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• Articles by R. Mendelsohn
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• Effect of Chain Length and Unsaturation on Elasticity of Lip...

  Effect of Chain Length and Unsaturation on Elasticity of Lipid Bilayers Biophysical Journal, Volume 79, Issue 1, 1 July 2000, Pages 328-339 W. Rawicz, K.C. Olbrich, T. McIntosh, D. Needham and E. Evans Abstract Micropipette pressurization of giant bilayer vesicles was used to measure both elastic bending and area stretch moduli of fluid-phase phosphatidylcholine (PC) membranes. Twelve diacyl PCs were chosen: eight with two 18 carbon chains and degrees of unsaturation from one double bond (C18:1/0, C18:0/1) to six double bonds per lipid (diC18:3), two with short saturated carbon chains (diC13:0, diC14:0), and two with long unsaturated carbon chains (diC20:4, diC22:1). Bending moduli were derived from measurements of apparent expansion in vesicle surface area under very low tensions (0.001–0.5mN/m), which is dominated by smoothing of thermal bending undulations. Area stretch moduli were obtained from measurements of vesicle surface expansion under high tensions (>0.5mN/m), which involve an increase in area per molecule and a small—but important—contribution from smoothing of residual thermal undulations. The direct stretch moduli varied little (< ±10%) with either chain unsaturation or length about a mean of 243mN/m. On the other hand, the bending moduli of saturated/monounsaturated chain PCs increased progressively with chain length from 0.56×10 J for diC13:0 to 1.2×10 J for diC22:1. However, quite unexpectedly for longer chains, the bending moduli dropped precipitously to ∼0.4×10 J when two or more double bonds were present in a chain (C18:0/2, diC18:2, diC18:3, diC20:4). Given nearly constant area stretch moduli, the variations in bending rigidity with chain length and polyunsaturation implied significant variations in thickness. To test this hypothesis, peak-to-peak headgroup thicknesses of bilayers were obtained from x-ray diffraction of multibilayer arrays at controlled relative humidities. For saturated/monounsaturated chain bilayers, the distances increased smoothly from diC13:0 to diC22:1 as expected. Moreover, the distances and elastic properties correlated well with a polymer brush model of the bilayer that specifies that the elastic ratio (/)=(−)/24, where ≈ 1nm accounts for separation of the headgroup peaks from the deformable hydrocarbon region. However, the elastic ratios and thicknesses for diC18:2, diC18:3, and diC20:4 fell into a distinct group below the correlation, which showed that poly- unsaturated chain bilayers are thinner and more flexible than saturated/monounsaturated chain bilayers. Abstract | Full Text | PDF (237 kb)

• Infrared Spectroscopic Study of Stratum Corneum Model Membra...

  Infrared Spectroscopic Study of Stratum Corneum Model Membranes Prepared from Human Ceramides, Cholesterol, and Fatty Acids Biophysical Journal, Volume 92, Issue 8, 15 April 2007, Pages 2785-2795 G.S. Gooris and J.A. Bouwstra Abstract The outermost layer of the skin, the stratum corneum, consists of corneocytes surrounded by lipid domains. The main lipid classes in stratum corneum are cholesterol, ceramides (CER), and free fatty acids forming two crystalline lamellar phases. However, only limited information is available on whether the various lipid classes participate in the same crystalline lattices or if separate domains are formed within the lipid lamellae. In this article infrared spectroscopic studies are reported of hydrated mixtures prepared from cholesterol, human CER, and free fatty acids. Evaluation of the methylene stretching vibrations revealed a conformational disordering starting at ∼60°C for all mixtures. Examination of the rotational ordering (scissoring and rocking vibrations) of mixtures prepared from equimolar cholesterol and CER with a variation in the level of free fatty acids showed that at lower free fatty acid content orthorhombic and hexagonal domains coexist in the lipid lamellae. Increasing the fatty acid level to an equimolar cholesterol/CER/fatty acid mixture reveals the dominant presence of an orthorhombic lattice, confirming x-ray diffraction studies. Replacing the protonated free fatty acid chains by their perdeuterated counterparts demonstrates that free fatty acids and CER participate in the same orthorhombic lattice up to a level of slightly less than 1:1:0.75 cholesterol/CER/free fatty acids molar ratio but that free fatty acids also form separate domains within the lipid lamellae at equimolar ratios at room temperature. However, no evidence for this has been observed at 32°C. Extrapolating these findings to the situation in stratum corneum led us conclude that in stratum corneum, fatty acids and CER participate in the orthorhombic lattice at 32°C, the skin temperature. Abstract | Full Text | PDF (257 kb)

• Liquid Domains in Vesicles Investigated by NMR and Fluoresce...

  Liquid Domains in Vesicles Investigated by NMR and Fluorescence Microscopy Biophysical Journal, Volume 86, Issue 5, 1 May 2004, Pages 2910-2922 S.L. Veatch, I.V. Polozov, K. Gawrisch and S.L. Keller Abstract We use H-NMR, H-MAS NMR, and fluorescence microscopy to detect immiscibility in three particular phospholipid ratios mixed with 30% cholesterol: 2:1 DOPC/DPPC, 1:1 DOPC/DPPC, and 1:2 DOPC/DPPC. Large-scale (≫160nm) phase separation into liquid-ordered () and liquid-crystalline () phases is observed by both NMR and fluorescence microscopy. By fitting superimposed H-NMR spectra, we quantitatively determine that the phase is strongly enriched in DPPC and moderately enriched in cholesterol. Tie-lines estimated at different temperatures and membrane compositions are based on both H-NMR observations and a previously published ternary phase diagram. H- and H-MAS NMR techniques probe significantly smaller length scales than microscopy experiments (submicron versus micron-scalp), and complex behavior is observed near the miscibility transition. Fluorescence microscopy of giant unilamellar vesicles shows micrometer-scale domains below the miscibility transition. In contrast, NMR of multilamellar vesicles gives evidence for smaller (∼80nm) domains just below the miscibility transition, whereas large-scale demixing occurs at a lower temperature, . A transition at is also evident in fluorescence microscopy measurements of the surface area fraction of ordered phase in giant unilamellar vesicles. Our results reemphasize the complex phase behavior of cholesterol-containing membranes and provide a framework for interpreting H-NMR experiments in similar membranes. Abstract | Full Text | PDF (432 kb)

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Abstract

We report on the gel-state microaggregation in binary mixtures of diacylphosphatidylcholines over temperatures ranging from -19 degrees C to near the gel-to-liquid crystal transition. Microaggregates with lateral dimensions in the range 1–100 chains were detected and measured with an isotope infrared method that relates the splitting or the shape of the methylene scissors band to aggregate size. Measurements were made on fully hydrated dispersions of diC18DPC/diC20HPC, diC18DPC/diC22HPC, and diC18DPC/diC24HPC at molar ratios of 4:1. Low levels of aggregation were determined with reference to the spectrum of the random mixture diC18DPC/diC18HPC. For diC18DPC/diC20HPC at -19 degrees C, which previous calorimetric measurements have indicated is a nearly ideal, we found about 4% of the minority component chains to be involved in aggregates. For diC18DPC/diC22HPC, the value increased to about 11%. DiC18DPC/diC24HPC was found to be highly fractionated, in agreement with the earlier studies. The unit subcell, which defines the type of acyl-chain packing, was determined for the components of the mixtures. The temperature behavior of the phases and the temperatures at which the minority component domains undergo dissolution were determined.