Membrane curvature and cholesterol effects on lipids packing and spin-labelled lipids conformational distributions

Abstract

Nitroxide spin-labelled lipid analogues are often used to study model membrane properties using EPR spectroscopy. Whereas in liquid phase membranes the spin label assumes, on average, its putative location, in gel phases and frozen membrane, depending on its position along the acyl chain, it may exhibit a different average location. Here we used ²H three-pulse Electron Spin Echo Envelope Modulation (ESEEM) of phospholipid spin probes, combined with various deuteration schemes to detect the effect of the model membrane curvature and cholesterol on vertical migrations of the spin label. We compared large and small unilamellar 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) vesicles with and without cholesterol (10%). The vertical displacement of the spin label was manifested as an apparently flat trans-membrane profile of water concentration and of label proximity to the head group choline. The spin-label propensity to migrate was found to increase with vesicle curvature and decrease in the presence of cholesterol. This in turn reflects the effect of packing and ordering of the membrane lipids. The results show that in curved vesicles lacking cholesterol, the label attached to carbon 16 may travel as far high along the membrane normal as the location of the label on carbon 5, due to the presence of U-shaped lipid conformations. This phenomenon must be taken into account when using spin-labelled lipids as membrane depth markers or to trace trans-membrane profiles.

Keywords:

• model membranes
• curvature
• EPR
• ESEEM
• spin-label conformational distributions

Supplementary information available

Demonstration of 3P-ESEEM data analysis, LPSVD reconstruction of the three-pulse ESEEM first modulation, best-fit simulations of ESEEM results, vesicles cryo-TEM images, ESEEM simulation χ² plots and best-fit traces, and comparison of ESEEM fits having different χ² values.

Acknowledgements

This research was supported by the Israel Science Foundation (ISF) and was made possible in part by the historic generosity of the Harold Perlman family. The electron microscopy studies were conducted at the Irving and Cherna Moskowitz Center for Nano and Bio-Nano Imaging at the Weizmann Institute of Science. D.G. holds the Erich Klieger Professorial Chair in Chemical Physics. The authors thank Ilia Kaminker for critical reading of the manuscript.

Notes

Since no replicates were prepared for this sample, the error percent was taken to be the same as for the bath-sonicated samples.