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Granek R, Hoffmann I, Kelley EG, Nagao M, Vlahovska PM, Zilman A. Dynamic structure factor of undulating vesicles: finite-size and spherical geometry effects with application to neutron spin echo experiments. THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER 2024; 47:12. [PMID: 38355850 DOI: 10.1140/epje/s10189-023-00400-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 10/20/2023] [Accepted: 12/11/2023] [Indexed: 02/16/2024]
Abstract
We consider the dynamic structure factor (DSF) of quasi-spherical vesicles and present a generalization of an expression that was originally formulated by Zilman and Granek (ZG) for scattering from isotropically oriented quasi-flat membrane plaquettes. The expression is obtained in the form of a multi-dimensional integral over the undulating membrane surface. The new expression reduces to the original stretched exponential form in the limit of sufficiently large vesicles, i.e., in the micron range or larger. For much smaller unilamellar vesicles, deviations from the asymptotic, stretched exponential equation are noticeable even if one assumes that the Seifert-Langer leaflet density mode is completely relaxed and membrane viscosity is neglected. To avoid the need for an exhaustive numerical integration while fitting to neutron spin echo (NSE) data, we provide a useful approximation for polydisperse systems that tests well against the numerical integration of the complete expression. To validate the new expression, we performed NSE experiments on variable-size vesicles made of a POPC/POPS lipid mixture and demonstrate an advantage over the original stretched exponential form or other manipulations of the original ZG expression that have been deployed over the years to fit the NSE data. In particular, values of the membrane bending rigidity extracted from the NSE data using the new approximations were insensitive to the vesicle radii and scattering wavenumber and compared very well with expected values of the effective bending modulus ([Formula: see text]) calculated from results in the literature. Moreover, the generalized scattering theory presented here for an undulating quasi-spherical shell can be easily extended to other models for the membrane undulation dynamics beyond the Helfrich Hamiltonian and thereby provides the foundation for the study of the nanoscale dynamics in more complex and biologically relevant model membrane systems.
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Affiliation(s)
- Rony Granek
- Avram and Stella Goldstein-Goren Department of Biotechnology Engineering, and Ilse Katz Institute for Nanoscale Science and Technology, Ben-Gurion University of the Negev, 84105, Beer Sheva, Israel.
| | - Ingo Hoffmann
- Institut Laue-Langevin (ILL), 71 Avenue des Martys, 38042, Grenoble, CEDEX 9, France.
| | - Elizabeth G Kelley
- Center for Neutron Research, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD, 20899, USA.
| | - Michihiro Nagao
- Center for Neutron Research, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD, 20899, USA
- Department of Materials Science and Engineering, University of Maryland, College Park, MD, 20742, USA
- Department of Physics and Astronomy, University of Delaware, Newark, DE, 19716, USA
| | - Petia M Vlahovska
- Department of Engineering Sciences and Applied Mathematics, Northwestern University, Evanston, IL, 60208, USA
| | - Anton Zilman
- Department of Physics, University of Toronto, 60 St George St, Toronto, ON, M5S 1A7, Canada
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2
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Machin JM, Kalli AC, Ranson NA, Radford SE. Protein-lipid charge interactions control the folding of outer membrane proteins into asymmetric membranes. Nat Chem 2023; 15:1754-1764. [PMID: 37710048 PMCID: PMC10695831 DOI: 10.1038/s41557-023-01319-6] [Citation(s) in RCA: 3] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/09/2022] [Accepted: 08/08/2023] [Indexed: 09/16/2023]
Abstract
Biological membranes consist of two leaflets of phospholipid molecules that form a bilayer, each leaflet comprising a distinct lipid composition. This asymmetry is created and maintained in vivo by dedicated biochemical pathways, but difficulties in creating stable asymmetric membranes in vitro have restricted our understanding of how bilayer asymmetry modulates the folding, stability and function of membrane proteins. In this study, we used cyclodextrin-mediated lipid exchange to generate liposomes with asymmetric bilayers and characterize the stability and folding kinetics of two bacterial outer membrane proteins (OMPs), OmpA and BamA. We found that excess negative charge in the outer leaflet of a liposome impedes their insertion and folding, while excess negative charge in the inner leaflet accelerates their folding relative to symmetric liposomes with the same membrane composition. Using molecular dynamics, mutational analysis and bioinformatics, we identified a positively charged patch critical for folding and stability. These results rationalize the well-known 'positive-outside' rule of OMPs and suggest insights into the mechanisms that drive OMP folding and assembly in vitro and in vivo.
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Affiliation(s)
- Jonathan M Machin
- Astbury Centre for Structural Molecular Biology, School of Molecular and Cellular Biology, Faculty of Biological Sciences, University of Leeds, Leeds, UK
| | - Antreas C Kalli
- Astbury Centre for Structural Molecular Biology, School of Molecular and Cellular Biology, Faculty of Biological Sciences, University of Leeds, Leeds, UK.
- Leeds Institute of Cardiovascular and Metabolic Medicine, School of Medicine, University of Leeds, Leeds, UK.
| | - Neil A Ranson
- Astbury Centre for Structural Molecular Biology, School of Molecular and Cellular Biology, Faculty of Biological Sciences, University of Leeds, Leeds, UK.
| | - Sheena E Radford
- Astbury Centre for Structural Molecular Biology, School of Molecular and Cellular Biology, Faculty of Biological Sciences, University of Leeds, Leeds, UK.
