ABSTRACT: Biological membrane function, in part, depends upon the local regulation of lipid composition. The spatial heterogeneity of membrane lipids has been extensively explored in the context of cholesterol and phospholipid acyl-chain-dependent domain formation, but the effects of lipid head groups and soluble factors in lateral lipid organization are less clear. In this contribution, the effects of divalent calcium ions on domain formation in monolayers containing phosphatidylinositol 4,5-bisphosphate (PIP2), a polyanionic, multi-functional lipid of the cytosolic leaflet of the plasma bilayer, are reported. In binarymonolayers of PIP2mixed with zwitterionic lipids, calcium induced a rapid, PIP2-dependent surface pressure drop, with the concomitant formation of laterally segregated, PIP2-rich domains. The effect was dependent upon head-group multi-valency, because lowered pH suppressed the surface-pressure effect and domain formation. In accordance with previous observations, inclusion of cholesterol in lipidmixtures induced coexistence of two liquid phases. Phase separation strongly segregated PIP2 to the cholesterol-poor phase, suggesting a role for cholesterol-dependent lipid demixing in regulating PIP2 localization and local concentration. Similar to binary mixtures, subphase calcium induced contraction of ternary cholesterol-containing monolayers; however, in these mixtures, calcium induced an unexpected, PIP2- andmultivalency-dependent decrease in the miscibility phase transition surface pressure, resulting in rapid dissolution of the domains. This result emphasizes the likely

The adsorption of proteins and synthetic polymers at the air-water interface (AWI) has broad significance in biomedicine and biotechnology. Protein behavior at the AWI can be guided to control the structure of the two-dimensional biopolymer film. In addition, synthetic polymers affect how plasma proteins act on implant or drug carrier surfaces, and can also decrease the potential for gas embolism. In this thesis, fluorescence microscopy was applied in combination with tensiometry and atomic force microscopy to study plasma proteins at the AWI alone and under the effect of synthetic polymers. First, the morphology of serum albumin layer controlled by the solution conditions was explored by fluorescence microscopy. Heterogeneity at the micron scale was observed for the protein film during adsorption and at reduced concentrations. Moreover, the competition for interfacial area between Pluronic surfactant F-127 and fibrinogen or serum albumin was studied by semi-quantitative confocal fluorescence methods. A transition stage where F-127 and protein underwent lateral phase separation was found. Two competing processes were revealed--the disintegration of protein-rich phase by F-127 and the coalescence of protein phase. Lastly, the interaction of immunoglobulin and the thin film of a widely used polydimethylsiloxane synthetic polymer was studied at the AWI. The compression state of the polymer film was shown to significantly affect protein adsorption and guide proteins