Spin-lattice relaxation times of phospholipid aminoxyl spin labels in cardiolipin–cytochrome c bilayers: a pulse saturation–recovery EPR study

A pulsed EPR saturation-recovery method has been used to measure spin-lattice relaxation times (T1, ) for aminoxyl spin labels in cardiolipin bilayers, with and without cytochrome c. The relaxation time for each spin label was determined at various positions in the bilayer, which included the bilayer surface and three positions within the hydrophobic interior in the membrane, as well as a position close to the glycerol backbone of phospholipids in the bilayer. A dynamic profile for the hydrocarbon chains in bilayers was found in agreement with results from other techniques. On addition of cytochrome c to cardiolipin bilayers, changes in the spin-lattice relaxation time and order parameter were observed around the unsaturated region in the cardiolipin acyl chains. Supported by complementary lineshape analysis, these effects were interpreted as changes in the molecular dynamics around the double bonds in the acyl chains of cardiolipin bilayers, induced upon the binding of cytochrome c.