MOLECULAR MOTIONS AND DYNAMICS OF A DIUNSATURATED ACYL CHAIN IN A LIPID BILAYER - IMPLICATIONS FOR THE ROLE OF POLYUNSATURATION IN BIOLOGICAL-MEMBRANES

The nature and dynamics of the motions of a diunsaturated fatty acyl chain in a lipid bilayer were examined using a comprehensive simulation program for H-2 NMR line shapes developed by Wittebort et al. [Wittebort, R. J., Olejniczak, E. T., & Griffin, R. G. (I 987) J. Chem. Phys. 36, 5411-5420]. A motional model in which the isolinoleoyl chain (18:2-DELTA-6,9) adopts two conformations consistent with the low energy structures proposed for 1,4-pentadiene [Applegate, K. R., & Glomset, J. A. (1986) J. Lipid Res. 27, 658-680], but undergoes a rapid jump between these states, is sufficient to account for the experimentally observed quadrupolar couplings, the H-2-H-2 and H-1-H-2 dipolar couplings, the longitudinal relaxation times, and the changes in the average conformation of the chain that occur with a variation in temperature. The jump motion originates via rotations about the C7-C8 and the C8-C9 carbon bonds and leads to the low order parameters assigned to the C8 methylene segment (0.18) and the C9-C10 double bond (0.11). In contrast, the C6-C7 double bond, which is not involved in the two-site jump, characterized by a relatively large order parameter (0.56). Fatty acyl chains containing three or more double bonds likely cannot undergo the same jump motion and consequently will be highly ordered structures. Correlation times for diffusion of the molecular long axis of the diunsaturated acyl chain about the bilayer normal (approximately 10(-10) s) and for the local jump motion (approximately 10(-10) s) were calculated. Relative to their rates of diffusion about their molecular long axes, the rate of the local jump motion in the diunsaturated bilayer is much slower than the rates of the local motions (trans/gauche isomerization) occurring in saturated bilayers. The presence of large amounts of highly unsaturated fatty acyl chains in biological membranes should create a dynamic state that allows considerable intermolecular motion but still maintain a high degree of local order within the hydrophobic region of the bilayer.