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.