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- Modulation of Concentration Fluctuations in Phase-Separated ...Modulation of Concentration Fluctuations in Phase-Separated Lipid Membranes by Polypeptide Insertion
Biophysical Journal, Volume 83, Issue 1, 1 July 2002, Pages 334-344
S. Fahsel, E.-M. Pospiech, M. Zein, T.L. Hazlet, E. Gratton and Roland Winter
AbstractThe lateral membrane organization and phase behavior of the binary lipid mixture DMPC (1,2-dimyristoyl--glycero-3-phosphatidylcholine) - DSPC (1,2-distearoyl--glycero-3-phosphatidylcholine) without and with incorporated gramicidin D (GD) as a model biomembrane polypeptide was studied by small-angle neutron scattering, Fourier-transform infrared spectroscopy, and by two-photon excitation fluorescence microscopy on giant unilamellar vesicles. The small-angle neutron scattering method allows the detection of concentration fluctuations in the range from 1 to 200nm. Fluorescence microscopy was used for direct visualization of the lateral lipid organization and domain shapes on a micrometer length scale including information of the lipid phase state. In the fluid-gel coexistence region of the pure binary lipid system, large-scale concentration fluctuations appear. Infrared spectral parameters were used to determine the peptide conformation adopted in the different lipid phases. The data show that the structure of the temperature-dependent lipid phases is significantly altered by the insertion of 2 to 5 mol% GD. At temperatures corresponding to the gel-fluid phase coexistence region the concentration fluctuations drastically decrease, and we observe domains in the giant unilamellar vesicles, which mainly disappear by the incorporation of 2 to 5 mol% GD. Further, the lipid matrix has the ability to modulate the conformation of the inserted polypeptide. The balance between double-helical and helical dimer structures of GD depends on the phospholipid chain length and phase state. A large hydrophobic mismatch, such as in gel phase one-component DSPC bilayers, leads to an increase in population of double-helical structures. Using an effective molecular sorting mechanism, a large hydrophobic mismatch can be avoided in the DMPC-DSPC lipid mixture, which leads to significant changes in the heterogeneous lipid structure and in polypeptide conformation.
Abstract | Full Text | PDF (502 kb) - Temperature Dependence of the Surface Topography in Dimyrist...Temperature Dependence of the Surface Topography in Dimyristoylphosphatidylcholine/Distearoylphosphatidylcholine Multibilayers
Biophysical Journal, Volume 86, Issue 4, 1 April 2004, Pages 2218-2230
Marie-Cécile Giocondi and Christian Le Grimellec
AbstractSimple lipid binary systems are intensively used to understand the formation of domains in biological membranes. The size of individual domains present in the gel/fluid coexistence region of single supported bilayers, determined by atomic force microscopy (AFM), generally exceeds by two to three orders of magnitude that estimated from multibilayers membranes by indirect spectroscopic methods. In this article, the topography of equimolar dimyristoylphosphatidylcholine/distearoylphosphatidylcholine (DMPC/DSPC) multibilayers, made of two superimposed bilayers supported on mica surface, was studied by AFM in a temperature range from room temperature to 45°C. In the gel/fluid phase coexistence region the size of domains, between ∼100nm and a few micrometers, was of the same order for the first bilayer facing the mica and the top bilayer facing the buffer. The gel to fluid phase separation temperature of the first bilayer, however, could be increased by up to 8°C, most likely as a function of the buffer layer thickness that separated it from the mica. Topography of the top bilayer revealed the presence of lipids in ripple phase up to 38–40°C. Above this temperature, a pattern characteristic of the coexistence of fluid and gel domains was observed. These data show that difference in the size of lipid domains given by AFM and spectroscopy can hardly be attributed to the use of multibilayers models in spectroscopy experiments. They also provide a direct evidence for metastable ripple phase transformation into a gel/fluid phase separated structure upon heating.
