Sandra Miskoski

INFLUENCE OF THE PEPTIDE BOND ON THE SINGLET MOLECULAR OXYGEN‐MEDIATED (O2[g]) PHOTOOXIDATION OF HISTIDINE and METHIONINE DIPEPTIDES. A KINETIC STUDY

The dye‐sensitized photooxidation of l‐histidine (His) and l‐methionine (Met) and their simplest dipeptides with glycine (Gly) (His‐Gly, Gly‐His, Gly‐Met) and Met‐methyl ester (Met‐ME) mediated by singlet molecular oxygen (O2[g]) was studied.

SINGLET MOLECULAR OXYGEN-MEDIATED PHOTO-OXIDATION OF TETRACYCLINES: KINETICS, MECHANISM AND MICROBIOLOGICAL IMPLICATIONS

Members of the biologically active series tetracyclines (TCs) suffer visible light-promoted photodynamic degradation to different extents, depending on their respective chemical structures and reaction conditions (solvent polarity and pH). The photo-oxidation is accompanied by a partial loss of the antimicrobial power. The photodamage is very fast in the alkaline pH range and less aggressive. although not negligible in kinetic terms, in the physiological pH region. Photo-oxidation quantum efficiencies, evaluated for eight TC derivatives, through singlet molecular oxygen [O2(1Delta(g))] phosphorescence detection, spectrophotometric and polarographic methods, range from 0.12 to 0.65 as upper limits in alkaline medium. The photo-oxidation essentially proceeds via a O2(1Delta(g)) mediated process, with rose bengal or eosine as dye-sensitizers, Nevertheless, as a minor reactive pathway,the excited triplet state of the dye sensitizers interacts with TCS in a competitive process with O2(1Delta(g) generation. The O2(1Delta(g)-mediated photo-oxidation of TCs appears to be a plausible mechanism to account for their phototransformations in biological media, in the presence of visible-absorbing pigments. In both highly and moderately polar media, the quenching of the excited oxygen species is mainly represented by a reactive interaction. It is exerted by the TC molecule through a cooperative effect from the different contributions of several nuclear and extranuclear O2(1Delta(g)-sensitive substituents, as discussed in detail in this paper. The TC lower than 0.03 in the most favourable cases. Nevertheless, the TC photoproduct, formed through direct irradiation, efficiently generates O2(1Delta(g) with Phi(Delta)=0.24. This important finding constitutes the first direct evidence of Type II sensitization by TC photoproducts, and could contribute to the elucidation of the mechanism of TC phototoxicity.

Effect of chlorophenolic pesticides on the photochemistry of riboflavin

The possibility of riboflavin sensitized photo‐oxidation of polychlorinated pesticides was investigated. The dye proved to be an inefficient sensitizer for that purpose, when irradiated with visible light, in aereated aqueous solutions. A quenching of singlet and triplet excited states of the chromophore (depending on the concentrations of the quencher) is responsible for this failure in the photo‐degradation of the pesticides, which resulted easily oxidizable, via singlet oxygen mechanism, when proflavine was employed as a sensitizer. Parallel, and as a direct consequence, the rate of aerobic riboflavin photobleaching decreased drastically in the presence of the chlorophenols. The implication of our results on the inhibition of riboflavin degradation by the presence of chlorinated phenolic pesticides in the environment, is discussed, from the point of view of a kinetic and mechanistic study.

Photostability and spectral properties of fluorinated fluoresceins and their biarsenical derivatives: a combined experimental and theoretical study.

New fluorinated biarsenical derivatives with improved optical properties based on highly photostable analogs of fluorescein were recently introduced. The photophysical parameters of the triplet excited states as well as photosensitized oxidation reactions of these dyes were determined in order to investigate the influence of molecular structure on the exceptional photostability of these fluorophores. The lack of correspondence between triplet quantum yields and lifetimes with the photobleaching rates of some of the fluorophores of the series suggests that differential reactivities of the excited states with ground state oxygen accounts for the different photodegradation resistances. The UV–visible absorption and emission spectra of the fluorinated fluoresceins and their biarsenical derivatives were evaluated using a TD‐DFT/BP86/6‐31G** approach, taking bulk solvent effects into account by means of the polarizable continuum model. The calculated properties are in good agreement with experimental data. The S0→S1 vertical excitation energies in the gas phase and in water were obtained with the optimized geometries of the excited states. This type of calculation could be used in the rational design of new dyes.

