A kinetic study of the photodynamic properties of the xanthene dye merbromin (mercurochrome) and its aggregates with amino acids in aqueous solutions

Abstract

The photophysical properties and photobleaching of the xanthene dye merbromin (MR), and the production and quenching of singlet molecular oxygen, O 2( 1Δ g), were investigated in aqueous solutions, in comparison with the xanthene dyes eosin (Eos) and rose bengal (RB). MR photobleaches in the absence of O 2 in a process involving the solvent (H 2O or alcohols). No participation of O 2· − or O 2( 1Δ g) could be established in the presence of O 2, although O 2 was consumed. In aqueous and alcoholic solutions, MR forms ground state, charge transfer (CT) complexes with biologically relevant electron-donor amino acids, such as histidine and tryptophan, and other indolic derivatives. These complexes are formed with relatively high association constants (3×10 3−5×10 3) and are mainly responsible for the photobleaching of MR and the amino acids in these solutions by a mechanism involving O 2( 1Δ g) generated probably by an excited state of the CT complex. MR generates O 2( 1Δ g) with a relatively high yield (Φ Δ=0.23±0.04) and quenches O 2( 1Δ g) only physically with k q=(2.3±0.05)×10 8 M −1 s −1. This process does not affect the photosensitizing ability of MR under the low concentrations needed for its action. Non-donor compounds such as the amino acid methionine and linoleic acid methyl ester are photodegraded by a type II mechanism without implication of a dye CT complex.