E/M dips. Evidence for lipids regularly distributed into hexagonal super-lattices in pyrene-PC/DMPC binary mixtures at specific concentrations.

We have examined the effect of 1-palmitoyl-2-(10-pyrenyl)decanoyl-sn-glycerol-3-phosphatidylcholine (Pyr-PC) concentration on the ratio of excimer fluorescence to monomer fluorescence (E/M) in L-alpha-dimyristoylphosphatidylcholine (DMPC) multilamellar vesicles at 30 degrees C, with special attention focussed on the smoothness of the curve. We observed a series of dips, in addition to kinks, in the plot of E/M versus the mole fraction of Pyr-PC (XPyrPC). The observation of dips is a new finding, perhaps unique for Pyr-PC in DMPC since only kinks were observed for Pyr-PC in L-alpha-dipalmitoylphosphatidylcholine (DPPC) and in egg yolk phosphatidylcholine (egg-PC) (Somerharju et al., 1985. Biochemistry. 24: 2773-2781). The dips/kinks observed here are distributed according to a well defined pattern reflecting a lateral order in the membrane, and distributed symmetrically with respect to 50 mol% Pyr-PC. Some of the dips appear at specific concentrations (YPyrPC) according to the hexagonal super-lattice model proposed by Virtanen et al. (1988. J. Mol. Electr. 4: 233-236). However, the observations of dips at XPyrPC > 66.7 mol% and the kink at 33.3 mol% cannot be interpreted by the model of Virtanen et al. (1988). These surprising results can be understood by virtue of an extended hexagonal super-lattice model, in which we have proposed that if the pyrene-containing acyl chains are regularly distributed as a hexagonal super-lattice in the DMPC matrix at a specific concentration YPyrPC, then the acyl chains of DMPC can form a regularly distributed hexagonal super-lattice in the membrane at a critical concentration (1-YPyrPC). The excellent agreement between the calculated and the observed dip/kink positions, except for the dip at 74 mol% and the kink at 40 mol%, provides most compelling evidence that lipids are regularly distributed into hexagonal super-lattices in Pyr-PC/DMPC mixtures at specific concentrations. The physical nature of the dips not only gives us a better understanding of lipid lateral organization in membranes but also will lead to new theoretical considerations and experimental designs for exploring the relationship between lipid regular distribution and membrane functions.