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Klaus Gawrisch

Publications and source records attributed to Klaus Gawrisch.

28 records · Page 2Linked to original sources
PubMed2004

Structure and fluctuations of charged phosphatidylserine bilayers in the absence of salt.

Using x-ray diffraction and NMR spectroscopy, we present structural and material properties of phosphatidylserine (PS) bilayers that may account for the well documented implications of PS headgroups in cell activity. At 30 degrees C, the 18-carbon monounsaturated DOPS in the fluid state has a cross-sectional area of 65.3 A(2) which is remarkably smaller than the area 72.5 A(2) of the DOPC analog, despite the extra electrostatic repulsion expected for charged PS headgroups. Similarly, at 20 degrees C, the 14-carbon disaturated DMPS in the gel phase has an area of 40.8 A(2) vs. 48.1 A(2) for DMPC. This condensation of area suggests an extra attractive interaction, perhaps hydrogen bonding, between PS headgroups. Unlike zwitterionic lipids, stacks of PS bilayers swell indefinitely as water is added. Data obtained for osmotic pressure versus interbilayer water spacing for fluid phase DOPS are well fit by electrostatic interactions calculated for the Gouy-Chapman regime. It is shown that the electrostatic interactions completely dominate the fluctuational pressure. Nevertheless, the x-ray data definitively exhibit the effects of fluctuations in fluid phase DOPS. From our measurements of fluctuations, we obtain the product of the bilayer bending modulus K(C) and the smectic compression modulus B. At the same interbilayer separation, the interbilayer fluctuations are smaller in DOPS than for DOPC, showing that B and/or K(C) are larger. Complementing the x-ray data, (31)P-chemical shift anisotropy measured by NMR suggest that the DOPS headgroups are less sensitive to osmotic pressure than DOPC headgroups, which is consistent with a larger K(C) in DOPS. Quadrupolar splittings for D(2)O decay less rapidly with increasing water content for DOPS than for DOPC, indicating greater perturbation of interlamellar water and suggesting a greater interlamellar hydration force in DOPS. Our comparisons between bilayers of PS and PC lipids with the same chains and the same temperature enable us to focus on the effects of these headgroups on bilayer properties.

Lipid Bilayers↗
PubMed2004

Domains in binary SOPC/POPE lipid mixtures studied by pulsed field gradient 1H MAS NMR.

We studied domain formation in mixtures of the monounsaturated lipids SOPC and POPE as a function of temperature and composition by NMR. Magic angle spinning at kHz frequencies restored resolution of (1)H NMR lipid resonances in the fluid phase, whereas the linewidth of gel-phase lipids remained rather broad and spinning frequency dependent. In regions of fluid- and gel-phase coexistence, spectra are a superposition of resonances from fluid and gel domains, as indicated by the existence of isosbestic points. Quantitative determination of the amount of lipid in the coexisting phases is straightforward and permitted construction of a binary phase diagram. Lateral rates of lipid diffusion were determined by (1)H MAS NMR with pulsed field gradients. At the onset of the phase transition near 25 degrees C apparent diffusion rates became diffusion time dependent, indicating that lipid movement is obstructed by the formation of gel-phase domains. A percolation threshold at which diffusion of fluid-phase lipid becomes confined to micrometer-size domains was observed when approximately 40% of total lipid had entered the gel phase. The results indicate that common phosphatidylethanolamines may trigger domain formation in membranes within a physiologically relevant temperature range. This novel NMR approach may aid the study of lipid rafts.

Biophysics↗
PubMed2003

Polyunsaturated docosahexaenoic vs docosapentaenoic acid-differences in lipid matrix properties from the loss of one double bond.

