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Lipid Bilayer Structure Determined by the Simultaneous Analysis of Neutron and X-Ray Scattering Data

Norbert Kuerka ^* , John F. Nagle , Jonathan N. Sachs , Scott E. Feller ^¶, Jeremy Pencer ^||, Andrew Jackson ^** and John Katsaras ^*  

^* Canadian Neutron Beam Centre, National Research Council, Chalk River, Ontario K0J 1J0, Canada; Department of Physical Chemistry of Drugs, Faculty of Pharmacy, Comenius University, 832 32 Bratislava, Slovakia; Department of Physics, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213; Biomedical Engineering, University of Minnesota, Minneapolis, Minnesota 55455; ^¶ Department of Chemistry, Wabash College, Crawfordsville, Indiana 47933; ^|| Atomic Energy of Canada, Chalk River Laboratories, Chalk River, Ontario K0J 1J0, Canada; ^** National Institute of Standards and Technology, Center for Neutron Research, Gaithersburg, Maryland 20899; Guelph-Waterloo Physics Institute and Biophysics Interdepartmental Group, University of Guelph, Guelph, Ontario N1G 2W1, Canada; and Department of Physics, Brock University, St. Catharines, Ontario L2S 3A1, Canada

Correspondence: Address reprint requests to Dr. Norbert Kuerka, National Research Council of Canada, Canadian Neutron Beam Centre, Chalk River Laboratories, Stn. 18, Chalk River, ON K0J 1P0, Canada. Tel.: 613-584-8811 ext. 4195; E-mail: norbert.kucerka{at}nrc.gc.ca.

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°C A = 63.0 Ų, whereas for DOPC at 30°C A = 67.4 Ų, with estimated uncertainties of 1 Ų. 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.