EVIDENCE FOR SUPERLATTICE ARRANGEMENTS IN FLUID PHOSPHATIDYLCHOLINE PHOSPHATIDYLETHANOLAMINE BILAYERS/

Recently, evidence for cholesterol and phosphatidylcholine (PC) molecu les to adapt superlattice arrangements in fluid lipid bilayers has bee n presented. Whether superlattice arrangements exist in other biologic ally relevant lipid membranes, such as phosphatidylethanolamine (PE)/P C, is still speculative. In this study, we have examined the physical properties of fluid 1-palmitoyl-2-oleoyl-PC (POPC) and 1-palmitoyl-2-o leoyl-PE (POPE) binary mixtures as a function of the POPE mole fractio n (X-PE) using fluorescence and Fourier transform infrared spectroscop y. At 30 degrees C, i.e., above the T-m of POPE and POPC, deviations, or dips, as well as local data scattering in the excimer-to-monomer fl uorescence intensity ratio of intramolecular excimer forming dipyrenyl phosphatidylcholine probe in POPE/POPC mixtures were detected at X-PE approximate to 0.04, 0.11, 0.16, 0.26, 0.33, 0.51, 0.66, 0.75, 0.82, 0 .91, and 0.94. The above critical values of X-PE coincide (within +/-0 .03) with the critical mole fractions X-HX,X-PE or X-R,X-PE predicted by a headgroup superlattice model, which assumes that the lipid headgr oups form hexagonal or rectangular superlattice, respectively, in the bilayer. Other spectroscopic data, generalized polarization of Laurdan and infrared carbonyl and phosphate stretching frequency, were also c ollected. Similar agreements between some of the observed critical val ues of X-PE from these data and the X-HX,X-PE or X-R,X-PE values were also found. However, all techniques yielded critical values of X-PE (e .g., 0.42 and 0.58) that cannot be explained by the present headgroup superlattice model. The effective cross-sectional area of the PE headg roup is smaller than that of the acyl chains. Hence, the relief of ''p acking frustration'' of PE in the presence of PC (larger headgroup tha n PE) may be one of the major mechanisms in driving the PE and PC comp onents to superlattice-like lateral distributions in the bilayer. We p ropose that headgroup superlattices may play a significant role in the regulation of membrane lipid compositions in cells.