We report here on the photophysical properties of the pyrene-labeled phospholipid molecule 1,2-bis-(1-pyrenyldecanoyl)-L-alpha-phosphatidylcholine (DPyPC) embedded in unlabeled phosphatidylcholine matrix monolayers at the gas/water interface. Phosphatidylcholines with varying acyl chain length such as dilauroyl-(DLPC), dimyristoyl- (DMPC), dipalmitoyl- (DPPC), distearyl- (DSPC), and diarachidylphosphatidylcholine (DAPC) were used as the matrix molecules. Surface pressure-, reflection-, and fluorescence-area isotherm measurements were employed for the investigation of photophysical properties of the pyrene chromophore. The reflection spectrum of the DPyPC monolayer (at the air/water interface) at high surface pressures shows four bands at 278, 317, 333, and 348 nm, respectively, which are slightly (2-4 nm) red shifted as compared to that for the monomer, indicating pyrene dimer formation. The reflection-area isotherms at 275 and 345 nm show that the longer molecular axis of the chromophore is not normal to the air/water interface. Increasing the acyl chain length of the matrix molecules leads to changes in the shape of the reflection-area isotherms, indicating that the pyrene chromophore tends to be more and more parallel to the acyl chains. The fluorescence spectrum of the mixed monolayers (DPyPC:DPPC = 1:50) at a surface pressure of < 0.5 mN/m shows both the monomer (378, 395, and 420 nm) and excimer (485 nm) emission bands. The fluorescence intensity and the ratio of monomer and excimer bands depend strongly on the acyl chain length of the matrix molecule and the surface pressure in the monolayer. In addition; it was found that the pyrene fluorescence (both monomer and excimer) is strongly and reversibly quenched by the presence of oxygen. Furthermore, the degree and kinetics of oxygen-induced quenching of pyrene emission are strongly dependent on the molecular packing density and the length of the acyl chain of the matrix molecule. For example, in the case of DAPC as the matrix molecule, hardly any quenching of pyrene fluorescence was observed. The results are analyzed and discussed in terms of molecular models.
