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 calcium in the subphases, were investigated by pressure-area measurements and simultaneous fluorescence microscopy. Condensed-phase domains were observed over both types of subphases and quantified by analyzing the microscope images. The domains' shape changed and area increased with the mole fraction of DMPG, and hysteresis effects in the compression-decompression cycles were more prevalent on the PG-rich side of the phase diagram. The origin of hysteresis and other nonequilibrium effects in monolayers is discussed, and the equilibrium pressure-composition phase diagram for this binary system is presented which shows a measured liquidus curve and calculated solidus curve. The overall effect of calcium is to facilitate formation of condensed-phase domains.