A CONSPICUOUS ORIENTATION AND LOCALIZATION OF PYRENE IN LANGMUIR LIPID MONOLAYERS

A previous study of pyrene solubilized in Langmuir lipid monolayers of eicosanoic acid (EA) suggests that pyrene is localized close to the w ater phase (Langmuir 1992, 8, 1366). In this work, we have examined th e orientation of pyrene in monolayers of EA and monolayers of 1,2-diol eoyl-sn-glycero-3-phosphocholine (DOPC) by means of polarized light sp ectroscopic methods. At low pyrene concentrations, fluorescence anisot ropy experiments reveal that the aromatic plane is preferentially orie nted perpendicular to the normal of the monolayer. The deviation from the theoretical value expected for a perfectly planar monolayer is com patible with the existence of slow director fluctuations. The experime ntal results strongly suggest that the pyrene molecules at low concent rations are located between the monolayer and water, so that the aroma tic plane forms an interface. With increasing pyrene concentrations, t he orientation of pyrene becomes gradually disordered.