Lateral diffusion in spin-labeled phosphatidylcholine multilayers.

The paramagnetic resonance spectrum of a highly concentrated region of spin-labeled phosphatidylcholine included in oriented bilayers of phosphatidylcholine (PC) changes dramatically in time. This time dependence of the spectra is due to the lateral diffusion of the oriented labeled molecules spreading in the planes of the corresponding monolayers. The resonance spectra can be analyzed in terms of a time-dependent superposition of spectra corresponding to the different concentrations of spin label. It is possible to estimate the diffusion constant: D ‘v 1.8 f 0.6 x cm2/sec at room temperature (25”) . If lateral diffusion is assumed to be due to pairwise exchange of neighboring molecules, then this diffusion coefficient corresponds to an exchange frequency which is of the order of lo7 sec-l. This rate is high enough to suggest that the lateral translation of molecules bound to membranes may sometimes have biological significance. he membrane of a biological cell is involved in many T essential processes that require molecular motion. For example, the active transport of ions and molecules through membranes doubtless requires the molecular motion of membrane components. There is much current interest in the question of the rate of lateral difSusion of molecules in membranes. This question arises in connection with membrane biosynthesis and, as discussed later, the possible lateral diffusion of “messenger” molecules in membranes. Since virtually all biological membranes contain lipids and many biological membranes are thought to cmtain lipid bilayers, we have carried out a study of the rate of lateral diffusion of a spin-labeled lipid in a phospholipid bilayer system. It is very difficult to make an a priori estimate of the rate of lateral diffusion of lipids in phospholipid bilayers. Much evidence indicates that in, for example, egg lecithin bilayers the hydrocarbon chains are in a highly “fluid” state. That is, chain isomerizations take place rapidly (2 107 sec-’) and with relatively high probability. However, this high degree of molecular motion is most pronounced toward the terminal methyl groups of the fatty acid chains, 3 , 4 whereas near the polar head groups the hydrocarbon chains appear to be relatively more rigid and tightly packed. The rate of inside-outside transitions of spin-labeled phospholipids through bilayers is remarkably of the order of -(24 hr)-l. Thus, the bilayer appears to have the combined properties of a fluid and a rigid structure. This makes it difficult to give any plausible estimate of how rapid lateral diffusion might be. The first attempt to measure the rate of lateral diffusion in phos(1) (a) Supported by the National Science Foundation under Grant No. GP 26456. (b) NATO Postdoctoral Fellow, on leave from Laboratoire de Physique des Solides, Faculti des Science de Paris. This work has benefited from facilities made available by the Advanced Research Projects Agency through the Center for Materials Research at Stanford University. (2) For a discussion of this subject and references to earlier work, see W. L. Hubbell and H. M. McConnell, J . Amer. Chem. Soc., 93,314 (1971). (3) B. G. McFarland and H. M. McConnell, Proc. Nut. Acud. Sci. U. S., 68, 1274 (1971). (4) P. Jost, L. J. Libertini, V. G. Hebert, and 0. H. Griffith, J. Mol. B i d , 59, 77 (1971). See also J. Seelig, J . Amer. Chem. Soc., 92, 3881 (1970). This increase in molecular motion toward the terminal methyl groups applies also to intact biological membranes. See W. L. Hubbell and H. M. McConnell, Proc. Nut. Acud. Sci. U . S., 64,20 (1969). (5) R. D. Kornberg and H. M. McConnell, Biochemistry, 10, 1111 (1971). pholipid bilayers was made by Kornberg and McConne1L6 These investigators studied the N-methyl proton line broadening in phosphatidylcholine vesicles brought about by low concentrations of the spin label PC I.