Toward Bicelle Stability with Ether-Linked Phospholipids: Temperature, Composition, and Hydration Diagrams by2H and31P Solid-State NMR

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Abstract
Phosphorus and deuterium wide line NMR was used to determine diagrams of binary mixtures of 1,2-di-O-tetradecyl-sn-glycero-3-phosphocholine (DIOMPC) and 1,2-di-O-hexyl-sn-glycero-3-phosphocholine (DIOHPC) ether-phospholipids. By varying the hydration, h, the temperature, T, and the mole fraction, X, of long-chain ether-phospholipids, we delineated the conditions for which such systems are oriented by the magnetic field, in the presence of 100 mM KCl. The 3D domain is found for X = 62−90%, T = 27−50 °C, and h = 70−98%. At 80% hydration, the domain shape (X = 70−90% and T = 27−42 °C) is close to that already observed for ester-phospholipids mixtures (Raffard, G.; Steinbruckner, S.; Arnold, A.; Davis, J. H.; Dufourc, E. J. Langmuir 2000, 16, 7655−7662) where disc-shaped bicelles of 300−600 Å have been found by electron microscopy (Arnold, A.; Labrot, T.; Oda, R.; Dufourc, E. J. Biophys. J.2002, 83, 2667−2680). Systems made of ether-linked lipids are much more stable on time and acidic conditions than those made of ester lipids. Assuming that the disc-shaped species are also found with ether lipids, their diameter as determined from integration of phosphorus NMR lines ranges from 240 to 440 Å ± 10%; it is generally independent of hydration and temperature but decreases with decreasing long-chain lipid content, X. The structure and the dynamics of water in the DIOMPC-DIOHPC were characterized by 2H NMR. Water exchanges between the membrane surface where it is bound and a bulk isotropic pool lead to an average ordered state for temperatures in the bicelle region and above, thus offering a larger thermal span for structural studies of dissolved molecules.

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