LONG-RANGE DIFFUSION-COEFFICIENTS IN, 2-DIMENSIONAL FLUID MEDIA MEASURED BY THE PYRENE EXCIMER REACTION

Diffusion-controlled bimolecular reactions in two-dimensional or quasi two-dimensional space are important in both surface and membrane reac tion kinetics. A model reaction that has been extensively used to stud y the dynamics and structure of phospholipid membranes is the formatio n of the excimer of pyrene. We present experimental evidence that in m ultilamellar liposomes of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocho line, in the fluid phase at temperatures above phase transition, T-L, and for a probe/phospholipid ratio of 1:75, or lower, the fluorescence decay of l-2-(1-pyrenodecanoyl)-sn-glycero-3-phosphocholine and the t ime-resolved emission of the excimer follow a kinetic law specific for two-dimensional diffusion-controlled reactions. The parameters obtain ed from the application of this law to the experimental results, eithe r from steady-state or from time-dependent fluorescence, give diffusio n coefficients, e.g., D(25 degrees C) = 3.1 x 10(-8) cm(2) s(-1) and a n activation energy for diffusion, E(a) = 35 kJ mol(-1), that are in a ccordance with the results of other independent techniques.