayer which has longer acyl chains was performed for comparison. The lipid adsorbs to the silicon surface as a continuous layer interrupted by irregularly shaped uncove red areas which are 100-500 Angstrom in size. The surface coverage was estimated to be 70 +/- 20%. The reflectivity measurements on DPPC at 60 degrees C show a silicon oxide layer with a thickness of the order of 4 Angstrom, a rough silicon oxide/water layer between silicon oxide and lipid with a thickness between 2 and 8 Angstrom, and a single lip id bilayer. Fitting resolved a central membrane layer with a thickness of 28 +/- 2 Angstrom which represents the lipid hydrocarbon chains. T his layer is sandwiched between interface membrane layers of lipid hea d groups and water which are 11.5 +/- 1 Angstrom in thickness. The ang strom-scale thickness changes of the central membrane layer as a funct ion of the phase state of the lipid and of the length of the hydrocarb on chains are easily detected.