INFLUENCE OF STRUCTURAL HETEROGENEITY ON THE FLUOROMETRIC RESPONSE CHARACTERISTICS OF LIPID-MEMBRANES CONTAINING BENZOXADIAZOLYLDIPALMITOYLPHOSPHATIDYLETHANOLAMINE

Structural alterations in lipid membranes that are induced by changes in pH or by selective enzyme-substrate or antibody-antigen interaction s can provide changes in fluorescence intensity from membranes contain ing benzoxadiazolyldipalmitoylphosphatidylethanolamine (NBD-PE). For e xample, fluorimetric responses of vesicles consisting of mixtures of p hosphatidic acid (PA) and phosphatidylcholine (PC) prepared either fro m egg yolk or as synthetic, dipalmitoyl-substituted moieties and conta ining 2.3 mol % NBD-PE consist of either an increase or decrease in to tal fluorescence yield upon alteration of supporting electrolyte pH. T his response was due to changes in NBD-PE self-quenching and the local environment of the fluorophore. The present work attempts to identify the factors that determine the fluorescence sensitivity of these lipi d membranes, The impact of phosphatidic acid loading in vesicular bila yer lipid membranes and microscopic structural heterogeneity in monola yers at the air-water interface were explored. The results indicated t hat monolayers that exhibited laterally-separated phase domains on the microscopic scale provided enhanced sensitivity to perturbations of b ulk pH and membrane surface charge redistribution through the alterati on of the degree of headgroup ionization. Variations in the surface po tential upon compression of either egg PA/egg PC or DPPA/DPPC monolaye rs in the liquid-expanded state were observed in order to facilitate t he qualitative correlation of trends in the fluorimetric response of s tructurally homogeneous and structurally heterogeneous vesicular membr anes with the existence of laterally-separated phase domains. The resu lts indicated a strong correlation between sensitivity and structurall y heterogeneity. The results also suggest the existence of ordered sub -microscopic domains in otherwise structurally homogeneous egg PA/egg PC membranes.