Lateral diffusion in binary mixtures of cholesterol and phosphatidylcholines.

Abstract

The lateral diffusion of a fluorescent-labeled phospholipid, phosphatidyl-N(4-nitrobenzo-2-oxa-1,3,-diazole)ethanolamine, has been measured in binary mixtures of cholesterol and dimyristoyl phosphatidylcholine at temperatures both above and below 23.80C, the chain-melting transition temperature of this phosphatidylcholine. There is a temperature-composition region, approximately temperature less than 230C and mole fraction of cholesterol (X) less than 0.20, in which the lateral diffusion coefficient of the fluorescent probe is at least an order of magnitude smaller than it is at points outside of this temperature-composition region. At temperatures above z230C there is a significant increasing cholesterol concentration, for X > 0.2.The physical properties of bilayer membranes containing cholesterol and phosphatidylcholines have been the subject of extensive investigations employing a large number of experimental techniques (1-16). From these studies certain generalizations emerge. The addition of cholesterol to phosphatidylcholine bilayers in the fluid state (above the chain-melting temperatures of the phosphatidylcholines-i.e., above 23.80C for dimyristoyl phosphatidylcholine), leads to a decrease in the "fluidity" of the bilayer membrane. On the other hand, inclusion of cholesterol in phosphatidylcholine membranes at temperatures below the chain-melting transition temperatures leads to "fluidization" of these membranes (1-16). These generalizations have been based in part on magnetic resonance studies (spin-label paramagnetic resonance and nuclear magnetic resonance) in which the spectra are sensitive to the rotational motions of the molecules. Another interesting property of binary mixtures of dimyristoyl phosphatidylcholine or dipalmitoyl phosphatidylcholine with cholesterol is that a phosphatidylcholinelike phase transition (at t230C or -420C) can be detected in the presence of mole fraction X < 0.20 cholesterol in the binary mixture (1,3,6,7,9,10,12,16). The chain-melting transition temperature of dimyristoyl phosphatidylcholine is 23.80C, and that of dipalmitoyl phosphatidylcholine is 41.40C. Although reliable phase diagrams have been established for a number of binary mixtures of phospholipids (17-20), it has been difficult to use the published phase diagrams for binary mixtures of cholesterol and phosphatidylcholines (3, 4, 7) to account for all of the observed physical properties of these mixtures. Some of this difficulty may originate in sample heterogeneity (4).In the present paper we give the results of determinations of the lateral diffusion coefficient of the fluorescent probe phosphatidyl-N-(4-nitrobenzo-2-oxa-1,3-diazole)ethanolamine (structure I; derived from egg phosphatidylcholine) in binarymixtures of cholesterol and dimyristoyl phosphatidyicholine as a function of temperature and composition. Some diffusion coefficients for this probe in binary mixtures of dipalmitoyl phosphatidylcholine and cholesterol are also given. As will be seen, such data shed considerable light on the nature...

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