Short range order of hydrocarbon chains in fluid phospholipid bilayers studied by x-ray diffraction from highly oriented membranes

We present a study of the short range ordering of hydrocarbon chains in phospholipid bilayers. The x-ray peak associated with the hydrocarbon chains has been probed by means of reciprocal space mappings. Using 20 keV undulator radiation and samples of negligible mosaicity ( orientational disorder), the intensity distribution is probed as a function of two coordinates, the momentum transfer parallel and perpendicular to the bilayer, over a wide range and at high resolution. Structural results are obtained concerning the distribution of tilted segments, the correlation length and the radial distribution function of the quasi two-dimensional liquid structure. A comparison is made with published molecular dynamics data ( H. Heller, M. Schaefer, and K. Schulten. 1993. J. Phys. Chem. 97: 8343 - 8360) by direct Fourier transformation of the atomic coordinates. The exact prefactor in the relationship between interchain distance and peak position is derived.