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

Abstract

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(2)O. 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 A(2), whereas for DOPC at 30 degrees C A = 67.4 A(2), with estimated uncertainties of 1 A(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.

Molecular Structures of Fluid Phosphatidylethanolamine Bilayers Obtained from Simulation-to-Experiment Comparisons and Experimental Scattering Density Profiles

“…Utilizing models then offers the advantage of combining information from different experiments, including simulations. 62 It is worth noting that the large discrepancies observed previously when standalone X-ray and neutron scattering data were evaluated separately 20 could be attributed to the fact that X-ray scattering is sensitive to the position of the phospholipid headgroup, while neutron scattering data accurately reflect the location of the bilayer/ water interface. These two results then lead to disparate structural parameters when used in conjunction with less than perfect models of the bilayer.…”

“…In order to improve the robustness of membrane structural parameters, we proposed to combine the simultaneous analysis of X-ray and neutron scattering data, the use of independently obtained volumetric data, and a bilayer parsing scheme, determined, in part, through simulation. 20 SDP Model for PEs. Data analysis is carried out using a single structural model which underlies all of the various scattering density profiles, or the so-called SDP model.…”

“…Data analysis is carried out using a single structural model which underlies all of the various scattering density profiles, or the so-called SDP model. The original model developed for PC 20 lipids has been extended recently for PG, 67 PS, 36 and ether 33 lipids. We continue this development by extending the SDP model to PE bilayers.…”

Cation-containing lipid membranes – experiment and md simulations

“…1 corresponds to the distribution of water whose amplitude achieves half maximum at ~21.6 Å. This value also indicates the lipid-water interface (i.e., the Gibbs dividing surface), or half the bilayer thickness D B (Kučerka et al, 2008a …”

“…Typically, the resultant distribution functions from such an analysis are smooth curves. In addition, we combined the distribution functions of individual atoms (united atoms) into components defined by the scattering density profile (SDP) model, which described the studied DPPC bilayers (Kučerka et al, 2008a). The probability distributions of the different components shown in Fig.…”