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3
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Nagao M, Seto H. Neutron scattering studies on dynamics of lipid membranes. BIOPHYSICS REVIEWS 2023; 4:021306. [PMID: 38504928 PMCID: PMC10903442 DOI: 10.1063/5.0144544] [Citation(s) in RCA: 5] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 01/30/2023] [Accepted: 05/01/2023] [Indexed: 03/21/2024]
Abstract
Neutron scattering methods are powerful tools for the study of the structure and dynamics of lipid bilayers in length scales from sub Å to tens to hundreds nm and the time scales from sub ps to μs. These techniques also are nondestructive and, perhaps most importantly, require no additives to label samples. Because the neutron scattering intensities are very different for hydrogen- and deuterium-containing molecules, one can replace the hydrogen atoms in a molecule with deuterium to prepare on demand neutron scattering contrast without significantly altering the physical properties of the samples. Moreover, recent advances in neutron scattering techniques, membrane dynamics theories, analysis tools, and sample preparation technologies allow researchers to study various aspects of lipid bilayer dynamics. In this review, we focus on the dynamics of individual lipids and collective membrane dynamics as well as the dynamics of hydration water.
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Affiliation(s)
| | - Hideki Seto
- Institute of Materials Structure Science, High Energy Accelerator Research Organization, Tsukuba, Ibaraki 305-0801, Japan
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4
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Gilbert J, Ermilova I, Nagao M, Swenson J, Nylander T. Effect of encapsulated protein on the dynamics of lipid sponge phase: a neutron spin echo and molecular dynamics simulation study. NANOSCALE 2022; 14:6990-7002. [PMID: 35470842 DOI: 10.1039/d2nr00882c] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/14/2023]
Abstract
Lipid membranes are highly mobile systems with hierarchical, time and length scale dependent, collective motions including thickness fluctuations, undulations, and topological membrane changes, which play an important role in membrane interactions. In this work we have characterised the effect of encapsulating two industrially important enzymes, β-galactosidase and aspartic protease, in lipid sponge phase nanoparticles on the dynamics of the lipid membrane using neutron spin echo (NSE) spectroscopy and molecular dynamics (MD) simulations. From NSE, reduced membrane dynamics were observed upon enzyme encapsulation, which were dependent on the enzyme concentration and type. By fitting the intermediate scattering functions (ISFs) with a modified Zilman and Granek model including nanoparticle diffusion, an increase in membrane bending rigidity was observed, with a larger effect for β-galactosidase than aspartic protease at the same concentration. MD simulations for the system with and without aspartic protease showed that the lipids relax more slowly in the system with protein due to the replacement of the lipid carbonyl-water hydrogen bonds with lipid-protein hydrogen bonds. This indicates that the most likely cause of the increase in membrane rigidity observed in the NSE measurements was dehydration of the lipid head groups. The dynamics of the protein itself were also studied, which showed a stable secondary structure of protein over the simulation, indicating no unfolding events occurred.
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Affiliation(s)
- Jennifer Gilbert
- Division of Physical Chemistry, Department of Chemistry, Naturvetarvägen 14, Lund University, 22362 Lund, Sweden.
- NanoLund, Lund University, Professorsgatan 1, 223 63 Lund, Sweden
| | - Inna Ermilova
- Department of Physics, Chalmers University of Technology, 412 96 Gothenburg, Sweden
| | - Michihiro Nagao
- NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
- Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742, USA
- Department of Physics and Astronomy, University of Delaware, Newark, Delaware 19716, USA
| | - Jan Swenson
- Department of Physics, Chalmers University of Technology, 412 96 Gothenburg, Sweden
| | - Tommy Nylander
- Division of Physical Chemistry, Department of Chemistry, Naturvetarvägen 14, Lund University, 22362 Lund, Sweden.
- NanoLund, Lund University, Professorsgatan 1, 223 63 Lund, Sweden
- Lund Institute of Advanced Neutron and X-Ray Science, Scheelevägen 19, 223 70 Lund, Sweden
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5
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Qian S, Zolnierczuk PA. Interaction of a Short Antimicrobial Peptide on Charged Lipid Bilayer: A Case Study on Aurein 1.2 Peptide. BBA ADVANCES 2022; 2:100045. [PMID: 37082600 PMCID: PMC10074906 DOI: 10.1016/j.bbadva.2022.100045] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/23/2021] [Revised: 01/21/2022] [Accepted: 02/01/2022] [Indexed: 12/27/2022] Open
Abstract
Aurein 1.2 (aurein) is a short but active α-helical antimicrobial peptide discovered in Australian tree frogs (Litoria aurea). It shows inhibition on a broad spectrum of bacteria and cancer cells. With well-defined helicity, amphipathicity, and cationic charges, it readily binds to membranes and causes membrane change and disruption. This study provides details on how aurein interacts with charged lipid membranes by using neutron membrane diffraction (NMD) and neutron spin echo (NSE) spectroscopy on complex peptide-membrane systems. NMD provides higher resolution lipid bilayer structures than solution scattering. NMD revealed the peptide is mostly associated in the lipid headgroup region. Even at moderately high concentrations (e.g., peptide:lipid ratio of 1:30), aurein is located at the acyl chain-headgroup region without deep penetration into the hydrophobic acyl chain. However, it does reduce the elasticity of the membrane at that concentration, which was corroborated by the NSE results. Furthermore, NSE shows that aurein first softens the membrane, like many other α-helical peptides at low concentration, but then makes the membrane much more rigid, even without membrane pore formation. Combining our previous studies, the evidence shows that aurein at relatively low concentrations still modifies lipid distribution significantly and can cause membrane thinning and lateral segregation of charged lipids. At the same time, the membrane's mechanical properties are modified with much slower lipid diffusion. This suggests that aurein can attack the microbial membrane without the need to form membrane pores or disintegrate membranes; instead, it promotes the formation of domains at low concentration.
