USE OF FOURIER-TRANSFORMS IN THE ANALYSIS OF FLUORESCENCE DATA .2. FLUORESCENCE OF PYRENE-LABELED PHOSPHATIDYLCHOLINE IN LIPID BILAYER-MEMBRANE - TEST OF THE BIRKS MODEL

Phase properties and lateral distribution of lipids in membranes are frequently studied by fluorescence of pyrene-labeled lipids. The analysis of the data derived from the fluorescence of pyrene-labeled lipids is usually performed by means of the Birks model (Birks et al. Proc. R. Soc. London A 1963, 275, 575-588). The Birks model has been proven correct in describing pyrene fluorescence in isotropic media at low pyrene concentrations, but it failed to describe properly the fluorescence of pyrene molecules embedded into lipid membranes. In this paper we test the validity of the Birks model directly in the case of pyrene-labeled lipid probes in membranes. The steady-state and phase-modulation fluorescence of N-[10-(1-pyrenyl)-decanoy]phosphatidylcholine in multilamellar vesicles of 1,2-dimyristoyl-L-alpha-phosphatidylcholine has been examined at a wide range of lipid compositions and different temperatures. The model parameter invariant predictions of the Birks model have been tested under the basis of the general methods developed in the preceding paper (part 1 of this series). The model proved to be inadequate at high modulation frequencies (> 3 MHz). Another indication of the inadequacy of the Birks model is that the fitted model parameters varied on an uncorrelated manner with varying membrane composition. The above limitations of the model could not be resolved by using distributed model parameter for the description of the excimer formation. A physically reasonable generalization of the Birks model has been proposed by introducing a new type of excited states.