Global small-angle scattering data analysis of inverted hexagonal phases.

Moritz P K Frewein,Michael Rumetshofer,Georg Pabst

doi:10.1107/s1600576719002760

Moritz P K Frewein, Michael Rumetshofer + Show 1 more

Open Access

https://doi.org/10.1107/s1600576719002760

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Abstract

A global analysis model has been developed for randomly oriented, fully hydrated, inverted hexagonal (HII) phases formed by many amphiphiles in aqueous solution, including membrane lipids. The model is based on a structure factor for hexagonally packed rods and a compositional model for the scattering length density, enabling also the analysis of positionally weakly correlated HII phases. Bayesian probability theory was used for optimization of the adjustable parameters, which allows parameter correlations to be retrieved in much more detail than standard analysis techniques and thereby enables a realistic error analysis. The model was applied to different phosphatidylethanolamines, including previously unreported HII data for diC14:0 and diC16:1 phosphatid-yl-ethanolamine. The extracted structural features include intrinsic lipid curvature, hydrocarbon chain length and area per lipid at the position of the neutral plane.

Highlights

Elastic small-angle scattering (SAS) techniques are unrivaled for providing detailed structural insight into aggregates formed by amphiphiles in aqueous solutions (Glatter, 2018)
The intrinsic lipid curvature C0 is given by the negative inverse of the curvature radius, À1/R0, of an unstressed monolayer at the position of the neutral plane, which corresponds to the location where molecular bending and stretching modes are decoupled (Kozlov & Winterhalter, 1991)
This constraint can lead to some systematic deviations from the experimental data, and Bayesian model comparison can be used for choosing the appropriate model

Summary

Introduction

Elastic small-angle scattering (SAS) techniques are unrivaled for providing detailed structural insight into aggregates formed by amphiphiles in aqueous solutions (Glatter, 2018). In the field of membrane biophysics, significant efforts have been devoted to the development of SAS analysis methods for the biologically most relevant fluid lamellar phases, including domain-forming lipid mixtures and asymmetric lipid bilayers (Heberle & Pabst, 2017). HII phases are highly amenable systems for deriving intrinsic lipid curvatures by small-angle X-ray scattering (SAXS) (Leikin et al, 1996; Di Gregorio & Mariani, 2005; Kollmitzer et al, 2013; Chen et al, 2015), which is the main focus of the present contribution. Major interest in obtaining reliable C0 values originates from its contribution to the stored elastic energy strain in planar bilayers (Marsh, 2006), transmembrane protein function (Dan & Safran, 1998; Frewein et al, 2016) and overall membrane shape (Frolov et al, 2011)

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