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Affiliation(s)
- Shuo Qian
- Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, United States
- Second Target Station, Oak Ridge National Laboratory, Oak Ridge, TN 37830, United States
- Corresponding author.
| | - Piotr A. Zolnierczuk
- Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37830, United States
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6
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Xie CZ, Chang SM, Mamontov E, Stingaciu LR, Chen YF. Uncoupling between the lipid membrane dynamics of differing hierarchical levels. Phys Rev E 2020; 101:012416. [PMID: 32069643 DOI: 10.1103/physreve.101.012416] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/03/2019] [Indexed: 11/07/2022]
Abstract
Diverse biological functions of biomembranes are made possible by their rich dynamic behaviors across multiple scales. While the potential coupling between the dynamics of differing scales may underlie the machineries regulating the biomembrane-involving processes, the mechanism and even the existence of this coupling remain an open question, despite the latter being taken for granted. Via inelastic neutron scattering, we examined dynamics across multiple scales for the lipid membranes whose dynamic behaviors were perturbed by configurational changes at two membrane regions. Surprisingly, the dynamic behavior of individual lipid molecules and their collective motions were not always coupled. This suggests that the expected causal relation between the dynamics of the differing hierarchical levels does not exist and that an apparent coupling can emerge by manipulating certain membrane configurations. The findings provide insight on biomembrane modeling and how cells might individually or concertedly control the multiscale membrane dynamics to regulate their functions.
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Affiliation(s)
- Cheng-Zhi Xie
- Department of Chemical and Materials Engineering, National Central University, Taoyuan 32001, Taiwan
| | - Shih-Min Chang
- Department of Chemical and Materials Engineering, National Central University, Taoyuan 32001, Taiwan
| | - Eugene Mamontov
- Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
| | - Laura R Stingaciu
- Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
| | - Yi-Fan Chen
- Department of Chemical and Materials Engineering, National Central University, Taoyuan 32001, Taiwan
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7
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Viscosity of Plasma as a Key Factor in Assessment of Extracellular Vesicles by Light Scattering. Cells 2019; 8:cells8091046. [PMID: 31500151 PMCID: PMC6769602 DOI: 10.3390/cells8091046] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/15/2019] [Revised: 08/19/2019] [Accepted: 09/04/2019] [Indexed: 12/31/2022] Open
Abstract
Extracellular vesicles (EVs) isolated from biological samples are a promising material for use in medicine and technology. However, the assessment methods that would yield repeatable concentrations, sizes and compositions of the harvested material are missing. A plausible model for the description of EV isolates has not been developed. Furthermore, the identity and genesis of EVs are still obscure and the relevant parameters have not yet been identified. The purpose of this work is to better understand the mechanisms taking place during harvesting of EVs, in particular the role of viscosity of EV suspension. The EVs were harvested from blood plasma by repeated centrifugation and washing of samples. Their size and shape were assessed by using a combination of static and dynamic light scattering. The average shape parameter of the assessed particles was found to be ρ ~ 1 (0.94–1.1 in exosome standards and 0.7–1.2 in blood plasma and EV isolates), pertaining to spherical shells (spherical vesicles). This study has estimated the value of the viscosity coefficient of the medium in blood plasma to be 1.2 mPa/s. It can be concluded that light scattering could be a plausible method for the assessment of EVs upon considering that EVs are a dynamic material with a transient identity.
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8
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Wang H, Zhang X, Liu Y, Liu J. Stabilization of Liposomes by Perfluorinated Compounds. ACS OMEGA 2018; 3:15353-15360. [PMID: 30556004 PMCID: PMC6288781 DOI: 10.1021/acsomega.8b02448] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Received: 09/20/2018] [Accepted: 10/30/2018] [Indexed: 05/05/2023]
Abstract
Perfluorinated compounds (PFCs) are emerging persistent environmental contaminants that may be toxic to animals and humans. To gain fundamental insights into the mechanism of their toxicity, the interactions of phosphocholine (PC) liposomes as model membranes were studied with three types of PFCs, including perfluorooctanoic acid, perfluorooctane sulfonate, and perfluorohexanesulfonic acid potassium salt, together with three common surfactants: sodium dodecyl sulfate (SDS), cetyltrimethylammonium bromide (CTAB), and sodium 1-heptanesulfonate (SHS). The interactions were systematically characterized by zeta potential measurement, dynamic light scattering, negative-stain transmission electron microscopy, and fluorescence spectroscopy. Unmodified liposomes, calcein-loaded liposomes, and Laurdan dye-embedded liposomes were all tested. By gradually increasing the temperature, the three PFCs and SHS decreased the leakage of calcein-loaded 1,2-dipalmitoyl-sn-glycero-3-phosphocholine liposomes, whereas SDS and CTAB increased the leakage. The PFCs that affected the lipid membranes stronger than SHS were attributable to their perfluoroalkyl carbon chains. Packing of the lipids was further studied using Laurdan dye as a probe. Calcein leakage tests also indicated that PFCs inhibited lipid membrane leakage induced by inorganic nanoparticles such as silica and gold nanoparticles. This study confirmed the similar effect of the PFCs as cholesterol in affecting membrane properties and would be helpful for understanding the interaction mechanism of PFCs and cell membranes.
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Affiliation(s)
- Heye Wang
- Jiangsu
Key Laboratory of Food Quality and Safety-State Key Laboratory Cultivation
Base of MOST, Institute of Food Safety and Nutrition, Jiangsu Academy of Agricultural Sciences, Nanjing, Jiangsu 210014, China
- Department
of Chemistry, Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
| | - Xiaohan Zhang
- Department
of Chemistry, Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
| | - Yibo Liu
- Department
of Chemistry, Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
| | - Juewen Liu
- Department
of Chemistry, Waterloo Institute for Nanotechnology, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada
- E-mail: (J.L.)