Insufficient supply to the developing brain of docosahexaenoic acid (22:6n3, DHA), or its omega-3 fatty acid precursors, results in replacement of DHA with docosapentaenoic acid (22:5n6, DPA), an omega-6 fatty acid that is lacking a double bond near the chain's methyl end. We investigated membranes of 1-stearoyl(d(35))-2-docosahexaenoyl-sn-glycero-3-phosphocholine and 1-stearoyl(d(35))-2-docosapentaenoyl-sn-glycero-3-phosphocholine by solid-state NMR, X-ray diffraction, and molecular dynamics simulations to determine if the loss of this double bond alters membrane physical properties. The low order parameters of polyunsaturated chains and the NMR relaxation data indicate that both DHA and DPA undergo rapid conformational transitions with correlation times of the order of nanoseconds at carbon atom C(2) and of picoseconds near the terminal methyl group. However, there are important differences between DHA- and DPA-containing lipids: the DHA chain with one additional double bond is more flexible at the methyl end and isomerizes with shorter correlation times. Furthermore, the stearic acid paired with the DHA in mixed-chain lipids has lower order, in particular in the middle of the chain near carbons C(10)(-)(12), indicating differences in the packing of hydrocarbon chains. Such differences are also reflected in the electron density profiles of the bilayers and in the simulation results. The DHA chain has a higher density near the lipid-water interface, whereas the density of the stearic acid chain is higher in the bilayer center. The loss of a single double bond from DHA to DPA results in a more even distribution of chain densities along the bilayer normal. We propose that the function of integral membrane proteins such as rhodopsin is sensitive to such a redistribution.

Computer Simulation↗
PubMed2003

Rhodopsin exhibits a preference for solvation by polyunsaturated docosohexaenoic acid.

An all-atom molecular dynamics simulation of rhodopsin in a membrane environment has been carried out with lipid composition similar to that of the retinal membrane. The initial conformation of the protein was taken from the X-ray crystallographic structure (1F88), while those of the lipids came from a previous molecular dynamics simulation. During the course of the 12.5 ns simulation, the initially randomly placed lipids adopt an anisotropic solvation structure around the protein. The lipids, having one saturated stearic acid chain and one polyunsaturated docosohexaenoic acid chain with a zwitterionic phosphatidylcholine headgroup, arrange themselves to maximize contact between the polyunsaturated chain and the protein surface. This organization is driven by energetically favorable interactions between the transmembrance helices and the docosohexaenoyl chains that are largely of the van der Waals type. These observations are consistent with various experimental studies on rhodopsin and other G-protein coupled receptors and with the picture of extreme flexibility in polyunsaturated fatty acid chains that has arisen from recent NMR and computational work.

Computer Simulation↗
PubMed2003

The structure of DHA in phospholipid membranes.

Early experiments and molecular simulations of PUFA favored a rigid arrangement of double bonds in U-shaped or extended conformations such as angle-iron or helical. Although results of recent solid-state NMR measurements and molecular simulations have confirmed the existence of these structural motifs, they portray an image of DHA (22:6n-3) as a highly flexible molecule with rapid transitions between large numbers of conformers on the time scale from picoseconds to hundreds of nanoseconds. The low barriers to torsional rotation about C-C bonds that link the cis-locked double bonds with the methylene carbons between them are responsible for this unusual flexibility. Both the amplitude and frequency of motion increase toward the terminal methyl group of DHA.

Docosahexaenoic Acids↗
PubMed2003

Lateral diffusion rates of lipid, water, and a hydrophobic drug in a multilamellar liposome.

The lateral diffusion constants of 1-palmitoyl-2-oleoyl-sn-glycero-3 phosphocholine (POPC), water, and ibuprofen were measured in multilamellar liposomes using pulsed field gradient magic-angle spinning (PFG-MAS) (1)H NMR. The analysis of diffusion data obtained in powder samples and a method for liposome curvature correction are presented. At 322 K POPC has a diffusion constant of (8.6 +/- 0.2) x 10(-12) m(2)/s when dehydrated (8.2 waters/lipid) and (1.9 +/- 0.1) x 10(-11) m(2)/s in excess water. The diffusion constant of water in dehydrated POPC was found to be (4.7 +/- 0.1) x 10(-10) m(2)/s. The radius of curvature is 21 +/- 2 microm for the dehydrated sample and 4.5 +/- 0.5 microm for POPC sample containing excess water. The activation energies of diffusion are 40.6 +/- 0.4 kJ/mole for dehydrated POPC, 30.7 +/- 0.9 kJ/mole for POPC with excess water, and 28.6 +/- 1.5 kJ/mole for water in dehydrated POPC. The diffusion constants and activation energies for a sample of POPC/ibuprofen/water (1:0.56:15) were also measured. The ibuprofen, which locates in the lipid-water interface, diffuses faster than POPC but has a slightly higher activation energy of lateral diffusion. Within certain restrictions, PFG-MAS NMR provides a useful method for characterizing membrane organization and mobility.

Anti-Inflammatory Agents, Non-Steroidal↗
PubMed2002

Polyunsaturated fatty acids in lipid bilayers: intrinsic and environmental contributions to their unique physical properties.

