Lipid bilayer structure determined by the simultaneous analysis of neutron and x-ray scattering data

Quantitative structures were obtained for the fully hydrated fluid phases of dioleoylphosphatidylcholine ( DOPC) and dipalmitoylphosphatidylcholine ( DPPC) bilayers by simultaneously analyzing x-ray and neutron scattering data. The neutron data for DOPC included two solvent contrasts, 50% and 100% D2O. For DPPC, additional contrast data were obtained with deuterated analogs DPPC_d62, DPPC_d13, and DPPC_d9. For the analysis, we developed a model that is based on volume probability distributions and their spatial conservation. The model's design was guided and tested by a DOPC molecular dynamics simulation. The model consistently captures the salient features found in both electron and neutron scattering density profiles. A key result of the analysis is the molecular surface area, A. For DPPC at 50 degrees C A 63.0 angstrom(2), whereas for DOPC at 30 degrees C A 67.4 angstrom(2), with estimated uncertainties of 1 angstrom(2). Although A for DPPC agrees with a recently reported value obtained solely from the analysis of x-ray scattering data, A for DOPC is almost 10% smaller. This improved method for determining lipid areas helps to reconcile long-standing differences in the values of lipid areas obtained from stand-alone x-ray and neutron scattering experiments and poses new challenges for molecular dynamics simulations.