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9
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Chakraborty S, Abbasi A, Bothun GD, Nagao M, Kitchens CL. Phospholipid Bilayer Softening Due to Hydrophobic Gold Nanoparticle Inclusions. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2018; 34:13416-13425. [PMID: 30350687 DOI: 10.1021/acs.langmuir.8b02553] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
Liposome-nanoparticle assemblies (LNAs) are vital in the context of novel targeted drug-delivery systems, in addition to investigating nanoparticle-lipid bilayer interactions. Quantifying membrane structural properties and dynamics in presence of nanoparticle inclusions provides a simple model to elucidate nanoparticle effects on membrane biophysical properties. We present experimental evidences of bilayer softening due to small hydrophobic gold nanoparticle inclusions. LNA structure has been investigated by a combination of cryo-transmission electron microscopy, dynamic light scattering, and small-angle neutron scattering. Neutron spin echo spectroscopy demonstrated a remarkable ∼15% bending modulus decrease for LNAs relative to pure liposomes. Clear dependence of bending modulus on gold nanoparticle diameter and concentration was observed from our observations. Our findings point toward local bilayer fluidization by nanoparticle inclusions leading to an overall bilayer softening. These findings add valuable information to liposomal drug-delivery vehicle design and membrane biophysics research.
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Affiliation(s)
- Saptarshi Chakraborty
- Department of Chemical and Biomolecular Engineering , Clemson University , Clemson , South Carolina 29634 , United States
| | - Akram Abbasi
- Department of Chemical Engineering , University of Rhode Island , Kingston , Rhode Island 02881 , United States
| | - Geoffrey D Bothun
- Department of Chemical Engineering , University of Rhode Island , Kingston , Rhode Island 02881 , United States
| | - Michihiro Nagao
- NIST Center for Neutron Research , National Institute of Standards and Technology , Gaithersburg , Maryland 20899 , United States
- Center for Exploration of Energy and Matter, Department of Physics , Indiana University , Bloomington , Indiana 47408 , United States
| | - Christopher L Kitchens
- Department of Chemical and Biomolecular Engineering , Clemson University , Clemson , South Carolina 29634 , United States
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10
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Ramakrishnan N, Bradley RP, Tourdot RW, Radhakrishnan R. Biophysics of membrane curvature remodeling at molecular and mesoscopic lengthscales. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2018; 30:273001. [PMID: 29786613 PMCID: PMC6066392 DOI: 10.1088/1361-648x/aac702] [Citation(s) in RCA: 25] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/19/2023]
Abstract
At the micron scale, where cell organelles display an amazing complexity in their shape and organization, the physical properties of a biological membrane can be better-understood using continuum models subject to thermal (stochastic) undulations. Yet, the chief orchestrators of these complex and intriguing shapes are a specialized class of membrane associating often peripheral proteins called curvature remodeling proteins (CRPs) that operate at the molecular level through specific protein-lipid interactions. We review multiscale methodologies to model these systems at the molecular as well as at the mesoscopic and cellular scales, and also present a free energy perspective of membrane remodeling through the organization and assembly of CRPs. We discuss the morphological space of nearly planar to highly curved membranes, methods to include thermal fluctuations, and review studies that model such proteins as curvature fields to describe the emergent curved morphologies. We also discuss several mesoscale models applied to a variety of cellular processes, where the phenomenological parameters (such as curvature field strength) are often mapped to models of real systems based on molecular simulations. Much insight can be gained from the calculation of free energies of membranes states with protein fields, which enable accurate mapping of the state and parameter values at which the membrane undergoes morphological transformations such as vesiculation or tubulation. By tuning the strength, anisotropy, and spatial organization of the curvature-field, one can generate a rich array of membrane morphologies that are highly relevant to shapes of several cellular organelles. We review applications of these models to budding of vesicles commonly seen in cellular signaling and trafficking processes such as clathrin mediated endocytosis, sorting by the ESCRT protein complexes, and cellular exocytosis regulated by the exocyst complex. We discuss future prospects where such models can be combined with other models for cytoskeletal assembly, and discuss their role in understanding the effects of cell membrane tension and the mechanics of the extracellular microenvironment on cellular processes.
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Affiliation(s)
- N Ramakrishnan
- Department of Bioengineering, University of Pennsylvania, Philadelphia, PA 19104, United States of America
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11
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Takechi-Haraya Y, Goda Y, Sakai-Kato K. Atomic Force Microscopy Study on the Stiffness of Nanosized Liposomes Containing Charged Lipids. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2018; 34:7805-7812. [PMID: 29869883 DOI: 10.1021/acs.langmuir.8b01121] [Citation(s) in RCA: 28] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
It has recently been recognized that the mechanical properties of lipid nanoparticles play an important role during in vitro and in vivo behaviors such as cellular uptake, blood circulation, and biodistribution. However, there have been no quantitative investigations of the effect of commonly used charged lipids on the stiffness of nanosized liposomes. In this study, by means of atomic force microscopy (AFM), we quantified the stiffness of nanosized liposomes composed of neutrally charged lipids combined with positively or negatively charged lipids while simultaneously imaging the liposomes in aqueous medium. Our results showed that charged lipids, whether negatively or positively charged, have the effect of reducing the stiffness of nanosized liposomes, independently of the saturation degree of the lipid acyl chains; the measured stiffness values of liposomes containing charged lipids are 30-60% lower than those of their neutral counterpart liposomes. In addition, we demonstrated that the Laurdan generalized polarization values, which are related to the hydration degree of the liposomal membrane interface and often used as a qualitative indicator of liposomal membrane stiffness, do not directly correlate with the physical stiffness values of the liposomes prepared in this study. However, our results indicate that direct quantitative AFM measurement is a valuable method to gain molecular-scale information about how the hydration degree of liposomal interfaces reflects (or does not reflect) liposome stiffness as a macroscopic property. Our AFM method will contribute to the quantitative characterization of the nano-bio interaction of nanoparticles and to the optimization of the lipid composition of liposomes for clinical use.
