Monomerization of photosensitizers by ultrasound irradiation in surfactant micellar solutions

The absorption and fluorescence properties of pheophorbide-a, Sodium salt of pheophorbide-a and its long chain (C20H39) ester (Pheophytine) were investigated in air-saturated micellar aqueous solutions before and after ultrasound irradiation (48 kHz, 10 min). The absorption spectra changed depending on the surfactant; cetyltrimethyl ammonium bromide (CTAB) or sodium dodecyl sulfate concentrations. The formation of different molecular species in various micellar solutions was estimated from the analysis of the absorption spectra. The absorption bands resulted from an aggregated form of the chromophore present in 50 mM phosphate buffer and in pre-micellar solutions. The specific bands of the aggregate disappeared with a simultaneous increase of the bands of monomer in normal micellar solution. The fluorescence spectra, the lifetimes and the fraction of each component (with a characteristic lifetime) of the chromophore in the micellar solutions changed significantly before and after ultrasound irradiation although the changes in absorption spectra were small. The fluorescence emission band at 710 nm due to the aggregate almost disappeared in the pre-micellar solution after ultrasound irradiation. The fraction of the short-lifetime component estimated for the aggregates decreased 55% in H2O or 85% in 2 mM CTAB, however the long-lifetime components increased after the ultrasound treatment. From these fluorescence properties, it was concluded that the aggregated molecules were converted to a stable monomeric form by ultrasound. Extrapolation of these data to in vivo situations suggests that pretreatment of certain photosensitizers with ultrasound in micellar solutions may lead to increased efficiency of photodynamic therapy since only the monomers are photodynamically active.