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Register a new userProbing Dynamics at Interfaces: Molecular Motions in Lipid Bilayers studied by Neutron Backscattering
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Added by
Maikel Rheinstaedter
Publication date
2004
fields
Physics
and research's language is
English
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Lipid membranes in a physiological context cannot be understood without taking into account their mobile environment. Here, we report on 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). This technique allows discriminating the Q-dependent onset of mobility and provides a benchmark test regarding the feasibility of dynamical neutron scattering investigations on these sample systems. Apart from freezing of the lipid acyl-chains, we could observe a second freezing temperature that we attribute to the hydration water in between the membrane stacks. The freezing is lowered several degrees as compared to (heavy) bulk water.
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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.
Inelastic neutron and X-ray scattering experiments on surfaces and interfaces are a challenging topic in modern physics. Particular interest arises regarding surfaces and interfaces of soft matter and biological systems. We review both neutron and x-ray spectroscopic techniques with view at their applicability to these samples. We discuss the different methods, namely neutron triple-axis, backscattering and spin-echo spectroscopy as well as x-ray photon correlation spectroscopy, in the context of planar lipid membrane models as an example. By the combination of the different methods, a large range in momentum and energy transfer is accessible.
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. Wave vector resolved quasi-elastic neutron scattering (QENS) 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 both by energy resolved and energy integrated in-situ diffraction. From a combined analysis of the inelastic data in the energy and time domain, the respective 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.
We have studied the collective short wavelength dynamics in deuterated DMPC bilayers by inelastic neutron scattering. The corresponding dispersion relation $hbaromega$(Q) is presented for the gel and 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 than that reported in a preceding inelastic x-ray scattering study [Phys. Rev. Lett. {bf 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 non-dispersive (optical) mode predicted by Molecular Dynamics (MD) simulations [Phys. Rev. Lett. {bf 87}, 238101 (2001)].
We use a long, all-atom molecular dynamics (MD) simulation combined with theoretical modeling to investigate the dynamics of selected lipid atoms and lipid molecules in a hydrated diyristoyl-phosphatidylcholine (DMPC) lipid bilayer. From the analysis of a 0.1 $mu$s MD trajectory we find that the time evolution of the mean square displacement, [delta{r}(t)]^2, of lipid atoms and molecules exhibits three well separated dynamical regions: (i) ballistic, with [delta{r}(t)]^2 ~ t^2 for t < 10 fs; (ii) subdiffusive, with [delta{r}(t)]^2 ~ t^{beta} with beta<1, for 10 ps < t < 10 ns; and (iii) Fickian diffusion, with [delta{r}(t)]^2 ~ t for t > 30 ns. We propose a memory function approach for calculating [delta{r}(t)]^2 over the entire time range extending from the ballistic to the Fickian diffusion regimes. The results are in very good agreement with the ones from the MD simulations. We also examine the implications of the presence of the subdiffusive dynamics of lipids on the self-intermediate scattering function and the incoherent dynamics structure factor measured in neutron scattering experiments.
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