Impact of membrane perturbants on the properties of phospholipid monolayers

Monolayers of phospholipid not only serve as effective models for biological membranes but also exhibit their own rich physical chemistry. Phase transitions in monolayers can be imaged using fluorescence microscopy or Brewster angle microscopy to reveal so-called “domains” of one phase dispersed in a second phase. The shapes of domains that form are unique to each phospholipid and sensitive to conditions at the interface. This sensitivity can be exploited by employing domain shape as a probe to examine the impact of a second component on the phospholipid monolayer. As model membranes, the controllability and sensitivity of phospholipid monolayers makes them powerful tools for the study of biologically-relevant materials. This thesis examines the action of several membrane-perturbing agents on the surface pressure/molecular area isotherms and domain shapes formed by dipalmitoylphosphatidylcholine (DPPC) monolayers. Four soluble surfactants were studied, chosen both for their application as membrane perturbants and for the range of electrostatic properties they span. The effect of surfactant action on DPPC monolayers was clearly stratified by the electrostatic nature of the surfactant: electrostatically neutral surfactants exhibited a class of behavior distinct from that of charged surfactants. The impact of lysophosphocholines, the membrane-perturbing single-chain analogues of DPPC, was also examined. Solubility, as dictated by the chain length of the lysolipid, largely determined the effect on DPPC monolayers; less soluble lysolipids induced the most drastic changes in DPPC behavior. Because each phospholipid exhibits different domain shapes in regions of phase coexistence, the molecular basis for domain shape has been a frequent focus for experimental and theoretical study. In this work, common traits between domains of related phospholipids were identified, establishing a connection between molecular structure and domain shape. The impact of these results on theoretical prediction of domain shape is discussed. Finally, an experimental station designed to study monolayers at fluid-fluid interfaces was demonstrated. For the first time, Brewster angle microscopy was applied to an interface between two immiscible fluids. This technique holds great promise for the examination of monolayer structure at a fluid-fluid boundary