RESUMO
The profusely employed drugs Piroxicam (Piro), Tenoxicam (Teno) and Meloxicam (Melo) belonging to the non-steroidal antiinflammatory drug (NSAID) family of the Oxicams (Oxis) were studied in the frame of two specific conditions: (a) their ROS scavenging ability, in relation to a possible biological antioxidant action and (b) their photodegradability under environmental conditions, in the context of Oxi-contaminated waters. Singlet molecular oxygen (O2((1)Δg)) and superoxide radical anion (O2(-)) were photogenerated through Riboflavin (Rf, vitamin B2)-photosensitization in aqueous and aqueous-methanolic solutions in the presence of Oxi concentrations in the range 50-500 µM. The visible-light absorber vitamin is currently present in all types of natural waters and constitutes the most frequent endogenous photosensitizer in mammals. Hence, it was employed in order to mimic both natural sceneries of interest. All three Oxis quench O2((1)Δg) with rate constants in the order of 10(8)M(-1)s(-1) showing a significant photodegradation efficiency given by a dominant reactive fashion for deactivation of the oxidative species. Although this is not a desirable property in the context of photoprotection upon prolonged photoirradiation, constitutes in fact a promissory aspect for the degradation NSAIDs, in waste waters. Indirect evidence indicates that Melo is also oxidized through a O2(-)-mediated component. The simultaneous presence of Piro plus tryptophan or tyrosine under Rf-photosensitizing conditions, which has taken the amino acids as photooxidizable model residues in a proteinaceous environment, indicates that the NSAID induces a protection of the biomolecules against photodynamic degradation.
Assuntos
Sequestradores de Radicais Livres/química , Piroxicam/análogos & derivados , Piroxicam/química , Espécies Reativas de Oxigênio/química , Tiazinas/química , Tiazóis/química , Anti-Inflamatórios não Esteroides/química , Luz , Meloxicam , Oxirredução , Fotólise/efeitos da radiação , Piridinas/química , Rodaminas/química , Riboflavina/química , Oxigênio Singlete/química , Superóxidos/química , Triptofano/química , Tirosina/química , Poluentes Químicos da Água/químicaRESUMO
The known O2((1)∆g)-sensitizer system Chitosan bounded Rose Bengal (CH-RB), with Rose Bengal (RB) immobilized by irreversible covalent bonding to the polymer Chitosan (CH), soluble in aquous acidic medium, was employed in the photodegradation of three tri-hydroxy benzene water-contaminants (THBs). The system sensitizes the O2((1)∆g)-mediated photodegradation of THBs by a process kinetically favored, as compared to that employing free RB dissolved in the same solvent. Additionally the free xanthene dye, degradable by O2((1)∆g) through self-sensitization upon prolonged light-exposure, is considerably protected when bonded to CH-polymer. The polymeric sensitizer, totally insoluble in neutral medium, can be removed from the solution after the photodegradative cycle by precipitation through a simple pH change. This fact constitutes an interesting aspect in the context of photoremediation of confined polluted waters. In other words, the sensitizing system could be useful for avoiding to dissolve dyestuffs in the polluted waters, in order to act as conventional sunlight-absorbing dye-sensitizers. In parallel the interaction CH-O2((1)∆g) in acidic solution was evaluated. The polymer quenches the oxidative species with a rate constant 2.4 × 10(8) M(-1) s(-1) being the process mostly attributable to a physical interaction. This fact promotes the photoprotection of the bonded dye in the CH-RB polymer.
Assuntos
Quitosana/química , Processos Fotoquímicos , Fármacos Fotossensibilizantes/química , Polímeros/química , Poluentes Químicos da Água/química , Hidroxilação , Oxirredução , SolubilidadeRESUMO
It is well known that endogenous daylight-absorbing compounds produce the sensitized photodegradation of biologically relevant substrates. In this context the photostability of a mixture of the indole neurotransmitter serotonin (Sero) and vitamin B2 (riboflavin, Rf) upon visible-light irradiation and the possible role of Sero and related compounds as generators or deactivators of reactive oxygen species (ROS) was investigated through a kinetic and mechanistic study. The work was done at pH 7 and under experimental conditions in which only the vitamin absorbs photoirradiation. Tryptamine (Trpa) and 5-hydroxyindole (OHIn) were included in the study as model compounds for the neurotransmitter. The visible light irradiation of aqueous Rf in the individual presence of Sero, Trpa and 5-OHIn, under aerobic conditions, induce degradative processes on the indole derivatives (In-der). At least two different mechanisms operate. Our analysis shows that the main reaction pathway is an electron-transfer-mediated quenching of triplet excited Rf ((3)Rf(*)) by the In-der. It produces the species Rf(-)/RfH() and the In-der radical cation that could react to form phenoxy and α-amino radicals. In a further reaction step the species O(2)(-) and OH() could be produced. In parallel, energy transfer from (3)Rf(*) to dissolved oxygen would generate O(2)((1)Δ(g)). Within the frame of the proposed mechanism, results suggest that Rf-sensitized degradation of Sero occurs via the mentioned ROS and non-oxygenated radical-mediated processes. The indole compound quenches O(2)((1)Δ(g)) in a dominant physical fashion. This fact constitutes a desirable property in antioxidants, provided that the quenching process practically does not eliminate the scavenger. Sero exerts a photoprotective effect towards tryptophan through the combined quenching of O(2)((1)Δ(g)) and (3)Rf(*), the latter excited species responsible for the generation of ROS. The amino acid can be taken as a target model of oxidizable biological substrates, particularly proteins.