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Lohner K, Sevcsik E, Pabst G. Chapter Five Liposome-Based Biomembrane Mimetic Systems: Implications for Lipid–Peptide Interactions. ADVANCES IN PLANAR LIPID BILAYERS AND LIPOSOMES 2008. [DOI: 10.1016/s1554-4516(07)06005-x] [Citation(s) in RCA: 48] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/04/2022]
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53
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54
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Partenskii MB, Miloshevsky GV, Jordan PC. The Theoretical Challenge Posed by Low-Voltage Membrane Electroporation, Viewed from the Perspective of Continuum and Molecular-Level Models. Isr J Chem 2007. [DOI: 10.1560/ijc.47.3-4.385] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
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55
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Hub JS, Salditt T, Rheinstädter MC, de Groot BL. Short-range order and collective dynamics of DMPC bilayers: a comparison between molecular dynamics simulations, X-ray, and neutron scattering experiments. Biophys J 2007; 93:3156-68. [PMID: 17631531 PMCID: PMC2025676 DOI: 10.1529/biophysj.107.104885] [Citation(s) in RCA: 70] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
We present an extensive comparison of short-range order and short wavelength dynamics of a hydrated phospholipid bilayer derived by molecular dynamics simulations, elastic x-ray, and inelastic neutron scattering experiments. The quantities that are compared between simulation and experiment include static and dynamic structure factors, reciprocal space mappings, and electron density profiles. We show that the simultaneous use of molecular dynamics and diffraction data can help to extract real space properties like the area per lipid and the lipid chain ordering from experimental data. In addition, we assert that the interchain distance can be computed to high accuracy from the interchain correlation peak of the structure factor. Moreover, it is found that the position of the interchain correlation peak is not affected by the area per lipid, while its correlation length decreases linearly with the area per lipid. This finding allows us to relate a property of the structure factor quantitatively to the area per lipid. Finally, the short wavelength dynamics obtained from the simulations and from inelastic neutron scattering are analyzed and compared. The conventional interpretation in terms of the three-effective-eigenmode model is found to be only partly suitable to describe the complex fluid dynamics of lipid chains.
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Affiliation(s)
- Jochen S Hub
- Computational Biomolecular Dynamics Group, Max-Planck-Institute for Biophysical Chemistry, Göttingen, Germany
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56
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Rheinstädter MC, Seydel T, Salditt T. Nanosecond molecular relaxations in lipid bilayers studied by high energy-resolution neutron scattering and in situ diffraction. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2007; 75:011907. [PMID: 17358184 DOI: 10.1103/physreve.75.011907] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/10/2006] [Indexed: 05/14/2023]
Abstract
We report a high energy-resolution neutron backscattering study to investigate slow motions on nanosecond time scales in highly oriented solid-supported phospholipid bilayers of the model system deuterated 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine, hydrated with heavy water. Wave-vector-resolved quasielastic neutron scattering is used to determine relaxation times tau , which can be associated with different molecular components, i.e., the lipid acyl chains and the interstitial water molecules in the different phases of the model membrane system. The inelastic data are complemented by both energy-resolved and energy-integrated in situ diffraction. From a combined analysis of the inelastic data in the energy and time domains, the corresponding character of the relaxation, i.e., the exponent of the exponential decay, is also determined. From this analysis we quantify two relaxation processes. We associate the fast relaxation with translational diffusion of lipid and water molecules while the slow process likely stems from collective dynamics.
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Affiliation(s)
- Maikel C Rheinstädter
- Institut Laue-Langevin, 6 rue Jules Horowitz, Boîte Postale 156, 38042 Grenoble Cedex 9, France.
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57
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Salditt T, Li C, Spaar A. Structure of antimicrobial peptides and lipid membranes probed by interface-sensitive X-ray scattering. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2006; 1758:1483-98. [DOI: 10.1016/j.bbamem.2006.08.002] [Citation(s) in RCA: 57] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/03/2006] [Revised: 08/04/2006] [Accepted: 08/04/2006] [Indexed: 10/24/2022]
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58
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de Candia A, Del Gado E, Fierro A, Sator N, Tarzia M, Coniglio A. Columnar and lamellar phases in attractive colloidal systems. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2006; 74:010403. [PMID: 16907047 DOI: 10.1103/physreve.74.010403] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/13/2006] [Revised: 06/06/2006] [Indexed: 05/11/2023]
Abstract
In colloidal suspensions, at low volume fraction and temperature, dynamical arrest occurs via the growth of elongated structures that aggregate to form a connected network at gelation. Here we show that, in the region of parameter space where gelation occurs, the stable thermodynamical phase is a crystalline columnar one. Near and above the gelation threshold, the disordered spanning network slowly evolves and finally orders to form the crystalline structure. At higher volume fractions the stable phase is a lamellar one, which seems to have a still longer ordering time.