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Affiliation(s)
- Yuki Takechi-Haraya
- Division of Drugs , National Institute of Health Sciences , 3-25-26 Tonomachi , Kawasaki-ku, Kawasaki City , Kanagawa 210-9501 , Japan
| | - Yukihiro Goda
- National Institute of Health Sciences , 3-25-26 Tonomachi , Kawasaki-ku, Kawasaki City , Kanagawa 210-9501 , Japan
| | - Kumiko Sakai-Kato
- Division of Drugs , National Institute of Health Sciences , 3-25-26 Tonomachi , Kawasaki-ku, Kawasaki City , Kanagawa 210-9501 , Japan
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12
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Sreij R, Dargel C, Geisler P, Hertle Y, Radulescu A, Pasini S, Perez J, Moleiro LH, Hellweg T. DMPC vesicle structure and dynamics in the presence of low amounts of the saponin aescin. Phys Chem Chem Phys 2018; 20:9070-9083. [DOI: 10.1039/c7cp08027a] [Citation(s) in RCA: 24] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/18/2023]
Abstract
Vesicle shape and bilayer parameters are studied by small-angle X-ray (SAXS) and small-angle neutron (SANS) scattering in the presence of the saponin aescin. Bilayer dynamics is studied by neutron spin-echo (NSE) spectroscopy.
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Affiliation(s)
- Ramsia Sreij
- Physical and Biophysical Chemistry
- Bielefeld University
- Bielefeld
- Germany
| | - Carina Dargel
- Physical and Biophysical Chemistry
- Bielefeld University
- Bielefeld
- Germany
| | - Philippe Geisler
- Cognitronics and Sensor Systems
- CITEC
- Bielefeld University
- Bielefeld
- Germany
| | - Yvonne Hertle
- Physical and Biophysical Chemistry
- Bielefeld University
- Bielefeld
- Germany
| | - Aurel Radulescu
- Jülich Centre for Neutron Science JCNS at Heinz Maier-Leibnitz Zentrum (MLZ)
- Forschungszentrum Jülich GmbH
- Garching
- Germany
| | - Stefano Pasini
- Jülich Centre for Neutron Science JCNS at Heinz Maier-Leibnitz Zentrum (MLZ)
- Forschungszentrum Jülich GmbH
- Garching
- Germany
| | | | - Lara H. Moleiro
- Physical and Biophysical Chemistry
- Bielefeld University
- Bielefeld
- Germany
| | - Thomas Hellweg
- Physical and Biophysical Chemistry
- Bielefeld University
- Bielefeld
- Germany
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13
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Sreij R, Dargel C, Moleiro LH, Monroy F, Hellweg T. Aescin Incorporation and Nanodomain Formation in DMPC Model Membranes. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2017; 33:12351-12361. [PMID: 28985678 DOI: 10.1021/acs.langmuir.7b02933] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
The saponin aescin from the horse chestnut tree is a natural surfactant well-known to self-assemble as oriented-aggregates at fluid interfaces. Using model membranes in the form of lipid vesicles and Langmuir monolayers, we study the mixing properties of aescin with the phase-segregating phospholipid 1,2-dimyristoyl-sn-glycero-phosphocholine (DMPC). The binary membranes are experimentally studied on different length scales ranging from the lipid headgroup area to the macroscopic scale using small-angle X-ray scattering (SAXS), photon correlation spectroscopy (PCS), and differential scanning calorimetry (DSC) with binary bilayer vesicles and Langmuir tensiometry (LT) with lipid monolayers spread on the surface of aescin solutions. The binary interaction was found to strongly depend on aescin concentration in two well differentiated concentration regimes. Below 7 mol %, the results reveal phase segregation of nanometer-sized aescin-rich domains in an aescin-poor continuous bilayer. Above this concentration, aescin-aescin interactions dominate, which inhibit vesicle formation but lead to the formation of new membrane aggregates of smaller sizes. From LT studies in monolayers, the interaction of aescin with DMPC was shown to be stronger in the condensed phase than in the liquid expanded phase. Furthermore, a destructuring role was revealed for aescin on phospholipid membranes, similar to the fluidizing effect of cholesterol and nonsteroidal anti-inflammatory drugs (NSAIDs) on lipid bilayers.
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Affiliation(s)
- Ramsia Sreij
- Physical and Biophysical Chemistry, Department of Chemistry, Bielefeld University , Universitässtraße 25, Bielefeld 33615, Germany
| | - Carina Dargel
- Physical and Biophysical Chemistry, Department of Chemistry, Bielefeld University , Universitässtraße 25, Bielefeld 33615, Germany
| | - Lara H Moleiro
- Physical and Biophysical Chemistry, Department of Chemistry, Bielefeld University , Universitässtraße 25, Bielefeld 33615, Germany
| | - Francisco Monroy
- Department of Physical Chemistry I, Complutense University , Avda. Complutense s/n, Madrid 28040, Spain
- Unit of Translational Biophysics, Institute of Biomedical Research Hospital Doce de Octubre (imas12) , Av. Andalucía s/n, Madrid 28041, Spain
| | - Thomas Hellweg
- Physical and Biophysical Chemistry, Department of Chemistry, Bielefeld University , Universitässtraße 25, Bielefeld 33615, Germany
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Karamdad K, Law RV, Seddon JM, Brooks NJ, Ces O. Studying the effects of asymmetry on the bending rigidity of lipid membranes formed by microfluidics. Chem Commun (Camb) 2016; 52:5277-80. [PMID: 27001410 DOI: 10.1039/c5cc10307j] [Citation(s) in RCA: 39] [Impact Index Per Article: 4.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
In this article we detail a robust high-throughput microfluidic platform capable of fabricating either symmetric or asymmetric giant unilamellar vesicles (GUVs) and characterise the mechanical properties of their membranes.
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Affiliation(s)
- K Karamdad
- Department of Chemistry, Imperial College London, Exhibition Road, London, SW7 2AZ, UK. and Institute of Chemical Biology, Imperial College London, Exhibition Road, London, SW7 2AZ, UK
| | - R V Law
- Department of Chemistry, Imperial College London, Exhibition Road, London, SW7 2AZ, UK. and Institute of Chemical Biology, Imperial College London, Exhibition Road, London, SW7 2AZ, UK
| | - J M Seddon
- Department of Chemistry, Imperial College London, Exhibition Road, London, SW7 2AZ, UK. and Institute of Chemical Biology, Imperial College London, Exhibition Road, London, SW7 2AZ, UK
| | - N J Brooks
- Department of Chemistry, Imperial College London, Exhibition Road, London, SW7 2AZ, UK. and Institute of Chemical Biology, Imperial College London, Exhibition Road, London, SW7 2AZ, UK
| | - O Ces
- Department of Chemistry, Imperial College London, Exhibition Road, London, SW7 2AZ, UK. and Institute of Chemical Biology, Imperial College London, Exhibition Road, London, SW7 2AZ, UK
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15
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Bradbury R, Nagao M. Effect of charge on the mechanical properties of surfactant bilayers. SOFT MATTER 2016; 12:9383-9390. [PMID: 27830216 DOI: 10.1039/c6sm01686c] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/24/2023]
Abstract
Charge effects on the mechanical properties of surfactant bilayers have been measured, for a system with a low ionic strength, using small-angle neutron scattering and neutron spin echo spectroscopy. We report that, not only does increasing the surface charge density lead to greater structural ordering and a stiffening of the membrane, which is consistent with classical theory of charge effects on membranes, but also that the relaxation rate of the membrane thickness fluctuations decreases without affecting the fluctuation amplitude. From the relaxation rate we demonstrate, using recent theory, that the viscosity of the surfactant membrane is increased with surface charge density, which suggests that the amount of charge controls the diffusion behavior of inclusions inside the membrane. The present results confirm that the thickness fluctuation relaxation rate and amplitude are tuned independently since the membrane viscosity is only influencing the relaxation rate. This work demonstrates that charge stabilization of lamellar bilayers is not merely affected by intermembrane interactions and structural ordering but that intramembrane dynamics also have a significant contribution.
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Affiliation(s)
- Robert Bradbury
- Center for Exploration of Energy and Matter, Department of Physics, Indiana University, Bloomington, Indiana, USA. and National Institute of Standards and Technology Center for Neutron Research, 100 Bureau Drive, Gaithersburg, Maryland, USA
| | - Michihiro Nagao
- Center for Exploration of Energy and Matter, Department of Physics, Indiana University, Bloomington, Indiana, USA. and National Institute of Standards and Technology Center for Neutron Research, 100 Bureau Drive, Gaithersburg, Maryland, USA
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16
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Lu L, Doak WJ, Schertzer JW, Chiarot PR. Membrane mechanical properties of synthetic asymmetric phospholipid vesicles. SOFT MATTER 2016; 12:7521-7528. [PMID: 27722472 PMCID: PMC5139623 DOI: 10.1039/c6sm01349j] [Citation(s) in RCA: 48] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/08/2023]
Abstract
Synthetic lipid vesicles have served as important model systems to study cellular membrane biology. Research has shown that the mechanical properties of bilayer membranes significantly affects their biological behavior. The properties of a lipid bilayer are governed by lipid acyl chain length, headgroup type, and the presence of membrane proteins. However, few studies have explored how membrane architecture, in particular trans-bilayer lipid asymmetry, influences membrane mechanical properties. In this study, we investigated the effects of lipid bilayer architecture (i.e. asymmetry) on the mechanical properties of biological membranes. This was achieved using a customized micropipette aspiration system and a novel microfluidic technique previously developed by our team for building asymmetric phospholipid vesicles with tailored bilayer architecture. We found that the bending modulus and area expansion modulus of the synthetic asymmetric bilayers were up to 50% larger than the values acquired for symmetric bilayers. This was caused by the dissimilar lipid distribution in each leaflet of the bilayer for the asymmetric membrane. To the best of our knowledge, this is the first report on the impact of trans-bilayer asymmetry on the area expansion modulus of synthetic bilayer membranes. Since the mechanical properties of bilayer membranes play an important role in numerous cellular processes, these results have significant implications for membrane biology studies.
