Ahmadi, D, Thompson, KC, García Sakai, V, Schweins, R, Moulin, M, Haertlein, M, Strohmeier, GA, Pichler, H, Forsyth, VT, Barlow, DJ, Lawrence, MJ and Foglia, F (2022) Nanoscale Structure and Dynamics of Model Membrane Lipid Raft Systems, Studied by Neutron Scattering Methods. Frontiers in Physics, 10. ISSN 2296-424X

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Quasi-elastic neutron scattering (QENS) and small angle neutron scattering (SANS), in combination with isotopic contrast variation, have been used to determine the structure and dynamics of three-component lipid membranes, in the form of vesicles, comprising an unsaturated [palmitoyl-oleoyl-phosphatidylcholine (POPC) or dioleoyl-phosphatidylcholine (DOPC)], a saturated phospholipid (dipalmitoyl-phosphatidylcholine (DPPC)), and cholesterol, as a function temperature and composition. SANS studies showed vesicle membranes composed of a 1:1:1 molar ratio of DPPC:DOPC:cholesterol and a 2:2:1 molar ratio of DPPC:POPC:cholesterol phase separated, forming lipid rafts of similar to 18 and similar to 7 nm diameter respectively, when decreasing temperature from 308 to 297 K. Phase separation was reversible upon increasing temperature. The larger rafts observed in systems containing DOPC are attributed to the greater mis-match in lipid alkyl chains between DOPC and DPPC, than for POPC and DPPC. QENS studies, over the temperature range 283-323K, showed that the resulting data were best modelled by two Lorentzian functions: a narrow component, describing the "in-plane" lipid diffusion, and a broader component, describing the lipid alkyl chain segmental relaxation. The overall "in-plane" diffusion was found to show a significant reduction upon increasing temperature due to the vesicle membranes transitioning from one containing rafts to one where the component lipids are homogeneously mixed. The use of different isotopic combinations allowed the measured overall reduction of in-plane diffusion to be understood in terms of an increase in diffusion of the saturated DPPC lipid and a corresponding decrease in diffusion of the unsaturated DOPC/POPC lipid. As the rafts are considered to be composed principally of saturated lipid and cholesterol, the breakdown of rafts decreases the exposure of the DPPC to cholesterol whilst increasing the exposure of cholesterol to unsaturated lipid. These results show the sensitivity of lipid diffusion to local cholesterol concentration, and the importance of considering the local, rather that the global composition of a membrane when understanding the diffusion processes of lipids within the membrane. The novel combination of SANS and QENS allows a non-intrusive approach to characterize the structure and dynamics occurring in phase-separated model membranes which are designed to mimic the lateral heterogeneity of lipids seen in cellular membranes-a heterogeneity that can have pathological consequences.

Item Type: Article
Additional Information: © 2022 Ahmadi, Thompson, García Sakai, Schweins, Moulin, Haertlein, Strohmeier, Pichler, Forsyth, Barlow, Lawrence and Foglia. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
Uncontrolled Keywords: QENS; SANS; lipid rafts; lipid; multi-component systems
Divisions: Faculty of Natural Sciences > School of Chemical and Physical Sciences
Depositing User: Symplectic
Date Deposited: 01 Jun 2022 11:23
Last Modified: 09 Jun 2022 15:24
URI: https://eprints.keele.ac.uk/id/eprint/10983

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