Magnetic resonance studies of eukaryotic cells. III. Spin labeled fatty acids in the plasma membrane

XC Sarcoma, Vero and Aedes aegypti plasma membranes have been studied in viable cells and in purified membrane of XC Sarcoma cells by the spin label method. The temperature dependence of the order parameter of fatty acid spin labels is found to be linear in all three cells and membrane and shows no evidence of a lipid phase transition. The order parameter of the fatty acid labels substituted at the 5-position is shown to increase as a function of the cholesterol: phospholipid molar ratio in cells that have been studied to date. Cells attached to their growing surface are studied for the first time by electron paramagnetic resonance spectroscopy (EPR). The resulting spectra are anisotropic due to the non-spherical shape of the cells and show that these labels orient preferentially perpendicular to the cell surface. The viscosity of the extracted XC cell membrane is estimated to be 2.5 P from rotational correlation time measurements of the spin label 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO).