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Stretched exponential dynamics in lipid bilayer simulations
KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical Biological Physics.
KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical Biological Physics.ORCID iD: 0000-0002-7448-4664
2010 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 133, no 11, 115101- p.Article in journal (Refereed) Published
Abstract [en]

The decay of fluctuations in fluid biomembranes is strongly stretched and nonexponential on nanometer lengthscales. We report on calculations of structural correlation functions for lipid bilayer membranes from atomistic and coarse-grained molecular dynamics simulations. The time scales extend up to microseconds, whereas the linear size of the largest systems is around 50 nm. Thus, we can cover the equilibrium dynamics of wave vectors over two orders of magnitude (0.2-20 nm(-1)). The time correlations observed in the simulations are best described by stretched exponential functions, with exponents of 0.45 for the atomistic and 0.60 for the coarse-grained model. Area number density fluctuations, thickness fluctuations, and undulations behave dynamically in a similar way and have almost exactly the same dynamics for wavelengths below 3 nm, indicating that in this regime undulations and thickness fluctuations are governed by in-plane density fluctuations. The out-of-plane height fluctuations are apparent only at the longest wavelengths accessible in the simulations (above 6 nm). The effective correlation times of the stretched exponentials vary strongly with the wave vector. The variation fits inverse power-laws that change with wavelength. The exponent is 3 for wavelengths smaller than about 1.25 nm and switches to 1 above this. There are indications for a switch to still another exponent, 2, for wavelengths above 20 nm. Compared to neutron spin-echo (NSE) experiments, the simulation data indicate a faster relaxation in the hydrodynamic limit, although an extrapolation of NSE data, as well as inelastic neutron scattering data, is in agreement with our data at larger wave vectors.

Place, publisher, year, edition, pages
2010. Vol. 133, no 11, 115101- p.
Keyword [en]
biological fluid dynamics, biomembranes, extrapolation, fluid oscillations, hydrodynamics, lipid bilayers, molecular dynamics method, nanofluidics, neutron spin echo
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:kth:diva-27123DOI: 10.1063/1.3478998ISI: 000282047500042Scopus ID: 2-s2.0-77956954404OAI: oai:DiVA.org:kth-27123DiVA: diva2:375362
Funder
Swedish e‐Science Research Center
Note
QC 20101208Available from: 2010-12-08 Created: 2010-12-06 Last updated: 2017-12-11Bibliographically approved
In thesis
1. Molecular Dynamics Simulations of Fluid Lipid Membranes
Open this publication in new window or tab >>Molecular Dynamics Simulations of Fluid Lipid Membranes
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Lipid molecules form thin biological membranes that envelop all living cells, and behave as two-dimensional liquid sheets immersed in bulk water. The interactions of such biomembranes with their environment lay the foundation of a plethora of biological processes rooted in the mesoscopic domain - length scales of 1-1000 nm and time scales of 1-1000 ns. Research in this intermediate regime has for a long time been out of reach for conventional experiments, but breakthroughs in computer simulation methods and scattering experimental techniques have made it possible to directly probe static and dynamic properties of biomembranes on these scales.

Biomembranes are soft, with a relatively low energy cost of bending, and are thereby influenced by random, thermal fluctuations of individual molecules. Molecular dynamics simulations show how in-plane (density fluctuations) and out-of-plane (undulations) motions are intertwined in the bilayer in the mesoscopic domain. By novel methods, the fluctuation spectra of lipid bilayers can be calculated withdirect Fourier analysis. The interpretation of the fluctuation spectra reveals a picture where density fluctuations and undulations are most pronounced on different length scales, but coalesce in the mesoscopic regime. This analysis has significant consequences for comparison of simulation data to experiments. These new methods merge the molecular fluctuations on small wavelengths, with continuum fluctuations of the elastic membrane sheet on large wavelengths, allowing electron density profiles (EDP) and area per lipid to be extracted from simulations with high accuracy.

Molecular dynamics simulations also provide insight on the small-wavelength dynamics of lipid membranes. Rapidly decaying density fluctuations can be described as propagating sound waves in the framework of linearized hydrodynamics, but there is a slow, dispersive, contribution that needs to be described by a stretched exponential over a broad range of length- and time scales - recent experiments suggest that this behavior can prevail even on micrometer length scales. The origin of this behavior is discussed in the context of fluctuations of the bilayer interface and the molecular structure of the bilayer itself. Connections to recent neutron scattering experiments are highlighted.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2011. xi, 90 p.
Series
Trita-FYS, ISSN 0280-316X ; 2011:48
Keyword
biological fluid dynamics, biomembranes, hydrodynamics, lipid bilayers, molecular dynamics, neutron spin echo
National Category
Biophysics
Identifiers
urn:nbn:se:kth:diva-42586 (URN)978-91-7501-125-7 (ISBN)
Public defence
2011-11-04, FB42, AlbaNova Universitetscentrum, Stockholm, 10:00 (English)
Opponent
Supervisors
Projects
Modelling of biological membranes
Funder
Swedish e‐Science Research Center
Note
QC 20111014Available from: 2011-10-14 Created: 2011-10-11 Last updated: 2012-05-24Bibliographically approved

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