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41.
Harroun TA Koslowsky M Nieh MP Raghunathan VA Katsaras J 《The European physical journal. E, Soft matter》2004,13(4):359-362
Finite-size effects in stacks of phospholipid bilayers, in the fluid
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.Received: 20 January 2004, Published online: 20 April 2004PACS:
68.08.Bc Wetting - 87.15.Ya Fluctuations - 87.16.Dg Membranes, bilayers, and vesicles - 87.64.Bx Electron, neutron, and X-ray diffraction and scattering 相似文献
42.
Using a streak camera we have measured the three Stokes polarization parameters during a polarization switch of a vertical-cavity semiconductor laser. The switch occurs along a corkscrew path on the Poincare sphere and takes on average a few nanoseconds; this value agrees with a theoretical treatment based upon the Fokker-Planck equation. 相似文献
43.
Mu-Ping Nieh V.A. Raghunathan Charles J. Glinka Thad Harroun John Katsaras 《Macromolecular Symposia》2005,219(1):135-146
The structures of bicelle mixtures composed of dimyristoyl and dihexanoyl phosphatidylcholines (DMPC and DHPC) with DMPC/DHPC molar ratios of 3.2 and 5 are characterized using polarized optical microscopy (POM) and small angle neutron scattering (SANS). Three phases, isotropic (I), chiral nematic (N*) and smectic (S) are observed as temperature (T) varies from 10 to 70 °C. The structure of the magnetically alignable N* phase, which was previously considered to be made up of discoidal micelles, is found to be composed of “ribbons”. Doping with the charged lipid, dimyristoyl phosphatidylglycerol (DMPG), which has the same 14:0 hydrocarbon chains as DMPC, results in a structural change of the aggregates where only the isotropic and smectic phases are observed. The smectic phase for the mixtures doped with DMPG is shear-alignable and follows one-dimensional swelling. However, at high-T zwitterionic DMPC/DHPC mixtures form multi-lamellar vesicles (MLV) with a relatively constant lamellar spacing of 66 Å, independent of water content. 相似文献
44.
Nieh MP Dolinar P Kučerka N Kline SR Debeer-Schmitt LM Littrell KC Katsaras J 《Langmuir : the ACS journal of surfaces and colloids》2011,27(23):14308-14316
Zwitterionic long-chain lipids (e.g., dimyristoyl phosphatidylcholine, DMPC) spontaneously form onion-like, thermodynamically stable structures in aqueous solutions (commonly known as multilamellar vesicles, or MLVs). It has also been reported that the addition of zwitterionic short-chain (i.e., dihexanoyl phosphatidylcholine, DHPC) and charged long-chain (i.e., dimyristoyl phosphatidylglycerol, DMPG) lipids to zwitterionic long-chain lipid solutions results in the formation of unilamellar vesicles (ULVs). Here, we report a kinetic study on lipid mixtures composed of DMPC, DHPC, and DMPG. Two membrane charge densities (i.e., [DMPG]/[DMPC] = 0.01 and 0.001) and two solution salinities (i.e., [NaCl] = 0 and 0.2 M) are investigated. Upon dilution of the high-concentration samples at 50 °C, thermodynamically stable MLVs are formed, in the case of both weakly charged and high salinity solution mixtures, implying that the electrostatic interactions between bilayers are insufficient to cause MLVs to unbind. Importantly, in the case of these samples small angle neutron scattering (SANS) data show that, initially, nanodiscs (also known as bicelles) or bilayered ribbons form at low temperatures (i.e., 10 °C), but transform into uniform size, nanoscopic ULVs after incubation at 10 °C for 20 h, indicating that the nanodisc is a metastable structure. The instability of nanodiscs may be attributed to low membrane rigidity due to a reduced charge density and high salinity. Moreover, the uniform-sized ULVs persist even after being heated to 50 °C, where thermodynamically stable MLVs are observed. This result clearly demonstrates that these ULVs are kinetically trapped, and that the mechanical properties (e.g., bending rigidity) of 10 °C nanodiscs favor the formation of nanoscopic ULVs over that of MLVs. From a practical point of view, this method of forming uniform-sized ULVs may lend itself to their mass production, thus making them economically feasible for medical applications that depend on monodisperse lipid-based systems for therapeutic and diagnostic purposes. 相似文献
45.
Abraham T Schooling SR Nieh MP Kucerka N Beveridge TJ Katsaras J 《The journal of physical chemistry. B》2007,111(10):2477-2483
Lipopolysaccharides (LPSs) are a major class of macromolecules populating the surface of Gram-negative bacteria. They contribute significantly to the bacterium's surface properties and play a crucial role in regulating the permeability of its outer membrane. Here, we report on neutron diffraction studies performed on aligned, self-assembled bilayers of LPS isolated from Pseudomonas aeruginosa PAO1. This LPS system is comprised of a mixture of rough and smooth A-band and B-band LPS, similar to that naturally found in P. aeruginosa. Temperature scans were conducted at various levels of hydration, and the phases adopted by LPS, along with their corresponding transition temperatures, have been identified. Because of LPS's chemical heterogeneity, the gel-to-liquid-crystalline transition was continuous and not abrupt as commonly observed in single-component phospholipid systems. From the construction of one-dimensional scattering length density profiles, we find that water penetrates into the hydrocarbon region up to and including the center of liquid-crystalline LPS bilayers. This permeability to water also extends to bilayers in the continuous phase transition region and could have far-reaching implications as to how small molecules penetrate the outer membrane of Gram-negative bacteria. 相似文献
46.
47.
We study the characteristics of the μe events due to e+e? → W+W? and the subsequent leptonic decay of W±, in the Weinberg-Salam model. The differential cross section for W production is given for non-polarized electron-positron beams as well as transversely polarized beams. The energy angle distributions as well as angular correlations of the final μe events are presented for beam energy of 100 + 100 GeV. Potential backgrounds are also discussed. 相似文献
48.
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50.
Nieh MP Kumar SK Fernando RH Colby RH Katsaras J 《Langmuir : the ACS journal of surfaces and colloids》2004,20(21):9061-9068
Aggregate structures of aqueous nonionic Gemini surfactant solutions, alpha,alpha'-[2,4,7,9-tetramethyl-5-decyne-4,7-diyl]bis[omega-hydroxyl-polyoxyethylene] with three different length polyoxyethylenes (i.e., 10, 20, and 30 ethylene oxide monomers, denoted from now on as S-10, S-20, and S-30, respectively), are investigated using small angle neutron scattering, dynamic light scattering, and fluorescence spectroscopy. For S-10 at low surfactant concentrations (Cs < 0.9 wt %), large "clusters", with an average hydrodynamic radius (RH) > 40 nm, are found to coexist with monomers. At intermediate Cs (0.9 < Cs < 2 wt %), some clusters break down forming micelles, with an (RH) approximately 2-3 nm, while the remaining clusters coexist with micelles. Increasing Cs further (>2 wt %) results in a pure micellar phase with little or no clusters present. S-20 and S-30 mixtures, on the other hand, differ from S-10 in that irrespective of surfactant concentration, large clusters and small monomers/dimers are found to coexist, while there is no direct evidence for the presence of micelles. 相似文献