Norman A. García

New trends in photobiology: Singlet-molecular-oxygen-mediated photodegradation of aquatic phenolic pollutants. A kinetic and mechanistic overview

Abstract The role of singlet molecular oxygen (O2(1Δg)) as an agent for the environmental degradation of aquatic phenolic pollutants is reviewed. Literature data on kinetic parameters, reaction mechanisms and reaction products for the photo-oxidation of phenols are compiled and critically evaluated.

Thermal-lensing measurements of singlet molecular oxygen (1Δg): quantum yields of formation and lifetimes

Abstract Absolute quantum yields φΔ for singlet molecular oxygen (1O2(1Δg)) production were determined by time-resolved thermal-lensing (TL) using several sensitizers. The φΔ value is smaller than the quantum yield φisc for intersystem crossing for tetraphenylporphyrin, zinc tetraphenylporphyrin and anthracene is air-saturated benzene solutions. For anthracene, the values φisc = 0.78 ± 0.08 and φΔ/ = 0.78 ± 0.16 were measured using this method. The lifetimes of 1O2, determined by time-resolved TL in several solvents at concentrations similar to those detected by emission, are in good agreement with literature data for values up to about 100 μs. For longer lifetimes the values are smaller than those in the literature. Various possible reasons for the discrepancies are discussed.

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.

Effect of the peptide bond on the singlet-molecular-oxygen-mediated sensitized photo-oxidation of tyrosine and tryptophan dipeptides. A kinetic study

Abstract The dye-sensitized photo-oxidation of tyrosine (tyr), tryptophan (trp) and their simplest dipeptides with glycine (gly) (tyr-gly, gly-tyr, trp-gly and gly-trp), mediated by singlet molecular oxygen (O2(1Δg)), was studied. The overall rate constants (kt) of O2(1Δg) quenching were measured by time-resolved IR detection. Reactive rate constants (kr) were determined in mixtures of polar organic solvents and in water to discriminate between the overall and physical contributions to the quenching. For trp and its two gly peptides, the peptide bond has practically no effect on the kinetics of photo-oxidation; kt is of the order of 107 M−1 s−1 in C2H5OHCH3CN (80:20) and the kq/kr ratio is in the range 11–13. In water (pH 7), kr is in the (3–5) × 107 M−1 s−1 range, the highest value corresponding to gly-trp. Loss of −NH2 in water suggests the formation of pyrroloindole-like photo-oxidation products in the case of trp and gly-trp. For tyr derivatives, photo-oxidation is hindered when the amino group of tyr is masked: kt and kr are significantly lower for gly-tyr. The kq/kr ratio in organic alkaline medium increases from 1 and 5 for tyr-gly and tyr respectively to 34 for gly-tyr. In buffered alkaline solutions, the kr values are in the range (1.4–5) × 107 M−1 s−1 with a small increase on going from pH 10 to 11.5. Only physical quenching of O2(1Δg) by tyr derivatives is observed in the acidic range. The results, together with the loss of −NH2, indicate a similar product distribution for tyr and tyr-gly on sensitized photo-oxidation.

Kinetic aspects of the rose bengal-sensitized photo-oxygenation of tryptophan alkyl esters. Ground state and photopromoted dye-tryptophan derivative interactions

The kinetics of the rose bengal (RB)-promoted photodynamic action on tryptophan (trp) and a series of tryptophan alkyl esters (methyl, butyl and octyl derivatives) was studied. The employment of auxiliary specific quenchers for superoxide ion and singlet molecular oxygen (O2(1Δg)) indicates that the O2(1Δg)-mediated process constitutes the major path in the photo-oxygenation process (with a contribution of about 70% to the total oxygen consumption upon RB-sensitized irradiation of the tryptophan derivatives). Blocking of the carboxylic group (esterification) greatly increases the O2(1Δg)physical quenching ability of the amino acid derivatives. The effect is more pronounced for those compounds possessing longer hydrocarbon chains. Besides, the pathway for chemical (reactive) quenching is slightly decreased in a general manner, this behaviour being independent of the blocking group. Results were corroborated by employing methylene blue as a dye sensitizer. Additionally, the interactions of the amino acid derivatives with RB excited and ground states were also studied. Laser flash photolysis determinations indicate that the tryptophan derivatives interact with the excited triplet state of the dye with rate constants of the order of 107 M−1 s−1. A ground state complexation between RB and the amino acid derivatives took place in the dark. Association constants, determined by static fluorescence quenching of the dye, increased with the length of the hydrocarbon chain of the ester. They ranged from 40 M−1 for trp to 18 000 M−1 for trp-octyl ester. The interaction was governed by an additive effect of charge transfer and hydrophobic bond formation.

