Abstract
A general model for nuclear magnetic resonance (NMR) relaxation studies of fluid bilayer systems is introduced, combining a mesoscopic Brownian dynamics description of the bilayer with atomistic molecular dynamics (MD) simulations. An example is given for dipalmitoylphosphatidylcholine in 2H2O solvent and compared with the experiment. Experimental agreement is within a factor of 2 in the water relaxation rates, based on a postulated model with fixed parameters, which are largely available from the MD simulation. Relaxation rates are particularly sensitive to the translational diffusion of water perturbed by the interface dynamics and structure. Simulation results suggest that a notable deviation in the relaxation rates may follow from the commonly used small-angle approximation of bilayer undulation. The method has the potential to overcome the temporal and spatial limitations in computing NMR relaxation with atomistic MD, as well as the shortcomings of continuum models enabling a consistent description of experiments performed on a solvent lipid and added spin probes. This work opens for possibilities to understand relaxation processes involving systems such as micelles, multilamellar vesicles, red blood cells, and so forth at biologically relevant timescales in great detail.
Highlights
Lyotropic phases display complex dynamics involving, in addition to the intramolecular degrees of freedom, thermally excited interface modes that feature collective molecular motion of lipids and solvents.[1]
We report a proof-of-principle study of the 2H2O solvent relaxation and line shape, in which the shortcomings of atomistic MD15 are largely overcome by combining it with a mesoscopic Brownian dynamics (BD) model
For more than four decades, the dynamical information from nuclear magnetic resonance (NMR) relaxation studies on lipid bilayer systems, relevant on the biological timescale, has presented a challenging modeling topic.[9,10,12,29,34,48]. During this period, atomistic molecular dynamics (MD) simulation has only played a minor contribution in confirming the static properties for the high-field magnetization relaxation, whereas the slow timescale dynamics relevant for the transverse relaxation rate has not been studied at a quantitative level of accuracy, in general.[14,15]
Summary
Lyotropic phases display complex dynamics involving, in addition to the intramolecular degrees of freedom (e.g., molecular rotation and vibrations), thermally excited interface modes that feature collective molecular motion of lipids and solvents.[1]. A spatially discretized bilayer sheet is used to simulate a time-dependent interface,[23] which allows for interface-correlated translational dynamics.[17] With a timescale separation in the relaxation model, all of the fast molecular processes are accounted for in atomistic detail This provides a close connection to both MD simulations and continuum models, avoiding, the approximations of the latter.[12] with the present trajectory-based method, generalization to the nonperturbative Liouville formalism is readily achievable, as has been illustrated with a water relaxation model with a trajectory length in the range of milliseconds.[20].
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