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ILL’s D16 instrument enables greater understanding of the unique interaction characteristics of glycolipid membranes. April 2017

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Chemical structures of a PC lipid (a) and of the glycolipid DGDG (b) as representatives of two fundamentally different lipid classes found in nature: Lipids with a headgroup chemistry dominated by one large electric dipole and lipids whose headgroups comprise multiple small electric dipoles in the form of OH groups. Both classes are schematically illustrated below the chemical structures. Dipoles are indicated by arrows. (c,d) Simulation snapshots of interacting DLPC and DGDG membranes, respectively, both at a large separation of Dw=2.3 nm. With periodic boundary conditions in all three directions, the simulations represent a periodic stack of membranes with adjustable hydration level. The simulation boxes are indicated with bright rectangles. For illustration, water molecules are only shown in the lower half of the box.

ILL’s D16 instrument enables greater understanding of the unique interaction characteristics of glycolipid membranes Glycolipids are essential constituents of membrane systems that naturally occur as densely packed membrane stacks, like in chloroplasts where photosynthesis takes place. A recent study conducted at the Institut Laue-Langevin (ILL) combined neutron diffraction experiments and atomistic molecular dynamics simulations to comparatively investigate the interaction mechanisms of glycolipid and phospholipid membranes.

 

Results explained the tight cohesion between glycolipid membranes at their swelling limit and their unique interaction characteristics in contrast to those of phospholipids, which are essential for the biogenesis of photosynthetic membranes. Structurally stable and densely packed membrane systems, such as thylakoids – the photosynthetic membranes in plants – comprise a high amount of glycolipids displaying multiple hydroxyl groups. Mono- (MGDG) and di-galactosyldiacylglycerol (DGDG) are uncharged glycolipids that represent over 80 per cent of the lipids in thylakoid lipid extracts.

Despite the similarities between glycolipids and phospholipids, the interaction characteristics of glycolipid and phospholipid membranes are different. This has been investigated extensively in experimental and theoretical studies. Recent research on membrane stacks reconstituted from natural thylakoid lipid extracts found that water uptake significantly depends on the lipid composition of membranes. Experimentally, repulsion between DGDG membranes is of much shorter range compared to that between phospholipid membranes, which exhibit swelling mechanisms of longer range. In excess water, the balance between DGDG attractive and repulsive forces governing the equilibrium separation between membranes.

A recent study conducted by an international collaboration of the ILL, Helmholtz-Zentrum Berlin für Materialien und Energie, Freie Universität Berlin, University of Groningen, Biosciences and Biotechnology Institute of Grenoble, and Max Planck Institute of Colloids and Interfaces used neutron diffraction experiments and molecular dynamics simulations to compare the interaction mechanisms of glycolipid and phospholipid membranes. The simulations quantitatively reproduced the experimentally observed [1], pressure-versus-distance curves of phospholipid and glycolipid membrane stacks. The study, published in Nature Communications and titled ‘Tight cohesion between glycolipid membranes results from balanced water-headgroup interactions’ [2], has shown water uptake into densely-pack glycolipid membrane stacks is solely driven by the hydrogen bond balance involved in non-ideal water/sugar mixing.

The differences in interaction mechanisms of glycolipid and phospholipid membranes this study found could be of particular relevance for protein-free regions in thylakoids, which exhibit membrane separations in a broad rangeand in the genesis of these membranes before protein complexes are incoporated. Whilst phospholipid membranes exhibit significant hydration repulsion, glycolipids’ hydration repulsion diminishes in this broad distance range, suggesting that protein-protein interactions are not the sole factor governing thylakoid structure. Researchers in this study have speculated that membrane interactions may therefore be relevant even for the evolution of lipid headgroup chemistry.

The neutron diffraction experiments were conducted on ILL’s D16 instrument; a small momentum transfer diffractometer upgraded recently. The two-circle diffractometer has a variety of applications across biology and biophysics, colloids and surfactants, polymer physics, material science and surface science, and is devoted to investigating partially ordered structures such as stacked membranes.

Bruno Demé, D16 Instrument Scientist and co-author of this study, commented: “The neutron diffraction experiments using D16 enabled us to sift through the similarities between phospholipids and glycolipids, and reveal the intricate differences between their membrane interaction mechanisms. We were able to conclude that the headgroup design has a great impact on membrane interactions, resulting in glycolipid and phospholipid membrane interactions differing in terms of water uptake and balance between attractive and repulsive forces. Headgroup chemistry evidently governs the interaction between lipid membranes, including the strength, range and mechanisms of repulsion. Our study has provided the physical explanation for the stronger cohesion of glycolipid membranes and their ability to form stable and regular membrane stacks.”

 


[1] Contribution of galactoglycerolipids to the 3-dimensional architecture of thylakoids, B. Demé, C. Cataye, M.A. Block, E. Maréchal and J. Jouhet, FASEB Journal 28 (2014) 3373-3383.

 

[2] Tight cohesion between glycolipid membranes results from balanced water–headgroup interactions, M. Kanduč, A. Schlaich, A.H. de Vries, J. Jouhet, E. Maréchal, B. Demé, R.R. Netz, and E. Schneck, Nature Communications 8 (2017) 14899. doi:10.1038/ncomms14899

 

 

 


Contact: Dr Bruno Demé, Scientist responsible for the D16 instrument, deme(at)ill.eu

Tel: +33 (0)4 76 20 73 11 | +33 (0)4 76 20 70 47