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Affiliation(s)
- A de Candia
- Dipartimento di Scienze Fisiche, Università di Napoli Federico II, Complesso Universitario di Monte Sant'Angelo, 80126 Napoli, Italy
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59
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Liu Y, Chen SH, Berti D, Baglioni P, Alatas A, Sinn H, Alp E, Said A. Effects of counterion valency on the damping of phonons propagating along the axial direction of liquid-crystalline DNA. J Chem Phys 2005; 123:214909. [PMID: 16356072 DOI: 10.1063/1.2128702] [Citation(s) in RCA: 13] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023] Open
Abstract
The phonon propagation and damping along the axial direction of films of aligned 40 wt % calf-thymus DNA rods are studied by inelastic x-ray scattering (IXS). The IXS spectra are analyzed with the generalized three effective eigenmode theory, from which we extract the dynamic structure factor S(Q,E) as a function of transferred energy E=variant Planck's over 2piomega, and the magnitude of the transferred wave vector Q. S(Q,E) of a DNA sample typically consists of three peaks, one central Rayleigh scattering peak, and two symmetric Stokes and anti-Stokes Brillouin side peaks. By analyzing the Brillouin peaks, the phonon excitation energy and damping can be extracted at different Q values from about 4 to 30 nm(-1). A high-frequency sound speed is obtained from the initial slope of the linear portion of the dispersion relation below Q=4 nm(-1). The high-frequency sound speed obtained in this Q range is 3100 ms, which is about twice faster than the ultrasound speed of 1800 ms, measured by Brillouin light scattering at Q approximately 0.01 nm(-1) at the similar hydration level. Our observations provide further evidence of the strong coupling between the internal dynamics of a DNA molecule and the dynamics of the solvent. The effect on damping and propagation of phonons along the axial direction of DNA rods due to divalent and trivalent counterions has been studied. It is found that the added multivalent counterions introduce stronger phonon damping. The phonons at the range between approximately 12.5 and approximately 22.5 nm(-1) are overdamped by the added counterions according to our model analyses. The intermediate scattering function is extracted and it shows a clear two-step relaxation with the fast relaxation time ranging from 0.1 to 4 ps.
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Affiliation(s)
- Yun Liu
- Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
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60
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Rheinstädter MC, Seydel T, Demmel F, Salditt T. Molecular motions in lipid bilayers studied by the neutron backscattering technique. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2005; 71:061908. [PMID: 16089766 DOI: 10.1103/physreve.71.061908] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 01/31/2005] [Indexed: 05/03/2023]
Abstract
We report a high energy-resolution neutron backscattering study to investigate slow motions on nanosecond time scales in highly oriented solid supported phospholipid bilayers of the model system DMPC-d54 (deuterated 1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine), hydrated with heavy water. This technique allows to discriminate the onset of mobility at different length scales for the different molecular components, as, e.g., the lipid acyl-chains and the hydration water in between the membrane stacks, respectively, and provides a benchmark test regarding the feasibility of neutron backscattering investigations on these sample systems. We discuss freezing of the lipid acyl-chains, as observed by this technique, and observe a second freezing transition which we attribute to the hydration water.
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Affiliation(s)
- Maikel C Rheinstädter
- Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France.
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61
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Rheinstädter MC, Ollinger C, Fragneto G, Demmel F, Salditt T. Collective dynamics of lipid membranes studied by inelastic neutron scattering. PHYSICAL REVIEW LETTERS 2004; 93:108107. [PMID: 15447459 DOI: 10.1103/physrevlett.93.108107] [Citation(s) in RCA: 67] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/16/2003] [Indexed: 05/24/2023]
Abstract
We have studied the collective short wavelength dynamics in deuterated 1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine (DMPC) bilayers by inelastic neutron scattering. The corresponding dispersion relation variant Planck's over 2pi omega(Q) is presented for the gel and the fluid phase of this model system. The temperature dependence of the inelastic excitations indicates a phase coexistence between the two phases over a broad range and leads to a different assignment of excitations from that reported in a preceding inelastic x-ray scattering study [Phys. Rev. Lett. 86, 740 (2001)]]. As a consequence, we find that the minimum in the dispersion relation is actually deeper in the gel than in the fluid phase. Finally, we can clearly identify an additional nondispersive (optical) mode predicted by molecular dynamics simulations [Phys. Rev. Lett. 87, 238101 (2001)]].
