ABSTRACT
Scattering techniques, in particular electron, neutron and X-ray scattering have played a major role in elucidating the static and dynamic structure of biologically relevant membranes. Importantly, neutron and X-ray scattering have evolved to address new sample preparations that better mimic biological membranes. In this review, we will report on some of the latest model membrane results, and the neutron and X-ray techniques that were used to obtain them.
Subject(s)
Cell Membrane/chemistry , Models, Molecular , Neutron Diffraction , X-Ray Diffraction , Membrane Proteins/chemistry , Phospholipids/chemistryABSTRACT
The influence of cholesterol on the structure of unilamellar-vesicle (ULV) phospholipid bilayers is studied using small-angle neutron scattering. ULVs made up of short-, mid- and long-chain monounsaturated phospholipids (diCn :1PC, n = 14 , 18, 22, respectively) are examined over a range (0-45 mol %) of cholesterol concentrations. Cholesterol's effect on bilayer structure is characterized through changes to the lipid's transmembrane thickness, lateral area and headgroup hydration. For all three lipids, analysis of the experimental data shows that the addition of cholesterol results in a monotonic increase of these parameters. In the case of the short- and mid-chain lipids, this is an expected result, however, such a finding was unexpected for the long-chain lipid. This implies that cholesterol has a pronounced effect on the lipid's hydrocarbon chain organization.
Subject(s)
Cholesterol/chemistry , Lipid Bilayers/chemistry , Membrane Fluidity , Models, Chemical , Models, Molecular , Unilamellar Liposomes/chemistry , Complex Mixtures/chemistry , Computer Simulation , Molecular ConformationABSTRACT
Finite-size effects in stacks of phospholipid bilayers, in the fluid L alpha phase, are investigated using samples oriented on silicon substrates. Recently in this journal, such effects have been suggested as the probable cause of reduced lamellar repeat spacings in very thin samples made up of a few (<10) bilayers. Our systematic studies on samples of different thicknesses do not support this conclusion. At full hydration all samples are found to have the same repeat spacing, irrespective of their thickness. At lower hydrations, on the other hand, very thin samples, consisting of only a few bilayers, have a slightly larger spacing.
Subject(s)
Biophysics/methods , Lipid Bilayers/chemistry , Phospholipids/chemistry , Silicon/chemistry , Biophysics/instrumentation , Electrons , Neutrons , Water/chemistry , X-Ray DiffractionABSTRACT
Using neutron diffraction and a specially constructed high pressure cell suitable for aligned multibilayer systems, we have studied, as a function of pressure, the much observed anomalous swelling regime in dimyristoyl- and dilauroyl-phosphatidylcholine bilayers, DMPC and DLPC, respectively. We have also reanalyzed data from a number of previously published experiments and have arrived at the following conclusions. (a). The power law behavior describing anomalous swelling is preserved in all PC bilayers up to a hydrostatic pressure of 240 MPa. (b). As a function of increasing pressure there is a concomitant decrease in the anomalous swelling of DMPC bilayers. (c). For PC lipids with hydrocarbon chains >or=13 carbons the theoretical unbinding transition temperature T small star, filled is coupled to the main gel-to-liquid crystalline transition temperature T(M). (d). DLPC is intrinsically different from the other lipids studied in that its T small star, filled is not coupled to T(M). (e). For DLPC bilayers we predict a hydrostatic pressure (>290 MPa) where unbinding may occur.
Subject(s)
Dimyristoylphosphatidylcholine/chemistry , Lipid Bilayers/chemistry , Membrane Fluidity , Neutron Diffraction/methods , Phosphatidylcholines/chemistry , Binding Sites , Gels/chemistry , Hydrostatic Pressure , Membranes, Artificial , Molecular Conformation , Phase Transition , Phospholipids/chemistry , Solutions , Transition TemperatureABSTRACT
Using small-angle neutron scattering and dynamic light scattering, we have constructed partial structural phase diagrams of lipid mixtures composed of the phosphatidylcholines dimyristoyl and dihexanoyl doped with calcium ions (Ca2+) and/or the negatively charged lipid, dimyristoyl phosphatidylglycerol (DMPG). For dilute solutions (lipid concentration < or =1 wt %), spontaneously forming unilamellar vesicles (ULVs) were found, and their polydispersity was determined to be approximately 20%. The stability of the Ca2+- or DMPG-doped ULVs was monitored over a period of 4 days and their structural parameters (e.g., average outer radius,
Subject(s)
Calcium/chemistry , Liposomes/chemistry , Micelles , Phosphatidylcholines/chemistry , Phosphatidylglycerols/chemistry , Neutron Diffraction/methodsABSTRACT
Thin stacks of lipid multibilayers supported on rigid silicon and mica substrates are found to exhibit finite-size effects. Using neutron diffraction we find that the repeat spacing (d) of stacks containing up to a few tens of bilayers depends on their thickness (D), with d increasing with decreasing D. Differences in d are larger in the low-temperature Lbeta' phase consisting of rigid bilayers than in the high-temperature Lalpha phase where the bilayers are more flexible. Various scenarios that may be responsible for this counterintuitive observation are discussed.
