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have been neglected relative to structure in simulation studies of membranes. We compared results from our simulation of a fluid phase bilayer with incoherent neutron scattering experiments that probed the motions of lipid H atoms on time scales of up to 100 ps. For the most part the simulation and experimental results were in good agreement, but the simulation predicted a slight anisotropy in the lipid motion that was not detected experimentally and overestimated the spatial dependence of the time scale of the slower of two dynamic processes resolved by the experiment. Having established a reasonable level of agreement between the simulation and neutron data, we examined the correspondence between the motions observed in the simulation and the dynamic models used to fit the experimental data. The center-of-mass motion and internal rearrangements are decoupled, and the former is well described as diffusion in a confined space (cylinder) on the 100 ps time scale, but not as Brownian motion. There were some significant discrepancies between the picture that emerged from the simulation and the models used to describe the internal motions. In particular, the simulation predicted a much weaker variation in the radii of the diffusion-in-a-sphere model and the involvement of more conformations in the acyl chain dynamics than the single kink assumed in the chain defect model.

Our simulation suggested that both the translational and rotational dynamics of water molecules in a fully hydrated, multilamellar lipid bilayer system depend on where the water molecules are located. As expected, both the translational and rotational mobilities of the ‘‘bulk’’ water molecules located in the middle of the interlamellar space are significantly greater than those of the ‘‘bound’’ water molecules located in the first solvation shell of the lipid polar groups (carbonyl, phosphate, and choline). The translational diffusion constants and the rotational rates of the bound water molecules increase in the order carbonyl-bound phosphate-bound choline-bound. On the time scale of tens of picoseconds, the bound water molecules exhibit largely isotropic translational motion, whereas the bulk water molecules diffuse approximately 50% faster in the plane of the membrane than out of the plane. None of the water molecules in the membrane–water system obey the Debye rotational diffusion model. The detailed picture of water dynamics near membranes derived from the simulation is in qualitative agreement with available data from NMR and neutron scattering experiments, but a more quantitative, model-independent comparison with neutron scattering would be useful for a better assessment.

ACKNOWLEDGMENTS

I am pleased to acknowledge that the simulation results presented in this chapter were obtained from calculations carried out in collaboration with Kechuan Tu, Mike Klein, and Kent Blasie. The calculations and fitting of the neutron scattering spectra benefited from discussions with Mounir Tarek. Financial support was provided by the School of Physical Sciences at the University of California at Irvine and a grant from the donors of The Petroleum Research Fund, administered by the American Chemical Society (ACS-PRF 33247-G7).

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