EXPLORATION OF PHYSICAL PRINCIPLES UNDERLYING LIPID REGULAR DISTRIBUTION - EFFECTS OF PRESSURE, TEMPERATURE, AND RADIUS OF CURVATURE ON E MDIPS IN PYRENE-LABELED PC/DMPC BINARY-MIXTURES/

In a previous study, we observed a series of dips in the plot of E/M ( the ratio of excimer to monomer fluorescence intensity) versus the mol e fraction of pyrenyl)decanoyl-sn-glycerol-3-phosphatidylcholine (Pyr- PC) in Pyr-PC/DMPC binary mixtures at 30 degrees C. In the present stu dy, we have characterized the physical nature of E/M dips in Pyr-PC/DM PC binary mixtures by varying pressure, temperature, and vesicle diame ter. The E/M dips at 66.7 and at 71.4 mol% PyrPC in DMPC multilamellar vesicles remain discernible at 30-43 degrees C. At higher temperature s (e.g., 53 degrees C), the depth of the dip abruptly becomes smaller. This result agrees with the idea that E/M dips appear as a result of regular distribution of pyrene-labeled acyl chains into hexagonal supe r-lattices at critical mole fractions. Regular distribution is a self- ordering phenomenon. Usually, in self-ordered systems, the number of s tructural defects increases with increasing temperature, and thermal f luctuations eventually result in an order-to-disorder transition. The effect of vesicle diameter on the E/M dip at 66.7 mol% Pyr-PC in DMPC has been studied at 37.5 degrees C by using unilamellar vesicles of va rying sizes. The E/M dip is observable in large unilamellar vesicles; however, the depth of the E/M dip decreases when the vesicle diameter is reduced. When the vesicle diameter is reduced to about 64 nm, the d ip becomes shallow and split. This result suggests that the curvature- induced increase in the separation of lipids in the outer monolayer de creases the tendency of regular distribution for pyrene-labeled acyl c hains. Regular distribution is believed to arise from the long-range r epulsive interaction between Pyr-PC molecules due to the elastic defor mation of the lipid matrix around the bulky pyrene moiety. When the ra dius of curvature becomes small, outer monolayer lipids are more separ ated. Therefore, pyrene-containing acyl chains fit better into the mem brane matrix, which alleviates the deformation of the lattice and dimi nishes the long-range repulsive interactions between pyrene-containing acyl chains. Furthermore, we have shown a striking difference in the pressure dependence of E/M at critical Pyr-PC mole fractions and at no ncritical mole fractions. In the pressure range between 0.001 and 0.7 kbar at 30 degrees C, E/M decreases steadily with increasing pressure at noncritical mole fractions; in contrast, E/M changes little with pr essure at critical mole fractions (e.g., 33.3 and 50.0 mol% Pyr-PC). T he pressure data suggest that membrane free volume in the liquid cryst alline state of the bilayer is less abundant at critical Pyr-PC mole f ractions than at noncritical mole fractions.