Scattering from laterally heterogeneous vesicles. II. The form factor

Despite growing interest in the formation of domains or `rafts' in cell and model membranes, there have been relatively few attempts to characterize such systems via scattering techniques. Previously [Pencer et al. (2006). J. Appl. Cryst. 39, 293-303], it was demonstrated that the Porod invariant, Q, could be used to detect lateral segregation. Here, the general theory for scattering from laterally heterogeneous vesicles is outlined and form factors are derived for vesicles containing either single circular or annular domains. These form factors are then applied to the analysis of neutron scattering data from heterogeneous vesicles. Potential advantages and limitations of this technique are also discussed.NRC publication: Ye

Spontaneously forming ellipsoidal phospholipid unilamellar vesicles and their interactions with helical domains of saposin C

We have observed a bimodal distribution of ellipsoidal unilamellar vesicles (ULVs) in a phospholipid mixture composed of dioleoyl phosphatidylserine (DOPS) and dipalmitoyl and dihexanoyl phosphatidylcholine, DPPC and DHPC, respectively. Dynamic light scattering and transmission electron microscopy data indicate a bimodal size distribution of these nanoparticles with hydrodynamic radii of approximately 200 and >500 nm, while small-angle neutron scattering data were fit using a model of coexisting monodisperse morphologies, namely, oblate and triaxial ellipsoidal vesicles. Unlike DOPS ULV formed by sonication, which can fuse days after being formed, these ULVs are stable over a period of 12 months at 4 degrees C. We also report on the structure of these ULVs associated with the two helical peptide domains (H1 and H2) of a glucosylprotein, namely, Saposin C, to gain some insight into protein-membrane interactions.NRC 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

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