Polyunsaturated lipids are an essential component of biological membranes, influencing order and dynamics of lipids, protein-lipid interaction, and membrane transport properties. To gain an atomic level picture of the impact of polyunsaturation on membrane properties, quantum mechanical (QM) and empirical force field based calculations have been undertaken. The QM calculations of the torsional energy surface for rotation about vinyl-methylene bonds reveal low barriers to rotation, indicating an intrinsic propensity toward flexibility. Based on QM and experimental data, empirical force field parameters were developed for polyunsaturated lipids and applied in a 16 ns molecular dynamics (MD) simulation of a 1-stearoyl-2-docosahexaenoyl-sn-glyerco-3-phosphocholine (SDPC) lipid bilayer. The simulation results are in good agreement with experimental data, suggesting an unusually high degree of conformational flexibility of polyunsaturated hydrocarbon chains in membranes. The detailed analysis of chain conformation and dynamics by simulations is aiding the interpretation of experimental data and is useful for understanding the unique role of polyunsaturated lipids in biological membranes. The complete force field is included as Supporting Information and is available from http://www.pharmacy.umaryland.edu/faculty/amackere/research.html.

Computer Simulation↗
PubMed2002

Nuclear Overhauser enhancement spectroscopy cross-relaxation rates and ethanol distribution across membranes.

Measurement of nuclear Overhauser enhancement spectroscopy cross-relaxation rates between ethanol and palmitoyloleoylphosphatidylcholine bilayers was combined with atomic-level molecular dynamics simulations. The molecular dynamics trajectories yielded autocorrelation functions of proton dipole-dipole interactions, and, consequently, relaxation times and cross-relaxation rates. These analyses allow the measured cross-relaxation rates to be interpreted in terms of relative interaction strengths with the various segments of the lipid molecule. We determined that cross-relaxation between ethanol and specific lipid resonances is primarily determined by the sites of interaction with some modulation due to lipid disorder and to local differences in intramolecular lipid dynamics. The rates scale linearly with the lifetime of temporary ethanol-lipid associations. Ethanol interacts with palmitoyloleoylphosphatidylcholine bilayers primarily via hydrophilic interactions, in particular the formation of hydrogen bonds to the lipid phosphate group. There is a weak contribution to binding from hydrophobic interaction with lipid chain segments near the glycerol. However, the strength of hydrophobic interactions is insufficient to compensate for the energetic loss of locating ethanol in an exclusively hydrophobic environment, resulting in a probability of locating ethanol in the bilayer center that is three orders of magnitude lower than locating ethanol at the lipid/water interface. The low cross-relaxation rates between terminal methyl protons of hydrocarbon chains and ethanol are as much the result of infrequent chain upturns as of brief excursions of ethanol into the region of lipid hydrocarbon chains near the glycerol. The combination of nuclear magnetic resonance measurements and molecular dynamics simulations offers a general pathway to study the interaction of small molecules with the lipid matrix at atomic resolution.

Biophysics↗
PubMed2002

Novel NMR tools to study structure and dynamics of biomembranes.

Nuclear magnetic resonance (NMR) studies on biomembranes have benefited greatly from introduction of magic angle spinning (MAS) NMR techniques. Improvements in MAS probe technology, combined with the higher magnetic field strength of modern instruments, enables almost liquid-like resolution of lipid resonances. The cross-relaxation rates measured by nuclear Overhauser enhancement spectroscopy (NOESY) provide new insights into conformation and dynamics of lipids with atomic-scale resolution. The data reflect the tremendous motional disorder in the lipid matrix. Transfer of magnetization by spin diffusion along the proton network of lipids is of secondary relevance, even at a long NOESY mixing time of 300 ms. MAS experiments with re-coupling of anisotropic interactions, like the 13C-(1)H dipolar couplings, benefit from the excellent resolution of 13C shifts that enables assignment of the couplings to specific carbon atoms. The traditional 2H NMR experiments on deuterated lipids have higher sensitivity when conducted on oriented samples at higher magnetic field strength. A very large number of NMR parameters from lipid bilayers is now accessible, providing information about conformation and dynamics for every lipid segment. The NMR methods have the sensitivity and resolution to study lipid-protein interaction, lateral lipid organization, and the location of solvents and drugs in the lipid matrix.

Animals↗