SPECTROSCOPY, IMAGING, OTHER TECHNIQUES |
Nanosecond Dynamics of a Mimicked Membrane-Water Interface Observed by Time-Resolved Stokes Shift of LAURDAN
Michel Vincent 1*, Beatrice de Foresta 2 and Jacques Gallay 1
1 CNRS
2 DBJC et URA 2096 (CNRS), CEA Saclay Section de Biophysique des Fonctions
* To whom correspondence should be addressed. E-mail: michel.vincent{at}ibbmc.u-psud.fr.
Submitted on December 3, 2004
Revised on March 5, 2005
Accepted on 11 March 2005
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Abstract |
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We studied the dipolar relaxation of the surfactant/water interface in reverse micelles sodium diethylhexyl sulfosuccinate (AOT)/water in isooctane in the nanosecond and subnanosecond time ranges by incorporating the amphipathic solvatochromic fluorescent probes 6-dodecanoyl-2-(N-dimethylaminonaphthalene) (LAURDAN) and tryptophan octyl ester (TOE). A negative component was observed in the fluorescence decays of the red edge 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 time scale, whereas the relaxation kinetics of water-soluble probes have been reported to be biphasic (on the subnanosecond and nanosecond time scales) 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 head group. The measured relaxation time values were in the range of the rotational motion of the AOT polar head group region as assessed by LAURDAN and TOE fluorescence anisotropy decays.
Key Words:
AOT, LAURDAN, Time-resolved fluorescence Stokes shift;, dipolar relaxation, reverse micelles