Vitamin B2-sensitized photooxidation of structurally related dihydroxyflavonoids

Abstract A kinetic study of the processes involved in the vitamin B2 (riboflavin, Rf)-sensitized photooxidation of selected flavonoids ( F ) was carried out in methanolic solution. Under aerobic visible-light-irradiation conditions a complex picture of competitive interactions takes place: the singlet and triplet excited states of Rf are quenched by three F , namely 5,7-dihydroxy-3′,4′-dimethoxyflavanone (FNN); 5,7-dihydroxy-4′,6,8-trimethoxyflavone (FLV) and 2′,4′-dihydroxychalcone (CHL). Concomitantly, the species singlet molecular oxygen [O 2 ( 1 Δ g )] and superoxide anion radical (O 2 •− ) are generated and interact with F and with Rf. CHN and FNN are photooxidised, probably by means of O 2 •− , whereas FVN, the poorest quencher of excited triplet Rf, allows the generation of O 2 ( 1 Δ g ), which oxidises the very flavonoid. The photodegradation of the vitamin is delayed due to an electron transfer process to ground state oxygen.

Sensitized Photooxidation of Thyroidal Hormones. Evidence for Heavy Atom Effect on Singlet Molecular Oxygen [O2(1Δg)]-mediated Photoreactions¶

Thyronine derivatives are essential indicators of thyroid gland diseases in clinical diagnosis and are currently used as standards for developing ordinary biochemical assays. Photooxidation of gland hormones of the thyronine (TN) family and structurally related compounds (TN, 3,5‐diiodo‐thyronine,3,3′,5‐triiodothyronine and 3,3′,5,5′‐tetraiodothyronine or thyroxine) was studied using rose bengal, eosin and perinaphthenone (PN) as dye sensitizers. Tyrosine (Tyr) and two iodinated derivatives (3‐iodotyrosine and 3,5‐diiodotyrosine) were also included in the study for comparative purposes. Irradiation of aqueous solutions of substrates containing xanthene dyes with visible light triggers a complex series of competitive interactions, which include the triplet excited state of the dye (3Xdye*) and singlet molecular oxygen [O2(1Δg)]‐mediated and superoxide ion‐mediated reactions. Rate constants for interaction with the 3Xdye*, attributed to an electron transfer process, are in the order of 108‐109M−1 s−1 depending on the dye and the particular substrate. The photosensitization using PN follows a pure Type‐II (O2(1Δg) mediated) mechanism. The presence of the phenolic group in Tyr, TN and iodinated derivatives dominates the kinetics of photooxidation of these compounds. The reactive rate constants, kr, and the quotient between reactive and overall rate constants (krlkt values, in the range of 0.7–0.06) behave in an opposite fashion compared with the overall rate constants and oxidation potentials. This apparent inconsistency was interpreted on the basis of an internal heavy atom effect, favoring the intersystem‐crossing deactivation route within the encounter complex with the concomitant reduction of effective photooxidation.

On the antioxidant properties of therapeutic drugs: quenching of singlet molecular oxygen by aminosalicylic acids

Abstract The ability of the widely employed therapeutic drugs 4-aminosalicylic acid and 5-aminosalicylic acid to act as singlet molecular oxygen (O2(1Δg)) scavengers was investigated at pH 7 and pH 12. The isomer 3-aminosalicylic acid was also included in the study for comparative purposes. All three compounds quench photochemically generated O2(1Δg) with rate constants in the range of 107-108 M-1s-1, depending on the experimental conditions. No chemical reaction (oxidation of the aminosalicylic acids) was detected at the neutral pH, whereas at pH 12 both chemical and physical interactions with O2(1Δg) operated. The physical process implies the de-activation of the oxidant species without destruction of the aminosalicylic acid. The quotients between the overall and reactive rate constants for O2(1Δg) quenching at pH 12 (kr/kt ratios), which account for the actual effectiveness of photodegradation, were relatively low (0.22, 0.04, and 0.06 for 3-, 4- and 5-aminosalicylic acids, respectively). This indicates that the drugs, particularly the 4- and 5-amino derivatives, de-activate the excited oxygen species, at both pH values studied, mainly in a physical fashion, preventing its photodegradation and providing an antioxidative protection for possible photo-oxidizable biological targets in the surroundings.

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.