The interaction of singlet molecular oxygen O2(1Δg) with indolic derivatives. Distinction between physical and reactive quenching

Abstract A number of indolic compounds were studied as quenchers of singlet molecular oxygen. O2(1Δg) generated by photosensitization. The ratio of the chemical (reactive) and physical quenching paths was a function of the substitution pattern of the quencher. Indoles substituted in position 3 gave measurable reaction rates, whereas the parent indole and those derivatives substituted in other nuclear positions predominantly gave physical interaction. The results are discussed in terms of the formation of an exciplex with partial polar character between O2(1Δg) and the indolic compound.

Sensitized photo-oxidation of dihydroxybenzenes and chlorinated derivatives. A kinetic study

Abstract The dye- sensitized photo-oxidation of dihydroxybenzenes and chlorinated derivatives was studied as a function of solvent polarity. Rate constants for the disappearance of singlet molecular oxygen (O 2 ( 1 Δ g )), determined by time-resolved phosphorescence at λ ⪢ 1050 nm, are in the range 5 × 10 5 to 3 × 10 8 M −1 s −1 depending on the solvent polarity and substitution pattern of the reactant. The ratio between the rate constant for reactive quenching k r and the total quenching constant k t is also strongly dependent on substitution pattern and solvent. The values of k r indicate that the O 2 ( 1 Δ g )-mediated photo-oxidation of the dihydroxybenzenes is a viable possibility for their degradation in an aqueous environment. A charge-transfer mechanism is suggested for the photo-oxygenation. Dihydroxybenzenes are fairly good O 2 ( 1 Δ g ) self-sensitizers of photo-oxidation on irradiation at 308 nm, with O 2 ( 1 Δ g ) quantum yields of approximately 0.2 in the most favourable case. For hydroquinones, this presumably occurs through the excitation of the corresponding quinone derivatives which are always present in traces in the aerated solutions.

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.

Dark and photoinduced interactions between Trolox, a polar-solvent-soluble model for vitamin E, and riboflavin

The aerobic riboflavin (Rf)-sensitized visible-light irradiation of Trolox (TX), a polar-solvent-soluble model for vitamin E, has been studied employing stationary photolysis, polarographic detection of oxygen uptake, stationary and time-resolved fluorescence spectroscopy, and laser flash photolysis. Results indicate that in methanolic solution, no dark complexation exists between Rf and TX. The latter quenches singlet and triplet states of Rf, with rate constants of 6.2 x 10(9) M(-1) s(-1) and 4.7 x 10(9) M(-1) s(-1), respectively. The photodecomposition of Rf, a known process taking place from triplet Rf, has been found to depend on the concentration of dissolved TX: at >/=30 mM very slight Rf photodecomposition occurs due to the massive quenching of excited singlet Rf, while at TX concentrations < or =1 mM triplet Rf is photogenerated and subsequently quenched either by oxygen, giving rise to O(2)((1)Delta(g)), or by TX, yielding semireduced Rf through an electron transfer process. Complementary experiments performed in pure water employing superoxide dismutase and sodium azide inhibition of the oxygen uptake, in coincidence with flash photolysis data, indicate that superoxide anion and singlet molecular oxygen are generated, likely by the reaction of the anion radical from Rf with dissolved oxygen, also yielding neutral, ground state Rf or by energy transfer from triplet Rf to ground-state oxygen, respectively. The final result is that both TX and Rf are photodegraded, likely through oxidation with activated oxygen species. In the absence of oxygen no degradation of TX can be detected, but Rf photodegradation is favoured because Rf regeneration is avoided.

Sensitized photooxidation of Di- and tripeptides of tyrosine.

This paper studies the dye‐sensitized photooxidation of tyrosine (tyr) and tyr di‐ and tripeptides (tyr‐tyr and tyr‐tyr‐tyr) mediated by singlet molecular oxygen (O,[l]) in alkaline media. Photooxidation quantum efficiencies (φr) were obtained by determining the overall and reactive rate constants of interaction with the oxidative species, employing the time‐resolved O,(l) phosphorescence detection method and static‐photolysis actinometric method, respectively. The interaction of O,(l)‐tyr derivatives occurs through an intermediate encounter complex with polar character. Ionization of the phenolic OH group of tyr derivatives and the polarity of the solvent favors the overall interaction. Nevertheless, & values decrease when changing from water to MeCN‐water medium. This indicates that the reactive deactivation of the encounter complex, probably an entropy‐controlled step, may be affected by solvent polarity in the same way as those processes in which charg‐ es are neutralized along the reaction pathway. Photooxidation quantum efficiencies indicate that the contribution to O,(′) physical quenching (a second alternative deactivation route for the encountered complex [O,(′)‐tyr derivatives]) increases with the complexity of the peptide. As a result, the selfprotection of the peptidic entity against physical quenching also increases. The information obtained from the fractional consumption mol OJmol tyr derivative (in tyr, the di‐ and tripeptides and the respective methyl ester of tyr and the tripeptide), together with the evolution (either consumption and/or generation) of primary amino groups upon photosensitized irradiation of the same compounds clearly indicates that the photooxidation of di‐ and tri‐tyr peptides proceeds with the breakage of peptidic bonds. As a consequence, in the final balance each tyr unity behaves as an independent photooxidizable target.