MOLECULAR-ORGANIZATION IN PHOSPHOLIPID MONOLAYER DOMAINS BY CORRELATIVE FLUORESCENCE MICROSCOPY AND ELECTRON-DIFFRACTION

Lipid monolayer is a half leaflet model for lipid bilayer, which forms the basis of biological membranes. Within a certain range of surface area per molecule of phospholipid monolayers at the air-water interfac e, where the compressibility was nearly infinite, two phases with diff erent molecular packings were observable by fluorescence microscopy. M ixed-phase monolayers of ipalmitoyl-N-monomethyl-3-phosphatidylethanol amine [DP(Me)PE] or dipalmitoyl-N-dimethyl-3-phosphatidyl-ethanolamine [DP(Me)(2)PE] were deposited on marker grids coated with Formvar film s. The molecular organization in the dark and bright fluorescent areas on the grids was investigated by low dose, selected area electron dif fraction. Sharp reflection arcs, at a spacing of 4.2 Angstrom and arra nged in a hexagon pattern, were detected from dark domains of both lip ids. A diffuse reflection ring at a spacing of 4.6 Angstrom was derive d from the bright background areas. Diffraction patterns were obtained from neighboring areas along selected dark domains of both lipids. Th e orientations of diffraction patterns from areas along smooth and cur ving boundaries of DP(Me),PE domains were found to turn with the bound aries. In the branching domains of DP(Me)PE, the orientations of diffr action patterns indicated that the branches were formed by twinning. E lectron diffraction thus provides an unique way to sample the local mo lecular packing order and orientation within individual domains in pho spholipid monolayers. (C) 1994 Wiley-Liss, Inc.