Neutron Reflectivity and Atomic Force Microscopy Studies of a Lipid Bilayer in Water Adsorbed to the Surface of a Silicon Single Crystal

• Published in: Langmuir Vol. 12; no. 5; pp. 1343 - 1350
• Main Authors: Koenig, B. W, Krueger, S, Orts, W. J, Majkrzak, C. F, Berk, N. F, Silverton, J. V, Gawrisch, K
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 06.03.1996
• Subjects: Artificial membranes and reconstituted systems; Biological and medical sciences; Chemistry; Colloidal state and disperse state; Exact sciences and technology; Fundamental and applied biological sciences. Psychology; General and physical chemistry; Membrane physicochemistry; Membranes; Molecular biophysics; Solid-liquid interface; Surface physical chemistry; Structure effect; Atomic force microscopy; Liquid solid interface; Liquid solid adsorption; Organic adsorbate; Chain length; Temperature effect; Phospholipid; Biomembrane; Inorganic adsorbent; Experimental study; Bilayer; Surface structure; Characterization; Neutron spectrometry; Specular reflection; Surface properties; Phosphatidylcholine; Membrane; Silicon; Aqueous solution; Surface area; Application; Labelled compound
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la950580r

Specular reflection of neutrons has been used to characterize the structure of single lipid bilayers adsorbed to a planar silicon surface from aqueous solution. We used a novel experimental setup which significantly decreased the incoherent background scattering and allowed us to measure neutron reflectivities as low as 5 × 10-7. Thicknesses and neutron scattering length densities were determined by a fitting procedure using (i) randomly generated smooth functions represented by parametric B-splines and (ii) stepped functions based on the theoretical lipid composition. The size of lipid domains at the surface and the degree of surface coverage were determined by atomic force microscopy. Chain-protonated and -deuterated dipalmitoylphosphatidylcholine (DPPC) bilayers were investigated in 2H2O and a mixture of 2H2O and H2O which matches the scattering density of silicon. Also, one measurement 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 uncovered areas which are 100−500 Å in size. The surface coverage was estimated to be 70 ± 20%. The reflectivity measurements on DPPC at 60 °C show a silicon oxide layer with a thickness of the order of 4 Å, a rough silicon oxide/water layer between silicon oxide and lipid with a thickness between 2 and 8 Å, and a single lipid bilayer. Fitting resolved a central membrane layer with a thickness of 28 ± 2 Å which represents the lipid hydrocarbon chains. This layer is sandwiched between interface membrane layers of lipid head groups and water which are 11.5 ± 1 Å in thickness. The angstrom-scale thickness changes of the central membrane layer as a function of the phase state of the lipid and of the length of the hydrocarbon chains are easily detected.