Abstract | Full Text | PDF (1554 kb) - Geometrical Properties of Gel and Fluid Clusters in DMPC/DSP...Geometrical Properties of Gel and Fluid Clusters in DMPC/DSPC Bilayers: Monte Carlo Simulation Approach Using a Two-State Model
Biophysical Journal, Volume 81, Issue 5, 1 November 2001, Pages 2425-2441
István P. Sugár, Ekaterina Michonova-Alexova and Parkson Lee-Gau Chong
AbstractIn this paper the geometrical properties of gel and fluid clusters of equimolar dimyristoylphosphatidylcholine/distearoylphosphatidylcholine (DMPC/DSPC) lipid bilayers are calculated by using an Ising-type model (Sugar, I. P., T. E. Thompson, and R. L. Biltonen. 1999. 76:2099–2110). The model is able to predict the following properties in agreement with the respective experimental data: the excess heat capacity curves, fluorescence recovery after photobleaching (FRAP) threshold temperatures at different mixing ratios, the most frequent center-to-center distance between DSPC clusters, and the fractal dimension of gel clusters. In agreement with the neutron diffraction and fluorescence microscopy data, the simulations show that below the percolation threshold temperature of gel clusters many nanometer-size gel clusters co-exist with one large gel cluster of size comparable with the membrane surface area. With increasing temperature the calculated effective fractal dimension and capacity dimension of gel and fluid clusters decrease and increase, respectively, within the (0, 2) interval. In the region of the gel-to-fluid transition the following geometrical properties are independent from the temperature and the state of the cluster: 1) the cluster perimeter linearly increases with the number of cluster arms at a rate of 8.2 nm/arm; 2) the average number of inner islands in a cluster increases with increasing cluster size, , according to a power function of 000427×; 3) the following exponential function describes the average size of an inner island versus the size of the host cluster, 1+109(1×). By means of the equations describing the average geometry of the clusters the process of the association of clusters is investigated.
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Copyright © 1998 The Biophysical Society. All rights reserved.
Biophysical Journal, Volume 74, Issue 5, 2443-2450, 1 May 1998
doi:10.1016/S0006-3495(98)77952-4
Direct Detection of Domains in Phospholipid Bilayers by Grazing Incidence Diffraction of Neutrons and Atomic Force Microscopy
C. Gliss*, H. Clausen-Schaumann#, R. Günther§, ¶, S. Odenbach§, O. Randl¶ and T.M. Bayerl*, 
, 
* Institut für Physik EP-5, Universität Würzburg, D-97074 Würzburg, Germany
# Lehrstuhl für Angewandte Physik, LMU-München, D-8799 Germany
§ Institut für Materialwissenschaften, Bergische Universität GH Wuppertal, D-42285 Wuppertal, Germany
¶ Institut Laue-Langevin, B.P. 156 X, F-38042 Grenoble Cedex, France
Address reprint requests to Professor Thomas M. Bayerl, Universität Würzburg, Physikalisches Institut EP-5, D-97074 Würzburg, Germany. Tel. 49-931-888-5863; Fax: 49-931-888-5851.Abstract
The geometry of domains in phospholipid bilayers of binary (1:1) mixtures of synthetic lecithins with a difference in chain length of four methylene groups has been studied by two independent, direct and complementary methods. Grazing incidence diffraction of neutrons provided gel domain sizes of less than 10nm in both the gel and the coexistence phase of the mixture, while no domains were detected for the fluid phase. For the coexistence region, the neutron data suggest that domains grow in number rather than in size with decreasing temperature. Atomic force microscopy was used to study gel phase size and shape of the domains. The domains imaged by atomic force microscopy exhibit a rather irregular shape with an average size of 10nm, thus confirming the neutron results for this phase. The good agreement between atomic force microscopy and neutron results, despite the completely different nature of their observables, has potential for the future development of refined models for the interpretation of neutron data from heterogeneous membranes in terms of regularly spaced and spatially extended scatterers.
