SPECTROSCOPIC AND DYNAMIC CHARACTERIZATION OF FMN IN REVERSED MICELLES ENTRAPPED WATER POOLS

Abstract— The encapsulation of FMN in surfactant entrapped water pools resulted into specific interactions of FMN with the polar head groups, the entrapped water molecules and the outer apolar solvent. Two positively charged surfactant/solvent systems were employed: dodecyl ammonium propionate (DAP) in toluene and hexadecyltrimethylammonium bromide (CTAB) in chloroform/ n -octane (6:5, vol/vol). Also a surfactant with a negatively charged polar head group, sodium bis (2-ethylhexyl) sulfosuccinate (AOT) in n -octane, was used. In CTAB and especially DAP reversed micellar systems the light absorption spectra revealed the localization of the flavin in a more apolar environment, while in AOT reversed micelles FMN appeared to reside mainly in the core of the water pool. The fluorescence spectra showed unresolved bands, which were blue-shifted in DAP and CTAB reversed micelles as compared to the spectra of aqueous FMN solutions. The fluorescence decay kinetics of FMN in enclosed water droplets is non-exponential. The heterogeneity can be explained assuming incomplete relaxation of partly immobilized water molecules during the lifetime of the excited singlet state. The relatively high anisotropy of the fluorescence of FMN in encapsulated water indicated a higher viscosity than in bulk water. This was confirmed by anisotropy decay measurements of FMN in DAP and AOT entrapped water, for which the rotational correlation times were much longer than for FMN in plain water.