J . Phys. Chem. 1992, 96, 1460-1463
1460
Timecorrelated Study of Fluorescent Probes in Lipid Aggregates Using Fractal Models Guy Duportail,t Jean-Claude Brocbon,* and Panagiotis Lianos**~ Centre de Recherches Pharmaceutiques. Laboratoire de Physique, UA CNRS 491, UniversitC Louis Pasteur, B.P. 24, 67401 IIIkirch Cedex, France, Laboratoire pour 1 ' Utilisation du Rayonnement Electromagnetique, CNRS-MEN- CEA, Bat. 2090, Centre Universitaire Paris-Sud, 91 405 Orsay, France, and University of Patras. School of Engineering, 26000 Patras, Greece (Received: May 13, 1991; In Final Form: September 1 1 , 1991) Diphenylhexatriene and pyrene as fluorescent probes and 12-doxylstearic acid methyl ester as a quencher have been used for time-correlated fluorescence studies in small unilamellar vesicles composed of dipalmitoylphosphatidylglyceroland/or L-a-phosphatidylglycerol.The analysis of the fluorescence decay profiles has been done by using a model obtained from the theory of random walks in fractal domains. Comparisons have been made between the diphenylhexatrienedata and those obtained with pyrene in the present or previous studies. The reaction rates have been calculated and expressed in a new and simple way involving a factor of dimensionality. The analysis permitted conclusions about probe and quencher distributions in the lipid bilayer. The spectral dimension for fluorescence quenching in the lipid bilayer depends mainly on the nature of the fluorophore-quencherpair and secondarily on the time scale of the reaction. The rate of quenching of excited diphenylhexatriene by 12-doxylstearic acid methyl ester is about 1 order of magnitude higher than that of pyrene by the same quencher. No detectable temperature effect on spectral dimension has been observed with diphenylhexatriene.
Introduction The study of reactions in microheterogeneous phases is a subject of prime importance from physical, chemical, or biophysical points of view, since reaction rates in such phases are largely influenced by the particular environment they provide. One of the main characteristics of this environment is its low dimensionalitywhich makes reactions proceed in a very different manner from homogeneous solutions. The recent discovery of fractal geometry has accelerated understanding of the effect of dimensionality on reaction rates and has helped unify the applied models by introducing the concept of noninteger dimension. In our previous works,'S2 we have used a model resulting from the theory of random walks in fractal structures to analyze the fluorescence decay profiles of monomer pyrene in the presence of an excimer in the bilayer of various phospholipid vesicles. Thus for infinitely short excitation pulses, the time profile of the fluorescence intensity was successfully described by the following equation2 I ( t ) = Io exp(-kot) exp(-Clrf+ C2t2r) 0 < f
