Lipid scattering densities

Fig. 3. Summary of the scattering densities of biological macromolecules in neutron and X-ray scattering. In neutron scattering, the increase in density for each class as the H20 content increases is the result of exchange. Neutron contrast variation by H20- H20 mixtures is able to match-out each of the five classes shown, while X-ray contrast variation can only be performed in the range between the electron densities of lipids and proteins. From [23].

To a first approximation, the sizes of isometric viruses can be estimated by comparing the experimental maxima and minima with the theoretical curves calculated for spheres and hollow spheres [492-494,504]. However, viruses are composed of protein shells and nucleic acid cores (with carbohydrate and lipid in more complex viral structures), so a full analysis requires the explicit consideration of non-uniform scattering densities. In addition, the principle of icosahedral symmetry in the assembly of the protein shell means that, at large Q, deviations from spherical symmetry will influence the scattering curve. The separation of the scattering curve... 

Fig. XV-4. Schematic drawing of four streptavidin molecules bound to biotinylated lipid in a monolayer above heavy water. The scattering length density for neutron reflectivity is shown at the side. (From Ref. 30.)...

Lins et al. 127 have studied five peptide sequences derived from the ApoB-100 protein, which is the protein moiety in low-density lipoproteins (LDC) that transport cholesterol. ApoB-100 is insoluble and binds to the surface of the LDC particle, and these selected sequences for this study have been implicated as being important in the lipid binding. ATR-FTIR studies showed the one core and three C-terminal originating sequences were mostly sheet-like in the presence of unilamellar vesicles but the N-terminal one was different, probably representing a complex mixture of conformers with some helical component. Furthermore, these workers were able to carry out ATR-LD measurements and determine the orientation of the peptide as being oblique to the membrane. These studies are in contrast to ultraviolet ECD results which were adversely affected by scattering artifacts. 

Appraisals of the optical properties of lipid solutions and dispersions will provide information on concentrations, aggregation and stability, phase transitions, densities, and repeating structuresJ Measurements of refractive index, scattered light intensity (polarized and depolarized), and birefringence are relatively easy laboratory methods on which certain product specifications may be based. Also, fluorescent techniques can readily provide information on lipid movements and transfer of lipid between particles. ... 

Fig. 18 a Neutron reflectometry data for lipid/PEG-lipid monolayers on a pure D2O subphase. The four reflectivity curves correspond to a pure DSPE monolayer and to mixtures of DSPE and DSPE-PEG2000. In this set of data, all of the DSPE and DSPE-PEG2000 lipid hydrocarbon chains were fully deuterated (case 1). Full lines represent free form fits to the individual measurements, b. Corresponding scattering length densities (J3 (z)) obtained from the fits shown in a [47] (reproduced with permission from the American Chemical Society)... 

The polyelectrolyte-tethered bilayers were investigated by means of time dependent surface plasmon spectroscopy, impedance spectroscopy, FRAP, as well as NR [27], The NR data given in Fig. 12 were first calculated based on a model that included the substrate and a box for the polyelectrolyte multilayer (A), and an additional box for the lipid bilayer (C). However, in order to fit the experimental NR curve, a top layer had to be added that was approximated by a uniform coating with average scattering length density b/V shown in the inset of Fig. 12. The thickness of... 

Fig. 12 Neutron reflectometry (NR) data of the polyelectrolyte multilayer (4 PSS/4 PAH) - coated solid support without lipid bilayer (A), and with a DMPC/DMPG (10 1) mixed membrane on top (C). The curves are shifted relative to each other for clarity. Solid lines represent model calculations of the data with scattering length densities, b/V, corresponding to the blank multilayer support (A), and to the tethered bilayer plus a nonspecific top layer (C), as given in the inset. The dotted line (B) represents a simulation of a lipid bilayer without an additional nonspecific layer on top...

In order to observe the concentration fluctuations caused by the gel-fluid phase coexistence in the above-mentioned binary phospholipid mixtures, SANS studies in combination with the H/D substitution technique were performed. Under these so-called matching conditions, no SANS signal is obtained for homogeneously mixed lipids in the all-gel or all-fluid phases, since then the scattering length density is constant over the whole sample. However, in the case of gel-fluid phase heterogeneities, SANS occurs due to the different compositions and... 

At the moment, very little is known about the coupling of collective density fluctuations in proteins and their hydration water (and lipids, in the case of membrane proteins). New instruments recently developed for coherent neutron scattering, combined with selective sample deuteration, may fill this gap soon, and MD simulations are expected to be helpful in the interpretation of the data [60,92]. 

Scattering properties of some lipids. The electron densities correspond to unhydrated residue volumes... 

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