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Affiliation(s)
- M C Rheinstädter
- Institut Laue-Langevin, 6 rue Jules Horowitz, BP 156, 38042 Grenoble Cedex 9, France.
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62
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Tommaseo G, Penciu RS, Fytas G, Economou EN, Hashimoto T, Hadjichristidis N. Phonon Propagation in Ordered Diblock Copolymer Solutions. Macromolecules 2004. [DOI: 10.1021/ma0496246] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Affiliation(s)
- G. Tommaseo
- Max Planck Institute of Polymer Research, P.O. Box 3148, 55128 Mainz, Germany, FORTH/Institute of Electronic Structure and Laser, P.O. Box 1527, 71110 Heraklion, Greece, Department of Polymer Chemistry Graduate School of Engineering, Kyoto University, Kyoto 6158510 Japan, and Department of Chemistry, University of Athens, 15701 Zografou, Athens, Greece
| | - R. S. Penciu
- Max Planck Institute of Polymer Research, P.O. Box 3148, 55128 Mainz, Germany, FORTH/Institute of Electronic Structure and Laser, P.O. Box 1527, 71110 Heraklion, Greece, Department of Polymer Chemistry Graduate School of Engineering, Kyoto University, Kyoto 6158510 Japan, and Department of Chemistry, University of Athens, 15701 Zografou, Athens, Greece
| | - G. Fytas
- Max Planck Institute of Polymer Research, P.O. Box 3148, 55128 Mainz, Germany, FORTH/Institute of Electronic Structure and Laser, P.O. Box 1527, 71110 Heraklion, Greece, Department of Polymer Chemistry Graduate School of Engineering, Kyoto University, Kyoto 6158510 Japan, and Department of Chemistry, University of Athens, 15701 Zografou, Athens, Greece
| | - E. N. Economou
- Max Planck Institute of Polymer Research, P.O. Box 3148, 55128 Mainz, Germany, FORTH/Institute of Electronic Structure and Laser, P.O. Box 1527, 71110 Heraklion, Greece, Department of Polymer Chemistry Graduate School of Engineering, Kyoto University, Kyoto 6158510 Japan, and Department of Chemistry, University of Athens, 15701 Zografou, Athens, Greece
| | - T. Hashimoto
- Max Planck Institute of Polymer Research, P.O. Box 3148, 55128 Mainz, Germany, FORTH/Institute of Electronic Structure and Laser, P.O. Box 1527, 71110 Heraklion, Greece, Department of Polymer Chemistry Graduate School of Engineering, Kyoto University, Kyoto 6158510 Japan, and Department of Chemistry, University of Athens, 15701 Zografou, Athens, Greece
| | - N. Hadjichristidis
- Max Planck Institute of Polymer Research, P.O. Box 3148, 55128 Mainz, Germany, FORTH/Institute of Electronic Structure and Laser, P.O. Box 1527, 71110 Heraklion, Greece, Department of Polymer Chemistry Graduate School of Engineering, Kyoto University, Kyoto 6158510 Japan, and Department of Chemistry, University of Athens, 15701 Zografou, Athens, Greece
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63
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Liu Y, Berti D, Faraone A, Chen WR, Alatas A, Sinn H, Alp E, Said A, Baglioni P, Chen SH. Inelastic X-ray scattering studies of phonons in liquid crystalline DNA. Phys Chem Chem Phys 2004. [DOI: 10.1039/b314462n] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/21/2022]
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64
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Chen PJ, Liu Y, Weiss TM, Huang HW, Sinn H, Alp EE, Alatas A, Said A, Chen SH. Studies of short-wavelength collective molecular motions in lipid bilayers using high resolution inelastic X-ray scattering. Biophys Chem 2003; 105:721-41. [PMID: 14499929 DOI: 10.1016/s0301-4622(03)00099-1] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
We summarize a series of experimental results made with the newly developed high resolution X-ray scattering (IXS) instrument on two pure lipid bilayers, including dimyristoylphosphatidylcholine (DMPC) and dilauroylphosphatidylcholine (DLPC) in both gel and liquid crystal phases, and lipid bilayers containing cholesterol. By analyzing the IXS data based on the generalized three effective eigenmode model (GTEE), we obtain dispersion relations of the high frequency density oscillations (phonons) of lipid molecules in these bilayers. We then compare the dispersion relations of pure lipid bilayers of different chain lengths among themselves and the dispersion relations of pure lipid bilayers with those of the cholesterol containing bilayers. We also compare our experimental results with collective dynamics data generated by computer molecular dynamics (MD) simulations for dipalmitoylphosphatidylcholine (DPPC) in gel phase and DMPC in liquid crystal phase.
