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

Abstract

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 'IF 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 Ts 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 if = 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.To establish a relationship among the structure, dynamics, and function of biological membranes requires knowledge of (i) the packing and motion of phospholipid molecules in a membrane and (ii) the mechanisms for lipid-sterol and lipidprotein interactions. During the last three decades, considerable efforts have been devoted to the biophysical studies of phospholipid bilayers, which are often viewed as a simplified model of natural membranes (1). Although the static properties of a lipid bilayer, such as the conformation and order ofphospholipids, have been carefully characterized (2, 3), the dynamic behavior of phospholipids in a bilayer environment is less well understood (4).The physical state of a phospholipid bilayer under physiological conditions is that of a lyotropic liquid crystal surrounded by water molecules. Due to the amorphous packing and prevalence of molecular motions, high-resolution x-ray diffraction cannot be applied to lipid bilayers in a liquid crystalline phase. Hence, magnetic resonance, which can give both static and dynamic information, is of vital importance in the field of membrane biophysics. In particular, when combined with isotopic labeling, nuclear relaxation becomes a unique technique which can monitor the microscopic dynamics of a specific site within a macromolecule or a supermolecular assembly. Normally, the spin-lattice relaxation in the laboratory frame (T1) is sensitive to molecular motions on the time scale of 10-1o to 10-1 sec, while the spin-lattice relaxation in the rotating frame (T1p) is sensitive to motions on the time scale of 10-6 to 10-4 sec (5).A number of nuclear magnetic resonance (NMR) investigations have focused on the molecular motions of phospholipids in a pure lipid bilayer, an...

New approach to study fast and slow motions in lipid bilayers: application to dimyristoylphosphatidylcholine-cholesterol interactions

“…In contrast, the presence of cholesterol seems to affect the polar head group region to a lesser extent. These observations are in agreement with other 'H, 2H, 13C, and '9F NMR studies (Seiter and Chan, 1973;Lancee-Hermkens and de Kruijff, 1977;Oldfield et al, 1978;Davis, 1983;Peng et al, 1989) that have indicated that the presence of cholesterol markedly restricts the local motions of the phospholipid molecules in unsonicated liposomes. In particular,2H NMR results have shown that the incorporation of cholesterol causes an increase of the quadrupolar splitting of the fatty acyl chains, which can be explained by a decrease in the extent and rate of trans-gauche isomerization (Siminovitch et al, 1988).…”

Lipid bilayer and water proton magnetization transfer: Effect of cholesterol

“…One preliminary study has revealed a cross-relaxation pathway between water and a lipid system, where selective irradiation of the water was shown to affect the T I of the lipid (13). Previous NMR studies of lipid vesicles have demonstrated the dependence of cross relaxation and spin diffusion within the lipid on the correlation times as well as the dipolar order parameter, SHH, of the bilayer system (10,11,19,20). As the correlation time increases, the energy conserving "zero quantum" dipoledipole processes will dominate the magnetization exchange pathway ( 10,19).…”

Slow motions in oriented phospholipid bilayers and effects of cholesterol or gramicidin. A 19F-NMR T1 rho study

“…The phospholipid and the additive were first codissolved in chloroform/ methanol mixture (3:1, vol/vol). The solution was directly deposited onto microscope cover slips, followed by evaporation in vacuum and hydration as described previously (Peng et al, 1988).…”

“…Thus, TI, measurements reflect only the averaged properties of the phospholipid molecule in both phases. For a more detailed discussion, see Peng et al (1989).…”

“…The commonly accepted model of biological membranes is the fluid mosaic model (Singer and Nicolson, 1972). In a recent publication (Peng et al, 1988), we have reported an investigation of the slow molecule motions (10-4_10-6 s) of the phospholipid acyl chain for lipid bilayers in the liquid-crystalline phase, using the nuclear spin-lattice relaxation in the rotatingframe (T,,) of '9F-labeled dimyristoylphosphatidylcholines ('9F-DMPCs). We have now extended our investigation to observe the changes on the slow motions of phospholipids that can be induced by adding cholesterol or gramicidin to the bilayer.…”