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Lipid bilayer - fluid

A biological membrane consists of a double layer of phospholipid molecules with a variety of embedded structures. X-ray crystallography has revealed the structures of many of the embedded components, including even such large complexes as the photosynthetic reaction center. The phospholipid bilayer, although cenceptually much simpler, offers serious obstacles to a complete structural characterization: it is fluid and so cannot be crystallized, and its properties depend on its composition and environment. John Nagle and co-workers, long-time CHESS users, have carried out many experiments to further the understanding of these membranes; here we mention two recent examples.

Closer Look at Structure of Fully Hydrated Fluid Phase DPPC Bilayers
N. Kuĉerka, S. Tristram-Nagle, and John Nagle,
Biophys. J. (2006), Vol. 90, L83-L85

Dipalmitoylphosphatidylcholine, or DPPC, is the most studied lipid bilayer. However, its structure depends on whether the lipid molecules are in a fluid or gel state, and on how much water is present; it has been difficult to obtain structural information for the fully hydrated fluid phase, which is the most biologically relevant form. The Nagle group has now succeeded in getting good data from both oriented stacks of bilayers and small unilamellar vesicles, and can report improved structural information for DPPC.

Several sets of diffuse scattering data, taken at CHESS on the two types of sample, were used. At low qz the vesicle data were most useful, while at high qz the oriented stacks gave better results; combining both types of dataset produced good, complete coverage out to 0.85 Å-1. A new hybrid model was used to generate electron density profiles (see the figure) and to calculate the area per phospholipid molecule. The quality of the results represents a distinct improvement over earlier experiments on this system.

Electron density as a function of depth in the DPPC bilayer, with the modeled contributions from different parts of the phospholipid molecules. Zero is at the middle of the bilayer, DC marks the end of the hydrocarbon portion of the phospholipid chains, and D'B is the full thickness of the membrane.

Full text of article (pdf)


Curvature Effect on the Structure of Phospholipid Bilayers.
N. Kuĉerka, J. Pencer, J. N. Sachs, J. F. Nagle, and J. Katsaras,
Langmuir (2007), Vol. 23, 1292-1299

In a biological membrane, a single phospholipid bilayer is present, of varying curvature depending on its location in the cell. For study of model membranes, however, samples consist of multiple layers, or a layer supported on a substrate, or small vesicles, and it can be difficult to extrapolate from such systems to the natural condition. In this study, confounding effects were minimized by using only well-characterized unilamellar vesicles of various sizes. Two phospholipids were used: dioleoyl-phosphatidylcholine (DOPC), which is neutral, and dioleoyl-phosphatidylserine (DOPS), which is charged. Dynamic light scattering was used to determine the size of the vesicles, and SAXS (at CHESS D-1 station) and neutron scattering (at the NIST Center for Neutron Research) were used to obtain structural information.

A small percentage of DOPS was added to DOPC to insure that all vesicles were unilamellar, without significantly affecting charge neutrality. These samples, contrary to indications from earlier experiments, showed no effect of vesicle size (i.e. bilayer curvature) on bilayer thickness, and no difference between inner and outer monolayers. Highly charged bilayers, for example pure DOPS, did show a pronounced asymmetry between inner and outer layers.

Scattering curves from various sizes and compositions of DOPC-DOPS vesicles. Samples with less than 4% DOPS (red and green curves) show structure due to the presence of some vesicles containing more than one layer, but with 4% DOPS (blue curves) present these are eliminated, and there is no apparent difference in the curves for unilamellar vesicles of different size.

Full text of article (pdf)