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% D 2O. 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°C A = 63.0 Å 2, whereas for DOPC at 30°C A = 67.4 Å2, with estimated uncertainties of 1 Å 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.
Bibliographical noteFunding Information:
The authors gratefully acknowledge the access to the instruments at the National Institute of Standards and Technology (NIST) Center for Neutron Research and the Cornell High Energy Synchrotron Source (CHESS, funded by National Science Foundation grant DMR-0225180). Computational work was supported by the Minnesota Supercomputer Institute and National Science Foundation. Support for J.F.N. and much of the x-ray data were obtained under National Institutes of Health grant GM 44976.
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