Subject(s)
Biomimetic Materials/chemistry , Dimyristoylphosphatidylcholine/chemistry , Lipid Bilayers/chemistry , Membrane Fluidity , Models, Chemical , Neutron Diffraction/methods , Computer Simulation , Macromolecular Substances/chemistry , Membranes, Artificial , Molecular Conformation , Phase Transition , Surface Properties , TemperatureABSTRACT
In this Letter we present small-angle neutron scattering data from a biomimetic system composed of the phospholipids dimyristoyl and dihexanoyl phosphorylcholine (DMPC and DHPC, respectively). Doping DMPC-DHPC multilamellar vesicles with either the negatively charged lipid dimyristoyl phosphorylglycerol (DMPG, net charge -1) or the divalent cation, calcium (Ca2+), leads to the spontaneous formation of energetically stabilized monodisperse unilamellar vesicles whose radii are concentration independent and in contrast with previous experimental observations.
Subject(s)
Biomimetic Materials/chemistry , Dimyristoylphosphatidylcholine/chemistry , Liposomes/chemistry , Phospholipid Ethers/chemistry , Calcium/chemistry , Cations, Divalent , Kinetics , ThermodynamicsABSTRACT
Using time-resolved small-angle neutron scattering, we have studied the kinetics of the recently observed bilayered-micelle (or so-called "bicelle") to perforated-lamellar transition in phospholipid mixtures. The data suggest that phase-ordering occurs via the early-time coalescence of bicelles into stacks of lamellae that then swell. Our measurements on this biomimetic system highlight the ubiquitous role of transient metastable states in the phase ordering of complex fluids.
Subject(s)
Biophysics/methods , Lipid Bilayers/chemistry , Micelles , Kinetics , Lipids/chemistry , Models, Chemical , Neutrons , Normal Distribution , Phospholipids/chemistry , Scattering, Radiation , Temperature , Time FactorsABSTRACT
Bilayered micelles, or bicelles, which consist of a mixture of long- and short-chain phospholipids, are a popular model membrane system. Depending on composition, concentration, and temperature, bicelle mixtures may adopt an isotropic phase or form an aligned phase in magnetic fields. Well-resolved (1)H NMR spectra are observed in the isotropic or so-called fast-tumbling bicelle phase, over the range of temperatures investigated (10-40 degrees C), for molar ratios of long-chain lipid to short-chain lipid between 0.20 and 1.0. Small angle neutron scattering data of this phase are consistent with the model in which bicelles were proposed to be disk-shaped. The experimentally determined dimensions are roughly consistent with the predictions of R.R. Vold and R.S. Prosser (J. Magn. Reson. B 113 (1996)). Differential paramagnetic shifts of head group resonances of dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholine (DHPC), induced by the addition of Eu(3+), are also consistent with the bicelle model in which DHPC is believed to be primarily sequestered to bicelle rims. Selective irradiation of the DHPC aliphatic methyl resonances results in no detectable magnetization transfer to the corresponding DMPC methyl resonances (and vice versa) in bicelles, which also suggests that DHPC and DMPC are largely sequestered in the bicelle. Finally, (1)H spectra of the antibacterial peptide indolicidin (ILPWKWPWWPWRR-NH(2)) are compared, in a DPC micellar phase and the above fast-tumbling bicellar phases for a variety of compositions. The spectra exhibit adequate resolution and improved dispersion of amide and aromatic resonances in certain bicelle mixtures.
