Using Neutron Spectroscopy to Study Collective Dynamics of Biological and Model Membrane Systems
Maikel C. RheinstŠdter,
Department of Physics and Astronomy and
University of Missouri Research Reactor
University of Missouri-Columbia, Columbia, MO 65211, USA
While most spectroscopic techniques, as, e.g., nuclear magnetic resonance or dielectric
spectroscopy probe macroscopic responses, neutron and within some restrictions
also x-ray scattering experiments give the unique access to microscopic dynamics
at length scales of intermolecular or atomic distances. Only recently, it has become
possible to study collective dynamics of planar lipid bilayers using neutron
spectroscopy techniques [1]. We determined the dispersion relation of the
coherent fast picosecond density fluctuations on nearest neighbor distances
of the phospholipid acyl chains in the gel and in the fluid phases of a DMPC
bilayer and could shed light on the evolution of structure and dynamics in the
range of the gel-fluid main phase transition.
The spectrum of fluctuations in biomimetic and biological membranes covers a large
range of time and length scales, ranging from the long wavelength undulation and
bending modes of the bilayer with typical relaxation times of nanoseconds and
lateral length scales of several hundred lipid molecules to the short wavelength
density fluctuations in the picosecond range on nearest neighbor distances of
lipid acyl chains. By combining different neutron scattering techniques, namely
three-axis, backscattering and spin-echo spectroscopy, we present measurements of
short and long wavelength collective fluctuations in biomimetic and biological
membranes in a large range in momentum and energy transfer, covering time scales
from about 0.1ps to almost 1¼s and length scales from 3
to about 0.1¼m [1-4]. Because of optimized setups and sample preparation, inelastic
neutron scattering experiments supply for the first time sufficiently strong
coherent inelastic signals for quantitative analysis. The measurements offer
a large window of length and time scales to test and refine theoretical
models of dynamics of biomimetic and biological membranes. From a smectic
hydrodynamic theory, the long wavelength dispersion relation give direct
access to the elasticity parameters of the membranes in the fluid phase [3],
i.e., the bilayer bending rigidity º and the compressional modulus B of the
stacks.
The overall objective of this project is to establish dynamics-function
relationships in artificial and biological membranes to relate in
particular the collective dynamics, i.e., phonons, to key functions of the
membranes, as, e.g., transport processes, pore opening, and membrane fusion.
[1] M.C. RheinstŠdter, C. Ollinger, G. Fragneto, F. Demmel, T. Salditt,
Phys. Rev. Lett. 93, 108107 (2004).
[2] Maikel C. RheinstŠdter, Tilo Seydel, Franz Demmel, Tim Salditt,
Phys. Rev. E 71, 061908 (2005).
[3] Maikel C. RheinstŠdter, Wolfgang HŠu§ler, Tim Salditt, Phys. Rev. Lett. 97, 048103 (2006).
[4] Maikel C. RheinstŠdter, Tilo Seydel, Tim Salditt, submitted to PRE, cond-mat/0607514.
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