title: Biochemical profiles of membranes from x-ray and neutron diffraction.
abstract: X-ray and neutron diffraction methods provide some information about the distribution of mass in biological membranes and lipid-water systems. Scattering density profiles obtained from these systems, however, usually are not directly interpretable in terms of the relative amounts of chemical constituents (e.g., lipid, protein, and water) as a function of position in the membrane. We demonstrate here that the combined use of x-ray and neutron-scattering profiles, together with information on the total amounts of each of the major membrane components, are sufficient to calculate unambiguously the volume fractions of these components at well-defined regions of the lamellar unit. Three cases are considered: a calculated model membrane pair, dipalmitoylphosphatidylcholine-water multilayers, and rabbit sciatic nerve myelin. For the model system, we discuss the limitations imposed by finite resolution in the diffraction patterns. For the lipid-water multilayers, we calculate water volume fractions in the hydrocarbon tail, lipid headgroup, and interlamellar regions; estimates of these values by various methods are in good agreement with our results. For the nerve myelin, we predict new results for the distribution of protein through the membrane.
mesh_terms: D000818:Animals; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D008051:Lipid Bilayers; D008433:Mathematics; D008954:Models, Biological; D009186:Myelin Sheath / Q000648:ultrastructure*; D009502:Neutrons; D011817:Rabbits; D012584:Sciatic Nerve / Q000033:anatomy & histology; D014961:X-Ray Diffraction
chemical_list: D008051:Lipid Bilayers