Scattering from laterally heterogeneous vesicles. III. Reconciling past and present work

A recent series of papers have devised and successfully used a methodology for the detection and characterization of domains in laterally heterogeneous vesicles via small-angle neutron scattering. This methodology is in seeming contradiction to similar work devised by Knoll, Haas, Stuhrmann, F\ufcldner, Vogel & Sackmann [J. Appl. Cryst. (1981), 14, 191-202]. The present paper shows how these results may be reconciled.NRC publication: Ye

Scattering from laterally heterogeneous vesicles. I. Model-independent analysis

This is the first of a series of papers considering the scattering from laterally heterogeneous vesicles. Here, it is shown that contrast variation studies on unilamellar vesicles can be analyzed in a model-independent manner to detect lateral segregation in model membranes. In particular, it is demonstrated that the Porod invariant, Q = fq2I(q) dq, is related to the scattering length density contrast between compositionally distinct regions in a heterogeneous membrane. The formation of domains and the concomitant identification of phase boundaries as a function of either membrane composition or temperature can therefore be detected in the changes taking place in Q.NRC publication: Ye

Neutron and X-ray Scattering for Biophysics and Biotechnology: Examples of Self-Assembled Lipid Systems

Membranes that surround cells and separate their contents fromthe external environment are ubiquitous in biological systems. Thesemembranes are organized assemblies consistingmainly of lipids and proteins, and are highly selective permeability barriers which control the flow of information between cells and their environment. It is accepted that the lipid bilayer is the underlying structure ofmost, if not all, biomembranes. As such, over the years scientists have exerted much effort in studying lipid bilayers and their biological relevance in hopes of understanding the functional mechanisms taking place at membrane interfaces. Neutron and X-ray scattering techniques are powerful tools for the characterization of the structure and dynamics of biomimetic systems as they provide unique access to microscopic structure and dynamics at length scales ranging from microns to intermolecular and/or atomic distances. The optimization of instruments and preparation techniques, as well as the new possibilities offered by protein deuteration, have opened up new avenues for the study of lipid/protein interactions that were not previously possible. One can now look at the insertion of biomolecules intomembranes and accurately determine the structure as well as the dynamics of the interaction. To illustrate the usefulness of diffraction and scattering techniques with regard to biologically relevant systems, we reviewsome of the leading edge studies that have taken place over the last couple of years in which these scattering techniques have played a central role.Peer reviewed: YesNRC publication: Ye

The study of liposomes, lamellae and membranes using neutrons and X-rays

Advances in colloid and interface science have stimulated a renewed interest in the study of lipid-water systems. In recent years, much progress has been achieved in the domains of sample preparation and sample environments, offering the unique possibility of studying these systems under physiologically relevant conditions. In the case of neutron reflectometry, new experimental protocols allow for the unique structural determination of one-dimensional membrane profiles, while the advantages offered by synchrotron radiation (e.g., high flux and spatial resolution) make X-rays an excellent tool for addressing questions pertaining to membrane interactions. Most recently, holographic techniques are evolving so that one day they may be able to resolve, to atomic resolution, the structure of poorly crystallized membrane associated proteins.NRC publication: Ye

Structure of fully hydrated fluid phase lipid bilayers with monounsaturated chains.

Quantitative structures are obtained at 30 degrees C for the fully hydrated fluid phases of palmitoyloleoylphosphatidylcholine (POPC), with a double bond on the sn-2 hydrocarbon chain, and for dierucoylphosphatidylcholine (di22:1PC), with a double bond on each hydrocarbon chain. The form factors F(qz) for both lipids are obtained using a combination of three methods. (1) Volumetric measurements provide F(0). (2) X-ray scattering from extruded unilamellar vesicles provides /F(qz)/ for low q(z). (3) Diffuse X-ray scattering from oriented stacks of bilayers provides /F(qz)/ for high q(z). Also, data using method (2) are added to our recent data for dioleoylphosphatidylcholine (DOPC) using methods (1) and (3); the new DOPC data agree very well with the recent data and with (4) our older data obtained using a liquid crystallographic X-ray method. We used hybrid electron density models to obtain structural results from these form factors. The result for area per lipid (A) for DOPC 72.4 +/- 0.5 A(2) agrees well with our earlier publications, and we find A = 69.3 +/- 0.5 A2 for di22:1PC and A = 68.3 +/- 1.5 A2 for POPC. We obtain the values for five different average thicknesses: hydrophobic, steric, head-head, phosphate-phosphate and Luzzati. Comparison of the results for these three lipids and for our recent dimyristoylphosphatidylcholine (DMPC) determination provides quantitative measures of the effect of unsaturation on bilayer structure. Our results suggest that lipids with one monounsaturated chain have quantitative bilayer structures closer to lipids with two monounsaturated chains than to lipids with two completely saturated chains.</p

What Determines the Thickness of a Biological Membrane

Membrane thickness is thought to play a key role in protein function. Thus understanding the cell\u2019s ability to modulate the thickness of its membranes is essential in elucidating the structure/ function relationship in biological membranes. We have investigated the influence of cholesterol on the structure of \u201cthin\u201d (diC14:1PC) and \u201cthick\u201d (diC22:1PC) phospholipid bilayers using oriented multibilayers and small angle neutron di raction. Neutron contrast variation was used to determine the structure factors and the distribution of water across the bilayers. We found that in response to cholesterol, bilayer thickness changed in a similar fashion in both systems. The thickening of bilayers was rationalized in terms of cholesterol\u2019s ordering effect on the lipid\u2019s acyl chains, which dominates over the other option of rectifying the hydrophobic mismatch, surprisingly even in the case of diC22:1PC and cholesterol.Peer reviewed: YesNRC publication: Ye

Curvature effect on the structure of phospholipid bilayers

High-resolution small-angle X-ray scattering (SAXS), complemented by small-angle neutron scattering (SANS) and dynamic light scattering (DLS) experiments, was used to study the effect of curvature on the bilayer structure of dioleoyl-phosphatidylcholine (DOPC) and dioleoyl-phosphatidylserine (DOPS) unilamellar vesicles (ULVs). Bilayer curvature, as a result of finite vesicle size, was varied as a function of vesicle radius and determined by DLS and SANS measurements. Unilamellarity of large DOPC ULVs was achieved by the addition of small amounts (up to 4 mol %) of the charged lipid, DOPS. A comparison of SANS data over the range of 0.02 < q <0.2 ?-1 indicated no change in the overall bilayer thickness as a function of ULV diameter (620 to 1840 ?). SANS data were corroborated by high-resolution (0.06 < q <0.6 ?-1) SAXS data for the same diameter ULVs and data obtained from planar samples of aligned bilayers. Both the inner and outer leaflets of the bilayer were found to be indistinguishable. This observation agrees well with simple geometric models describing the effect of vesicle curvature. However, 1220-?-diameter pure DOPS ULVs form asymmetric bilayers whose structure can most likely be rationalized in terms of geometrical constraints coupled with electrostatic interactions, rather than curvature alone.Peer reviewed: YesNRC publication: Ye