Magnetically aligned phospholipid bilayers with large q ratios stabilize magnetic alignment with high order in the gel and Lα phases

The magnetic alignment behavior of bicelles (magnetically alignable phospholipid bilayered membranes) as a function of the q ratio (1,2-dihexanoyl-sn-glycerol phosphatidylcholine/1,2-dimyristoyl-sn-glycerol phosphatidylcholine mole ratio) and temperature was studied by spin-labeled X-band electron paramagnetic resonance (EPR) spectroscopy and solid-state H-2 and P-31 NMR spectroscopy. Well-aligned bicelle samples were obtained at 45 degrees C for q ratios between 2.5 and 9.5 in both the EPR and NMR spectroscopic studies. The molecular order of the system, S-mol, increased as the q ratio increased and as the temperature decreased. For higher q ratios (>= 5.5), bicelles maintained magnetic alignment when cooled below the main phase transition temperature (similar to 30 degrees C when in the presence of lanthanide cations), which is the first time, to our knowledge, that bicelles were magnetically aligned in the gel phase. For the 9.5 q ratio sample at 25 degrees C, S-mol was calculated to be 0.83 (from H-2 NMR spectra, utilizing the isotopic label perdeuterated 1,2-dimyristoyl-sn-glycerol phosphatidylcholine) and 0.911 (from EPR spectra utilizing the spin probe 3 beta-doxyl-5 alpha-cholestane). The molecular ordering of the high q ratio bicelles is comparable to literature values of S-mol for both multilamellar vesicles and macroscopically aligned phospholipid bilayers on glass plates. The order parameter S-bicelle revealed that the greatest degree of bicelle alignment was found at higher temperatures and larger q ratios (S-bicelle = -0.92 for q ratio 8.5 at 50 degrees C).