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

The behavior of N-(1-pyrenesulfonyl)dipalmitoyl-L-alpha-phosphatidylethanolamine (pyrene-DPPE) embedded in dimyristoyl-L-alpha-phosphatidylcholine (DMPC) monolayers at the gas-water interface has been examined by surface pressure-area isotherm measurements and steady-state and time-resolved fluorescence spectroscopy. The pyrene moiety of pyrene-DPPE markedly alters the packing characteristics from those of the pure lipid DPPE, contributing significantly to the area per molecule. Steady-state, fluorescence spectra showed monomer emission only, and a nonlinear increase in fluorescence intensity with concentration of pyrene-DPPE was observed, which can be attributed to oxygen quenching. Time-resolved fluorescence measurements yielded single-exponential decays for the pyrene chromophore, providing evidence that pyrene-DPPE was not aggregated in the monolayer film. The lifetime of pyrene-DPPE was also found to increase with monolayer compression. The presence of iodide ions in the subphase efficiently quenched the fluorescence of pyrene-DPPE in the monolayer. The degree of quenching was found to be independent of the molecular packing density, suggesting that the pyrene chromophore is located in the headgroup region of the monolayer. Measurements of the steady-state fluorescence intensity as a function of area per molecule showed pyrene-DPPE to be strongly susceptible to oxygen quenching. The degree of oxygen quenching was found to decrease with compression, accounting for the increase in the fluorescence lifetime of the excited pyrene with increasing surface pressure.