Model-based approaches for the determination of lipid bilayer structure from small-angle neutron and X-ray scattering data
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Some of our recent work has resulted in the detailed structures of fully hydrated, fluid phase phosphatidylcholine (PC) and phosphatidylglycerol (PG) bilayers. These structures were obtained from the joint refinement of small-angle neutron and X-ray data using the scattering density profile (SDP) models developed by Kučerka et al. (Biophys J 95:2356–2367, 2008; J Phys Chem B 116:232–239, 2012). In this review, we first discuss models for the standalone analysis of neutron or X-ray scattering data from bilayers, and assess the strengths and weaknesses inherent to these models. In particular, it is recognized that standalone data do not contain enough information to fully resolve the structure of naturally disordered fluid bilayers, and therefore may not provide a robust determination of bilayer structure parameters, including the much-sought-after area per lipid. We then discuss the development of matter density-based models (including the SDP model) that allow for the joint refinement of different contrast neutron and X-ray data, as well as the implementation of local volume conservation within the unit cell (i.e., ideal packing). Such models provide natural definitions of bilayer thicknesses (most importantly the hydrophobic and Luzzati thicknesses) in terms of Gibbs dividing surfaces, and thus allow for the robust determination of lipid areas through equivalent slab relationships between bilayer thickness and lipid volume. In the final section of this review, we discuss some of the significant findings/features pertaining to structures of PC and PG bilayers as determined from SDP model analyses.
KeywordsLipid bilayer Bilayer structure Area per lipid Bilayer thickness Molecular dynamics simulations Fluid phase
This work acknowledges the support of the office of Biological and Environmental Research (BER) at Oak Ridge National Laboratory’s (ORNL) Center for Structural Molecular Biology (CSMB) through the utilization of facilities supported by the US Department of Energy, managed by UT-Battelle, LLC under contract no. DE-AC05-00OR2275. Facilities located at the National Institute of Standards and Technology (NIST) are supported in part by the National Science Foundation under agreement no. DMR- 0944772. Facilities located at the Cornell High Energy Synchrotron Source (CHESS) are supported by the National Science Foundation and the National Institutes of Health/National Institute of General Medical Sciences under National Science Foundation award DMR-0225180. JK is supported by ORNL’s Program Development (PD) and Laboratory Directed Research and Development (LDRD) programs. RFS is supported by ORNL’s LDRD program.
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