Graner Adsorbed and free lipid bilayers at the solid ± liquid interface viewed by specular and off-specular re ̄ ectivity

Content Provider	Semantic Scholar
Author	Braslau Charitat, Thierry Daillant, Jean Fragneto, Giovanna
Abstract	membrane interactions is growing fast. Simple lipid bilayers deposited on a substrate are often used as biological model membranes [1±3]. The interest in them lies in the fact that they are studied in a bulk aqueous environment and the e€ect of a small amount of material (e.g. peptides or other small molecules) may be detected by techniques such as neutron or X-ray re ̄ectivity. The limit of this model is the interaction of the bilayer with the solid substrate. We have recently succeeded in depositing two bilayers on ̄at substrates [4]. The second one, called ``free'' bilayer, ̄oats 20±30 AÊ above the ®rst. By interacting less strongly with the substrate, it represents a better model for biological membranes. Double bilayers have been characterised by specular neutron re ̄ectivity [5] and structural information (thickness and composition) has been obtained in the direction normal to the interface [4]. Following these results, a study was started to determine the in-plane structure and ̄uctuations of the membrane. In the case of a single bilayer a good probe for in-plane imaging is atomic force microscopy (AFM) in noncontact mode. In the case of the free bilayer such measurement is not yet possible. The most promising technique for exploring in-plane structure is o€-specular re ̄ectivity [6]. We present here some preliminary results obtained from specular and o€-specular re ̄ectivity on single and double bilayers deposited on silicon substrates and studied in bulk water. Since neutron intensities are so far too noisy to extract o€-specular data at solid/liquid interfaces, synchrotron radiation was used. Distearyl phosphatidyl choline lipids were used. They were deposited on 2.5 ́ 2.5 cm silicon substrates with the Langmuir±Blodgett and Langmuir±Schae€er techniques. Details of the deposition procedure are given in Ref. [4]. The specular and o€-specular measurements were performed on the BM32 beamline at the ESRF (Grenoble, France). If we assume that the interface is perfectly smooth, the re ̄ection of the beam on the di€erent interfaces is purely specular. The momentum transfer q, is perpendicular to the plane of the membrane and gives information on the structure in this direction. As a surface is not perfectly smooth the specular signal decreases and a part of the Progr Colloid Polym Sci (2000) 115 : 171±173 Ó Springer-Verlag 2000 BIOSYSTEMS
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