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Nanosecond Dynamics of a Mimicked Membrane-Water Interface Observed by Time-Resolved Stokes Shift of LAURDAN

Michel Vincent ^*, Béatrice de Foresta  and Jacques Gallay ^*

^* LURE Laboratoire pour l'Utilisation du Rayonnement Electromagnétique, Université Paris-Sud, Bâtiment 209D, BP 34, F-91898 Orsay Cedex, France; and Service de Biophysique des Fonctions Membranaires, CEA DSV/DBJC and URA CNRS 2096, Centre d'Etudes de Saclay, 91191 Gif sur Yvette Cedex, France

Correspondence: Address reprint requests to Michel Vincent, Institut de Biophysique et Biologie Moléculaire et Cellulaire, UMR CNRS 8619, Université Paris-Sud, Bâtiment 430, F-91405 Orsay Cedex, France. E-mail: michel.vincent{at}ibbmc.u-psud.fr.

We studied the dipolar relaxation of the surfactant-water interface in reverse micelles of AOT-water in isooctane in the nanosecond and subnanosecond time ranges by incorporating the amphipathic solvatochromic fluorescent probes LAURDAN and TOE. A negative component was observed in the fluorescence decays in the red edge of the emission spectrum—the signature of an excited state reaction—with LAURDAN but not for TOE. The deconvolution of the transient reconstructed spectra of LAURDAN based on a model constructed by adding together three log-normal Gaussian equations made it possible to separate the specific dynamic solvent response from the intramolecular excited state reactions of the probe. The deconvoluted spectrum of lowest energy displayed the largest Stokes shift. This spectral shift was described by unimodal kinetics on the nanosecond timescale, whereas the relaxation kinetics of water-soluble probes have been reported to be biphasic (on the subnanosecond and nanosecond timescales) due to the heterogeneous distribution of these probes in the water pool. Most of this spectral shift probably resulted from water relaxation as it was highly sensitive to the water to surfactant molar ratio (w₀) (60–65 nm at w₀ = 20–30). A small part of this spectral shift (9 nm at w₀ = 0) probably resulted from dipolar interaction with the AOT polar headgroup. The measured relaxation time values were in the range of the rotational motion of the AOT polar headgroup region as assessed by LAURDAN and TOE fluorescence anisotropy decays.

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