Abstract
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. (Kučerka et al. 2012; Kučerka et al.
2008). 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 in these models.
In particular, it is recognized that standalone data do not contain enough information to fully
resolve the structure of inherently disordered fluid bilayers, and therefore may not provide a
robust determination of bilayer structural 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 sets, as
well as the implementation of local volume conservation in 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.