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Affiliation(s)
- Li Lu
- Department of Mechanical Engineering, State University of New York at Binghamton, Binghamton, NY, USA. and Binghamton Biofilm Research Center, State University of New York at Binghamton, Binghamton, NY, USA
| | - William J Doak
- Department of Mechanical Engineering, State University of New York at Binghamton, Binghamton, NY, USA.
| | - Jeffrey W Schertzer
- Binghamton Biofilm Research Center, State University of New York at Binghamton, Binghamton, NY, USA and Department of Biological Sciences, State University of New York at Binghamton, Binghamton, NY, USA
| | - Paul R Chiarot
- Department of Mechanical Engineering, State University of New York at Binghamton, Binghamton, NY, USA. and Binghamton Biofilm Research Center, State University of New York at Binghamton, Binghamton, NY, USA
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17
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West A, Brummel BE, Braun AR, Rhoades E, Sachs JN. Membrane remodeling and mechanics: Experiments and simulations of α-Synuclein. BIOCHIMICA ET BIOPHYSICA ACTA 2016; 1858:1594-609. [PMID: 26972046 PMCID: PMC5081225 DOI: 10.1016/j.bbamem.2016.03.012] [Citation(s) in RCA: 37] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/18/2015] [Revised: 03/05/2016] [Accepted: 03/07/2016] [Indexed: 01/24/2023]
Abstract
We review experimental and simulation approaches that have been used to determine curvature generation and remodeling of lipid bilayers by membrane-bending proteins. Particular emphasis is placed on the complementary approaches used to study α-Synuclein (αSyn), a major protein involved in Parkinson's disease (PD). Recent cellular and biophysical experiments have shown that the protein 1) deforms the native structure of mitochondrial and model membranes; and 2) inhibits vesicular fusion. Today's advanced experimental and computational technology has made it possible to quantify these protein-induced changes in membrane shape and material properties. Collectively, experiments, theory and multi-scale simulation techniques have established the key physical determinants of membrane remodeling and rigidity: protein binding energy, protein partition depth, protein density, and membrane tension. Despite the exciting and significant progress made in recent years in these areas, challenges remain in connecting biophysical insights to the cellular processes that lead to disease. This article is part of a Special Issue entitled: Membrane Proteins edited by J.C. Gumbart and Sergei Noskov.
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Affiliation(s)
- Ana West
- Department of Biomedical Engineering, University of Minnesota, 312 Church St. SE, Minneapolis, MN 55455, USA
| | - Benjamin E Brummel
- Department of Biomedical Engineering, University of Minnesota, 312 Church St. SE, Minneapolis, MN 55455, USA
| | - Anthony R Braun
- Department of Neuroscience, University of Minnesota, 321 Church St. SE, Minneapolis, MN 55455, USA
| | - Elizabeth Rhoades
- Department of Chemistry, University of Pennsylvania, 231 S 34th St., Philadelphia, PA 19104, USA
| | - Jonathan N Sachs
- Department of Biomedical Engineering, University of Minnesota, 312 Church St. SE, Minneapolis, MN 55455, USA.
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18
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Marquardt D, Heberle FA, Nickels JD, Pabst G, Katsaras J. On scattered waves and lipid domains: detecting membrane rafts with X-rays and neutrons. SOFT MATTER 2015; 11:9055-72. [PMID: 26428538 PMCID: PMC4719199 DOI: 10.1039/c5sm01807b] [Citation(s) in RCA: 52] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/21/2015] [Accepted: 09/21/2015] [Indexed: 05/28/2023]
Abstract
In order to understand the biological role of lipids in cell membranes, it is necessary to determine the mesoscopic structure of well-defined model membrane systems. Neutron and X-ray scattering are non-invasive, probe-free techniques that have been used extensively in such systems to probe length scales ranging from angstroms to microns, and dynamics occurring over picosecond to millisecond time scales. Recent developments in the area of phase separated lipid systems mimicking membrane rafts will be presented, and the underlying concepts of the different scattering techniques used to study them will be discussed in detail.
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Affiliation(s)
- Drew Marquardt
- University of Graz, Institute of Molecular Biosciences, Biophysics Division, NAWI Graz, Humboldtstr. 50/III, Graz, Austria. and BioTechMed-Graz, Graz, Austria
| | - Frederick A Heberle
- Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. and Joint Institute for Neutron Sciences, Oak Ridge, Tennessee 37831, USA
| | - Jonathan D Nickels
- Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. and Joint Institute for Neutron Sciences, Oak Ridge, Tennessee 37831, USA
| | - Georg Pabst
- University of Graz, Institute of Molecular Biosciences, Biophysics Division, NAWI Graz, Humboldtstr. 50/III, Graz, Austria. and BioTechMed-Graz, Graz, Austria
| | - John Katsaras
- Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA. and Joint Institute for Neutron Sciences, Oak Ridge, Tennessee 37831, USA
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19
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Mell M, Moleiro LH, Hertle Y, López-Montero I, Cao FJ, Fouquet P, Hellweg T, Monroy F. Fluctuation dynamics of bilayer vesicles with intermonolayer sliding: experiment and theory. Chem Phys Lipids 2014; 185:61-77. [PMID: 25455136 DOI: 10.1016/j.chemphyslip.2014.11.005] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/04/2014] [Revised: 11/24/2014] [Accepted: 11/27/2014] [Indexed: 10/24/2022]
Abstract
The presence of coupled modes of membrane motion in closed shells is extensively predicted by theory. The bilayer structure inherent to lipid vesicles is suitable to support hybrid modes of curvature motion coupling membrane bending with the local reorganization of the bilayer material through relaxation of the dilatational stresses. Previous experiments evidenced the existence of such hybrid modes facilitating membrane bending at high curvatures in lipid vesicles [Rodríguez-García, R., Arriaga, L.R., Mell, M., Moleiro, L.H., López-Montero, I., Monroy, F., 2009. Phys. Rev. Lett. 102, 128201.]. For lipid bilayers that are able to undergo intermonolayer sliding, the experimental fluctuation spectra are found compatible with a bimodal schema. The usual tension/bending fluctuations couple with the hybrid modes in a mechanical interplay, which becomes progressively efficient with increasing vesicle radius, to saturate at infinity radius into the behavior expected for a flat membrane. Grounded on the theory of closed shells, we propose an approximated expression of the bimodal spectrum, which predicts the observed dependencies on the vesicle radius. The dynamical features obtained from the autocorrelation functions of the vesicle fluctuations are found in quantitative agreement with the proposed theory.
