BEHAVIOR OF A PYRENE-LABELED PHOSPHOLIPID IN MONOLAYERS OF DIMYRISTOYL-ALPHA-PHOSPHATIDYLCHOLINE AT THE GAS WATER INTERFACE - A FLUORESCENCE QUENCHING STUDY

The behavior of fonyl)dipalmitoyl-L-alpha-phosphatidylethanolamine (py rene-DPPE) embedded in dimyristoyl-L-alpha-phosphatidylcholine (DMPC) monolayers at the gas-water interface has been examined by surface pre ssure-area isotherm measurements and steady-state and time-resolved fl uorescence spectroscopy. The pyrene moiety of pyrene-DPPE markedly alt ers the packing characteristics from those of the pure lipid DPPE, con tributing significantly to the area per molecule. Steady-state, fluore scence spectra showed monomer emission only, and a nonlinear increase in fluorescence intensity with concentration of pyrene-DPPE was observ ed, which can be attributed to oxygen quenching. Time-resolved fluores cence measurements yielded single-exponential decays for the pyrene ch romophore, providing evidence that pyrene-DPPE was not aggregated in t he monolayer film. The lifetime of pyrene-DPPE was also found to incre ase with monolayer compression. The presence of iodide ions in the sub phase efficiently quenched the fluorescence of pyrene-DPPE in the mono layer. The degree of quenching was found to be independent of the mole cular packing density, suggesting that the pyrene chromophore is locat ed in the headgroup region of the monolayer. Measurements of the stead y-state fluorescence intensity as a function of area per molecule show ed pyrene-DPPE to be strongly susceptible to oxygen quenching. The deg ree of oxygen quenching was found to decrease with compression, accoun ting for the increase in the fluorescence lifetime of the excited pyre ne with increasing surface pressure.