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Biophysical Journal 84:3767-3776 (2003)
© 2003 The Biophysical Society

Collective Chain Dynamics in Lipid Bilayers by Inelastic X-Ray Scattering

Thomas M. Weiss *, Poe-Jou Chen {dagger}, Harald Sinn {ddagger}, Ercan E. Alp {ddagger}, Sow-Hsin Chen {dagger} and Huey W. Huang *

* Department of Physics and Astronomy, Rice University, Houston, Texas; {dagger} Department of Nuclear Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts; and {ddagger} Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois

Correspondence: Address reprint requests to Dr. Huey W. Huang, Dept. of Physics and Astronomy, Rice University, Houston, TX 77251-1892. Tel.: 713-348-4899; Fax: 713-348-4150; E-mail: hwhuang{at}rice.edu.

We investigated the application of inelastic x-ray scattering (IXS) to lipid bilayers. This technique directly measures the dynamic structure factor S(q,{omega}) which is the space-time Fourier transform of the electron density correlation function of the measured system. For a multiatomic system, the analysis of S(q,{omega}) is usually complicated. But for multiple bilayers of lipid, S(q,{omega}) is dominated by chain-chain correlations within individual bilayers. Thus IXS provides a unique probe for the collective dynamics of lipid chains in a bilayer that cannot be obtained by any other method. IXS of dimyristoyl phosphatidylcholine and dimyristoyl phosphatidylcholine + cholesterol at two different concentrations were measured. S(q,{omega}) was analyzed by three-mode hydrodynamic equations, including a thermal diffusive mode and two propagating acoustic modes. We obtained the dispersion curves for the phonons that represent the collective in-plane excitations of lipid chains. The effect of cholesterol on chain dynamics was detected. Our analysis shows the importance of having a high instrument resolution as well as the requirement of sufficient signal-to-noise ratio to obtain meaningful results from such an IXS experiment. The requirement on signal-to-noise also applies to molecular dynamics simulations.




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