Origin of laurdan sensitivity to the vesicle-to-micelle transition of phospholipid-octylglucoside system: A time-resolved fluorescence study

The fluorescent probe laurdan has been shown to be sensitive to the vesicle -to-micelle transition of phosphatidylcholine/octylglucoside (M. Paternostr e, O. Meyer, C. Grabielle-Madelmont, S, Lesieur, and M. Ollivon, 1995, Biop hys. J. 69,2476-2488). On the other hand, a study on the photophysics of la urdan in organic solvents has shown that the complex de-excitation pathway of the probe can be described by two successive processes, i.e., an intramo lecular charge transfer followed by dielectric relaxation of the solvent if polar. These two excited-state reactions lead to three emitting states, i. e., a locally excited state, a charge transfer state, and a solvent relaxed state (M. Viard, J. Gallay, M. Vincent, B. Robert and M. Paternostre, 1997 , Biophys. J. 73:2221-2234), Experiments have been performed using time-res olved fluorescence on the probe inserted in amphiphile aggregates (mixed li posomes, mixed micelles) different in detergent-to-lipid ratios. The result s have been compared with those obtained for laurdan inserted in dipalmitoy l phosphatidylcholine liposomes in the gel and in the fluid lamellar phase. Except for laurdan in dipalmitoyl phosphatidylcholine liposomes in the gel lamellar phase, the red part of the emission spectra originates from the d e-excitation of the relaxed excited state of laurdan, indicating that indee d the dielectric relaxation process is an important phenomena in the ground -state return pathway of this probe. On the other hand, the maximization en tropy method (MEM) analysis of the fluorescence decay recorded in the blue part of the emission spectra indicates that the dielectric relaxation is no t the only reaction occurring to the excited state of laurdan. Moreover, th e analysis of the fluorescence decays of laurdan inserted in gel lamellar d ipalmitoylphosphatidylcholine (DPPC) liposomes indicates excited-state reac tions, although dielectric relaxation is impossible. These results are in a greement with the de-excitation pathway determined from laurdan behavior in organic solvent even if, in most of the aggregates studied in this work, t he major phenomenon is the dielectric relaxation of the solvent. All along the vesicle-to-micelle transition, we have observed that the lifetime of th e relaxed excited state of laurdan continuously decreases probably due to a dynamic quenching process by water molecules. On the other hand, the time constant of the dielectric relaxation process remains almost unchanged in t he lamellar part of the transition but abruptly decreases as soon as the fi rst mixed micelle is formed. This decrease is continuous all over the rest of the transition even if it is more pronounced in the mixed liposomes' and mixed micelles' coexistence. The increase of the octylglucoside-to-lipid r atio of the mixed micelles via the change of the size and the shape of the aggregates may facilitate the penetration and the mobility of water molecul es. Therefore, during the vesicle-to-micelle transition, laurdan probes the evolution of both the amphiphile packing in the aggregates and the increas e of the interface polarity. This study finally shows that the detergent-to -lipid ratio of the mixed micelles is an important parameter to control to limit the penetration and the mobility of water within the amphiphile aggre gates and that laurdan is a nice tool to monitor this phenomenon.