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.
Tang
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Department of Biochemistry, Meharry Medical College, Nashville, Tennessee 37208.
Chong
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D015060:1,2-Dipalmitoylphosphatidylcholine / Q000737:chemistry; D004134:Dimyristoylphosphatidylcholine / Q000737:chemistry*; D008433:Mathematics; D008954:Models, Biological; D008961:Models, Structural; D008968:Molecular Conformation; D010713:Phosphatidylcholines / Q000737:chemistry*; D013050:Spectrometry, Fluorescence / Q000379:methods
D010713:Phosphatidylcholines; D015060:1,2-Dipalmitoylphosphatidylcholine; C021248:1-palmitoyl-2-pyrenedecanoylphosphatidylcholine; D004134:Dimyristoylphosphatidylcholine