We report a high energy-resolution neutron backscattering study, combined with in-situ diffraction, to investigate slow molecular motions on nanosecond time scales in the fluid phase of phospholipid bilayers of 1,2-dimyristoyl-sn-glycero-3-phoshatidylcholine (DMPC) and DMPC/40% cholesterol (wt/wt). A cooperative structural relaxation process was observed. From the in-plane scattering vector dependence of the relaxation rates in hydrogenated and deuterated samples, combined with results from a 0.1 µs long all atom molecular dynamics simulation, it is concluded that correlated dynamics in lipid membranes occurs over several lipid distances, spanning a time interval from pico-to nanoseconds.PACS numbers: 87.16.dj, 87.14.Cc, 83.85.Hf, 87.15.ap, 83.10.Mj It is speculated that atomic and molecular motions in regions of biomolecular systems with strong local interactions are highly correlated on certain range of time and length scales [1,2]. In proteins intra-protein correlations are believed to be essential for their biological functioning, such as protein folding, domain motion and conformational changes. Very recently, inter-protein correlations in protein crystals and also membranes have been reported from experiment and simulation [3,4]. Experimental and computational effort has been invested to study collective molecular motions in phospholipid model membranes [5,6,7,8] to understand the possible impact on physiological and biological functions of the bilayers, such as transport processes [9], and eventually their implication on function of membrane-embedded proteins. While fast (picosecond) propagating collective microscopic fluctuations in the plane of the bilayer can be understood as sound waves [10,11], the slow (nanosecond) in-plane mesoscopic fluctuations (undulations) are governed by the elasticity properties of the bilayers [12].We studied dynamical modes at nearest neighbor distances of the lipid molecules using the neutron backscattering technique [13]. These modes are too fast to be accessed by x-ray photon correlation spectroscopy and the lateral length scales involved are too small to be resolved by dynamic light scattering or the neutron spinecho technique. Selective deuteration was used to discriminate relaxations due to collective molecular motions from relaxations arising from localized, single molecule excitations. In this Letter, we examine results of inelastic neutron scattering experiments that demonstrate the existence of slow coherent motion of lipid molecules in the fluid phase of phospholipid bilayers. From the inplane scattering vector dependence (q || ) of the measured relaxation rates, combined with results of a 0.1 µs long all atom molecular dynamics (MD) simulation, we find that the cooperative structural dynamics in lipid membranes occurs over several lipid distances, spanning a time interval from pico-to nanoseconds.The experiments were carried out at the cold neutron backscattering spectrometer IN16 [14] at the Institut Laue-Langevin (ILL) with an energy resolution of about 0....
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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