Scavenging of photogenerated singlet molecular oxygen and superoxide radical anion by sulpha drugs - Kinetics and mechanism

摘要

The ability of the sulfanilic antibiotics (SDs),dapsone (DAP),sulfisoxazole (SFX),sulfadiazine (SFD),and sulfanilic acid (SFNA) to act as scavengers of the visible-light-photogenerated species superoxide radical anion (O_2~(centre dot-)) and singlet molecular oxygen (O_2(~1DELTA_g)) was studied employing the natural pigment riboflavin (Rf) and the artificial dye Rose Bengal as photosensitisers.A complex mechanism,common to all the SDs studied,was elucidated through stationary photolysis,polarographic detection of oxygen uptake,fluorescence spectroscopy,time-resolved phosphorescence detection of O_2(~1DELTA_g),and laser flash photolysis.Visible-light irradiation of aqueous and aqueous methanolic solutions of Rf (ca.0.02 mmol/L) plus SD (ca.0.5 mmol/L) photogenerated excited singlet and triplet Rf (~1Rf and ~3Rf).Under these experimental conditions,only ~3Rf is quenched either by oxygen,giving rise to O_2(~1DELTA_g) by electronic energy transfer to dissolved ground-state oxygen,or by SD,yielding semireduced Rf through an electron-transfer process.Complementary experiments performed in pure water employing superoxide dismutase and sodium azide inhibition of oxygen uptake,in parallel with laser flash photolysis data,showed that O_2~(centre dot-) is also formed,probably due to the reaction of the anion radical from Rf with dissolved oxygen,yielding also neutral,ground-state Rf.Both active oxygen species,namely,O_2~(centre dot-) and O_2(~1DELTA_g),are quenched by the SDs and,as a result,photodegradation of the SDs - each to a different extent - and photodegradation of the sensitiser itself were observed.The SDs that kinetically behave as the better physical quenchers of O_2(~1DELTA_g),which are in principle good candidates as photoprotectors,namely,DAP and SFD,suffer photooxidation,exhibiting high to moderate oxygen consumption rates due to the O_2~(centre dot-) oxidative pathway,whereas for SFNA and SFX,oxidation predominantly occurs through an O_2(~1DELTA_g)-mediated mechanism.Microbiological results for SFX,taken as a representative SD,indicate that the photodegradation of the drug,upon visible-light Rf-sensitised irradiation,is accompanied by a net loss in bacteriostatic activity.