Use of Fourier transforms in the analysis of fluorescence data. 3. Fluorescence of pyrene-labeled phosphatidylcholine in lipid bilayer membrane. A three-state model

In this paper a three-state model has been developed to describe the fluorescence of pyrene-labeled phospholipids in lipid membranes. The three-state model is a generalization of the Birks model (Birks et al. Proc. R. Soc. London, A 1963, 275, 575-588), which proved to fail in the case of membrane systems. In contrast to the Birks model, the three-state model makes a distinction between two types of the excited monomers: excited monomers that have and that have no ground-state probe molecules in their nearest neighbor. The presence of ground-state probe molecules in the nearest neighbor of an excited monomer may result in excimer formation at proper orientations of the respective pyrene rings. The kinetics of the fluorescence process is represented in the model by three coupled, first-order, linear differential equations with coefficients depending on the probe concentration. The analytical forms of the concentration dependencies of the coefficients are determined by means of an Ising type model of the two-component membranes. This combined application of reaction kinetics and statistical mechanics provides a proper description of the pyrene fluorescence in the cases of both ideal and nonideal mixtures composed of pyrene-labeled probes and matrix lipid molecules. By using the explicit solutions of the three state model, the global analysis of the steady-state and phase-modulation fluorescence data provides informations on the fluorescence rate parameters and on the static and dynamic parameters of the lateral distribution of the pyrene-labeled lipid molecules in the membrane. The analysis of the fluorescence of N-[10-(1-pyrenyl)decanoyl]phosphatidylcholine (PyrPC) in multilamellar vesicles of 1,2-dimyristoyl-L-α-phosphatidylcholine (DMPC) shows the following: the PyrPC molecules tend to be maximally separated in the lipid matrix forming regular rather than completely random distribution in the membrane; the lateral diffusion coefficient of the PyrPC is 23 μm² s^-1 in the liquid crystalline phase at 30.3°C and 2.6 μm² s^-1 in the gel phase at 10°C; the rotational diffusion rate of the pyrene moiety is 5.1 × 10⁸ s^-1 in the liquid-crystalline phase and 9 × 10⁷ s^-1 in the gel phase.