Film balance and fluorescence microscopic investigation of the effects of Ca2+ on mixed DMPC/DMPG monolayers

Dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol ( DMPG) mixed monolayers at the air-water interface, with and without 5 mM ca lcium in the subphases, were investigated by pressure-area measurements and simultaneous fluorescence microscopy. Condensed-phase domains were observe d over both types of subphases and quantified by analyzing the microscope i mages. The domains' shape changed and area increased with the mole fraction of DMPG, and hysteresis effects in the compression-decompression cycles we re more prevalent on the PG-rich side of the phase diagram. The origin of h ysteresis and other nonequilibrium effects in monolayers is discussed, and the equilibrium pressure-composition phase diagram for this binary system i s presented which shows a measured liquidus curve and calculated solidus cu rve. The overall effect of calcium is to facilitate formation of condensed- phase domains.