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Affiliation(s)
- Poe-Jou Chen
- Department of Nuclear Engineering, Massachusetts Institute of Technology, 24-209, MIT, Cambridge, Massachusetts 02139, USA
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65
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Salditt T. Lipid-peptide interaction in oriented bilayers probed by interface-sensitive scattering methods. Curr Opin Struct Biol 2003; 13:467-78. [PMID: 12948776 DOI: 10.1016/s0959-440x(03)00113-1] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/19/2022]
Abstract
Oriented lipid membranes deposited on solid substrates offer unique experimental opportunities to study lipid bilayer structure and lipid-peptide interaction in suitable model systems. In particular, modern interface-sensitive X-ray and neutron scattering methods can be used to probe the short-range order and molecular conformations of peptides and lipids in the fluid state of the bilayer.
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Affiliation(s)
- Tim Salditt
- Institüt für Röntgenphysik, Universität Göttingen, Geiststrasse 11, D-37037 Göttingen, Germany.
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66
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Mermet A, David L, Lorenzen M, Krisch M. Inelastic x-ray scattering from stretch-oriented polyethylene. J Chem Phys 2003. [DOI: 10.1063/1.1579681] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022] Open
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67
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Weiss TM, Chen PJ, Sinn H, Alp EE, Chen SH, Huang HW. Collective chain dynamics in lipid bilayers by inelastic x-ray scattering. Biophys J 2003; 84:3767-76. [PMID: 12770883 PMCID: PMC1302959 DOI: 10.1016/s0006-3495(03)75105-4] [Citation(s) in RCA: 36] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022] Open
Abstract
We investigated the application of inelastic x-ray scattering (IXS) to lipid bilayers. This technique directly measures the dynamic structure factor S(q,omega) which is the space-time Fourier transform of the electron density correlation function of the measured system. For a multiatomic system, the analysis of S(q,omega) is usually complicated. But for multiple bilayers of lipid, S(q,omega) is dominated by chain-chain correlations within individual bilayers. Thus IXS provides a unique probe for the collective dynamics of lipid chains in a bilayer that cannot be obtained by any other method. IXS of dimyristoyl phosphatidylcholine and dimyristoyl phosphatidylcholine + cholesterol at two different concentrations were measured. S(q,omega) was analyzed by three-mode hydrodynamic equations, including a thermal diffusive mode and two propagating acoustic modes. We obtained the dispersion curves for the phonons that represent the collective in-plane excitations of lipid chains. The effect of cholesterol on chain dynamics was detected. Our analysis shows the importance of having a high instrument resolution as well as the requirement of sufficient signal-to-noise ratio to obtain meaningful results from such an IXS experiment. The requirement on signal-to-noise also applies to molecular dynamics simulations.
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Affiliation(s)
- Thomas M Weiss
- Department of Physics and Astronomy, Rice University, Houston, Texas 77251-1892, USA
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68
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Ge L, Möhwald H, Li J. Polymer-stabilized phospholipid vesicles formed on polyelectrolyte multilayer capsules. Biochem Biophys Res Commun 2003; 303:653-9. [PMID: 12659869 DOI: 10.1016/s0006-291x(03)00391-7] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Phospholipid vesicles on polyelectrolyte multilayer shells can be stabilized against ethanol by coating a single cationic polyelectrolyte. Confocal laser scanning microscopy (CLSM) proved that the lipids were stabilized by cationic polyelectrolytes and the permeability to small hydrophilic dyes was decreased. Measurements of fluorescence recovery after photo-bleaching (FRAP) with individual capsules enable quantification of release profiles.
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Affiliation(s)
- Liqin Ge
- International Joint Lab, The Center for Molecular Science, Institute of Chemistry, Chinese Academy of Science, Beijing 100080, China
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69
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Urbas AM, Thomas EL, Kriegs H, Fytas G, Penciu RS, Economou LN. Acoustic excitations in a self-assembled block copolymer photonic crystal. PHYSICAL REVIEW LETTERS 2003; 90:108302. [PMID: 12689040 DOI: 10.1103/physrevlett.90.108302] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/24/2002] [Indexed: 05/24/2023]
Abstract
High resolution Brillouin light scattering can sensitively detect acoustic phonons in concentrated solutions of a high molecular weight poly(styrene-b-isoprene) symmetric copolymer in toluene. This block copolymer lamellar forming system also possesses a photonic stop band in the visible spectrum. Based on the low but finite contrast in mechanical properties between the styrene and isoprene components and taking into account the geometrical characteristics of the layered microstructure, we calculate the acoustic band structure and represent the observed acousticlike and opticlike phonons.
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Affiliation(s)
- Augustine M Urbas
- Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, USA.
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70
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Partenskii MB, Jordan PC. Membrane deformation and the elastic energy of insertion: Perturbation of membrane elastic constants due to peptide insertion. J Chem Phys 2002. [DOI: 10.1063/1.1519840] [Citation(s) in RCA: 46] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/14/2022] Open
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71
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Surface-sensitive X-ray and neutron scattering characterization of planar lipid model membranes and lipid/peptide interactions. PEPTIDE-LIPID INTERACTIONS 2002. [DOI: 10.1016/s1063-5823(02)52007-x] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
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72
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Tarek M, Tobias DJ, Chen SH, Klein ML. Short wavelength collective dynamics in phospholipid bilayers: a molecular dynamics study. PHYSICAL REVIEW LETTERS 2001; 87:238101. [PMID: 11736477 DOI: 10.1103/physrevlett.87.238101] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/17/2001] [Indexed: 05/23/2023]
Abstract
Short wavelength density fluctuations of hydrated multilamellar phospholipid bilayers have been studied by molecular dynamics simulations in both the gel and liquid crystalline phases, and the results compared to recent inelastic x-ray scattering data [Phys. Rev. Lett. 86, 740 (2001)]. We confirm the existence of a highly dispersive sound mode, whose frequency and damping depend on the lipid phase, that the scattering arises mainly from in-plane motion of hydrocarbon chains, and we also identify a nondispersive mode attributed to motions of the chain terminal carbons.
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Affiliation(s)
- M Tarek
- NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899-8562, USA
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73
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Liao C, Chen SH. Theory of the generalized dynamic structure factor of polyatomic molecular fluids measured by inelastic x-ray scattering. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2001; 64:021205. [PMID: 11497570 DOI: 10.1103/physreve.64.021205] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/02/2000] [Indexed: 05/23/2023]
Abstract
We describe a theory for the calculation of the generalized dynamic structure factor S(k,omega) as measured by an inelastic x-ray scattering (IXS) experiment on single-component molecular or polyatomic molecular fluids. IXS spectrum of a simple fluid is proportional to the dynamic structure factor of a single species of atom. In the case of a molecular fluid, however, IXS spectrum is a weighted sum of partial dynamic structure factors of pairs of atomic species. The weighting factors are products of the atomic form factors of the pairs. We call this weighted average dynamic structure factor the generalized dynamic structure factor. We extend the formalism of a three effective eigenmode theory (TEE) developed previously for simple fluids to derive an approximate evolution equation for the generalized dynamic structure factor, which can be considered as a generalized hydrodynamic equation for molecular fluids. As examples, we first study the contributions of the partial dynamic structure factor to the generalized dynamic structure factor computed from molecular dynamics simulation of SPC/E model water. We found that the generalized dynamic structure factor of water measured by IXS can be well approximated by the center of mass or the oxygen atom dynamic structure factors. The generalized TEE model was then employed to analyze IXS spectra of nearly fully hydrated dilauroylphosphatidylcholine. The theory is able to fit all of the spectra in the k range from 5 to 32 nm(-1) quantitatively and gives their deconvoluted generalized dynamic structure factors.
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Affiliation(s)
- C Liao
- Department of Nuclear Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA
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