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Affiliation(s)
- Michael Mell
- Departamento de Química Física I, Universidad Complutense de Madrid, E-28040 Madrid, Spain
| | - Lara H Moleiro
- Departamento de Química Física I, Universidad Complutense de Madrid, E-28040 Madrid, Spain; Physikalische Chemie I, Univeristät Bayreuth, Universitätsstraße 30, D-95447 Bayreuth, Germany
| | - Yvonne Hertle
- Physikalische und Biophysikalische Chemie I, Universität Bielefeld, Universitätsstraße 25, D-33615 Bielefeld, Germany
| | - Iván López-Montero
- Departamento de Química Física I, Universidad Complutense de Madrid, E-28040 Madrid, Spain
| | - Francisco J Cao
- Departamento de Física Atómica, Molecular y Nuclear, Universidad Complutense de Madrid, E-28040 Madrid, Spain
| | - Peter Fouquet
- TOF/HR Group, Institut Laue Langevin, 6 Rue Jules Horowitz, BP156, F-38042 Grenoble Cedex 9, France
| | - Thomas Hellweg
- Physikalische und Biophysikalische Chemie I, Universität Bielefeld, Universitätsstraße 25, D-33615 Bielefeld, Germany
| | - Francisco Monroy
- Departamento de Química Física I, Universidad Complutense de Madrid, E-28040 Madrid, Spain.
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20
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Brüning BA, Prévost S, Stehle R, Steitz R, Falus P, Farago B, Hellweg T. Bilayer undulation dynamics in unilamellar phospholipid vesicles: effect of temperature, cholesterol and trehalose. BIOCHIMICA ET BIOPHYSICA ACTA-BIOMEMBRANES 2014; 1838:2412-9. [PMID: 24950248 DOI: 10.1016/j.bbamem.2014.06.006] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 04/16/2014] [Revised: 06/02/2014] [Accepted: 06/09/2014] [Indexed: 10/25/2022]
Abstract
We report a combined dynamic light scattering (DLS) and neutron spin-echo (NSE) study on the local bilayer undulation dynamics of phospholipid vesicles composed of 1,2-dimyristoyl-glycero-3-phosphatidylcholine (DMPC) under the influence of temperature and the additives cholesterol and trehalose. The additives affect vesicle size and self-diffusion. Mechanical properties of the membrane and corresponding bilayer undulations are tuned by changing lipid headgroup or acyl chain properties through temperature or composition. On the local length scale, changes at the lipid headgroup influence the bilayer bending rigidity κ less than changes at the lipid acyl chain: We observe a bilayer softening around the main phase transition temperature Tm of the single lipid system, and stiffening when more cholesterol is added, in concordance with literature. Surprisingly, no effect on the mechanical properties of the vesicles is observed upon the addition of trehalose.
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Affiliation(s)
- Beate-Annette Brüning
- Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin, Hahn-Meitner Platz 1, 14109 Berlin, Germany; Radiation Science and Technology, Delft University of Technology, Mekelweg 15, JB 2629 Delft, The Netherlands.
| | - Sylvain Prévost
- Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin, Hahn-Meitner Platz 1, 14109 Berlin, Germany
| | - Ralf Stehle
- Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin, Hahn-Meitner Platz 1, 14109 Berlin, Germany
| | - Roland Steitz
- Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin, Hahn-Meitner Platz 1, 14109 Berlin, Germany
| | - Peter Falus
- Time-of-Flight and High Resolution, Institut Laue Langevin, B. P. 156, 38042 Grenoble, Cedex 9, France
| | - Bela Farago
- Time-of-Flight and High Resolution, Institut Laue Langevin, B. P. 156, 38042 Grenoble, Cedex 9, France
| | - Thomas Hellweg
- Physical and Biophysical Chemistry, Bielefeld University, Universitätsstr. 25, 33615 Bielefeld, Germany
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21
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Brüning B, Farago B. Perfluorooctanoic acid rigidifies a model lipid membrane. PHYSICAL REVIEW. E, STATISTICAL, NONLINEAR, AND SOFT MATTER PHYSICS 2014; 89:040702. [PMID: 24827173 DOI: 10.1103/physreve.89.040702] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/09/2014] [Indexed: 05/21/2023]
Abstract
We report a combined dynamic light scattering and neutron spin-echo (NSE) study on vesicles composed of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine under the influence of varying amounts of perfluorooctanoic acid. We study local lipid bilayer undulations using NSE on time scales up to 200 ns. Similar to the effect evoked by cholesterol, we attribute the observed lipid bilayer stiffening to a condensing effect of the perfluorinated compound on the membrane.
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Affiliation(s)
- B Brüning
- Soft Matter and Functional Materials, Helmholtz-Zentrum Berlin, Berlin, Germany and Reactor Institute Delft, Delft University of Technology, Delft, The Netherlands
| | - B Farago
- Time-of-Flight and High Resolution, Institut Laue-Langevin, Grenoble, France
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22
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Fragneto G, Gabel F. Editorial on the topical issue "neutron biological physics". THE EUROPEAN PHYSICAL JOURNAL. E, SOFT MATTER 2013; 36:81. [PMID: 23893263 DOI: 10.1140/epje/i2013-13081-4] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/07/2013] [Indexed: 06/02/2023]
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