Effects of cholesterol or gramicidin on slow and fast motions of phospholipids in oriented bilayers.

Nuclear spin-lattice relaxation both in the rotating frame and in the laboratory frame is used to investigate the slow and fast molecular motions of phospholipids in oriented bilayers in the liquid crystalline phase. The bilayers are prepared from a perdeuterated phospholipid labeled with a pair of 19F atoms at the 7 position of the 2-sn acyl chain. Phospholipid-cholesterol or phospholipid-gramicidin interactions are characterized by measuring the relaxation rates as a function of the bilayer orientation, the locking field, and the temperature. Our studies show that cholesterol or gramicidin can specifically enhance the relaxation due to slow motions in phospholipid bilayers with correlation times tau s longer than 10(-8) sec. The perturbations of the geometry of the slow motions induced by cholesterol are qualitatively different from those induced by gramicidin. In contrast, the presence of cholesterol or gramicidin slightly suppresses the fast motions with correlation times tau f = 10(-9) to 10(-10) sec without significantly affecting their geometry. Weak locking-field and temperature dependences are observed for both pure lipid bilayers and bilayers containing either cholesterol or gramicidin, suggesting that the motions of phospholipid acyl chains may have dispersed correlation times.