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The aerial part of Aloysia gratissima(Gillies & Hook. ex Hook.) Tronc., "Usillo", is used as aromatic and medicinal. It is a shrub of up to 3 meters, distributed in North America, from the south of the USA to the north of Mexico, and in South America up to the 37th parallel. As in other native species, the demand is covered by harvesting in wild populations, which brings about the deterioration of the resource and lack of homogeneity of the harvested product. The variability was characterized in nine populations of A. gratissimavar. gratissima from the northeast of San Luis, Argentina, based on morphological and phytochemical characters. It was found that the species in the region presents considerable heterogeneity. Diversity was detected in the chemical characteristics of the essential oils analyzed and the prevalence of mono and sesquiterpenes was related to the olfactory identities identified. The main components were the sesquiterpene spatulenol and the monoterpene 1,8 cineole.

The photodynamic effect on tryptophan methyl ester (trpME) and tryptophan octyl ester (trpOE), using the O(2)((1)Delta(g))-photosensitizers Rose Bengal (RB) and Perinaphthenone (PN) has been studied in large unilamellar vesicles (LUVs) of the phospholipid 1,2-di-oleoyl-sn-glycero-3-phosphatidylcholine (DOPC) by stationary photolysis and time-resolved methods. This work reports on the influence of both the site (O(2)((1)Delta(g))) generation and the location of the tryptophan derivatives (trpD), on the photo-oxidation process in a compartmentalized system. The apparent rate constant values for chemical quenching of O(2)((1)Delta(g)) by trpOE (k(r,app)), was higher in vesicles than in water. Also, the ratio between apparent reactive and overall rate constant values for the deactivation of O(2)((1)Delta(g)) (k(r,app)/k(t,app)), increases in vesicles as compared with water, when the oxidative species is generated in the lipidic region or at the interface. