Gabriela V. Ferrari

The Employment of a Removable Chitosan‐Derivatized Polymeric Sensitizer in the Photooxidation of Polyhydroxylated Water‐Pollutants

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 × 108 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.

The NSAIDs Indomethacin and Diflunisal as scavengers of photogenerated reactive oxygen species

Diflunisal (DFNS) and Indomethacin (IMTC) are two profusely employed NSAIDs that provide anti‐inflammatory and analgesic effects in humans. The scavenging of reactive oxygen species (ROS) by both NSAIDs was systematically studied in pH 7 aqueous solution. The ROS O2(1Δg), O2•− and H2O2, generated by visible light irradiation of Riboflavin (Rf) in the presence of DFNS and IMTC, are deactivated by the NSAIDs. The ROS scavenging action by both NSAIDs constitutes an interesting result and adds one more positive aspect to the beneficial actions attributed to these drugs. Nevertheless it should be taken into account that several NSAIDs, in particular IMTC, have been connected to the pathogenesis of gastric mucosal lesions, which in some cases includes ROS generating‐ability. DFNS quenches ROS in a dominant physical fashion. It constitutes an excellent protective‐antioxidant provided that is practically not destroyed/oxidized after the ROS scavenging action. IMTC, being also an efficient interceptor of ROS, belong to the so‐called group of sacrificial‐ROS quenchers: It is easily degraded by the oxidative species in the scavenging action. Although this property is negative in the context of prolonged ROS elimination, exhibits a promissory aspect for the degradation of pharmaceutical contaminants, such as NSAIDs, in waste waters.

On the generation and quenching of reactive-oxygen-species by aqueous vitamin B2 and serotonin under visible-light irradiation

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.

Scavenging of photogenerated ROS by Oxicams. Possible biological and environmental implications

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.

Effect of Cu2 +-complexation on the scavenging ability of chrysin towards photogenerated singlet molecular oxygen (O2(1Δg)). Possible biological implications

Visible-light irradiation of aqueous-ethanolic solutions of Riboflavin (Rf) in the individual presence of the flavone chrysin (Chr) and its complex with Cu(2+) ([Chr2Cu]; 2:1 L:M) generates singlet molecular oxygen O2((1)Δg), that concomitantly interact with both flavone derivatives. Overall (kt) and reactive (kr) rate constants in the order of 10(7)M(-1)s(-1) were determined for the process. Metal chelation greatly enhances the scavenging ability of [Chr2Cu] towards O2((1)Δg) through a mechanism dominated, in >80%, by the physical component. In this way, practically all O2((1)Δg) is deactivated by the complex without significant loss of the quencher. The isolated flavone quenches O2((1)Δg) in a prevailing reactive fashion. The very low value exhibited by [Chr2Cu] for the kr/kt ratio constitutes a positive quality for antioxidative protectors in biological media, where elevated local concentration and high reactivity of significant molecules make them initial targets for O2((1)Δg) aggression. Finally, two interesting properties in the field of free radicals scavenging by [Chr2Cu] must be mentioned. In first place metal chelation itself, in the obvious sense of free metal ion withdrawal from the oxidizable medium, prevents the initiation of a free radical-mediated oxidation processes through mechanisms of Fenton or lipid peroxidation. In addition, the incorporation of Cu adds to [Chr2Cu] the ability of a free radical scavenger, already described for similar Cu-chelate compounds. This collection of beneficial properties positions the complex as a remarkably promising bioprotector towards ROS-mediated oxidation. A quantification of the efficiency on the initial anti-oxidative effect exerted by Chr and [Chr2Cu] towards tryptophan was carried out. The amino acid is an archetypal molecular model, commonly employed to monitor oxidative degradation of proteinaceous media. It was efficiently photoprotected against O2((1)Δg)-mediated photooxidation by [Chr2Cu].

Chalcones as Analytical Reagents of Aluminum: Stability, Thermodynamic and Kinetic Study

Abstract Chalcones, a group of polyphenolic compounds, has been studied for a long time due to their biological properties and their ability as analytical reagents. In this work the stability and thermodynamic parameters concerning the fotmation in ethanolic medium of two chalcone–aluminium complexes (2′,3-dihydroxychalcone-Al(III) and 2′,4′,3-trihydroxychalcone-Al(III)) were studied and a kinetic study of the formation reaction was aslo performed. Both systems showed 1:1 L : M stoichiometry and stability constants at four temperatures were determined. Thermodynamic parameters indicate that both formation reactions are endothermic and driving force is entropic. Kinetic study revealed that 2′,4′,3-trihydroxychalcone-Al(III) formation is faster than 2′,3-dihydroxyclacone-Al(III), a factor which can be interesting when proposing new analytical reagents.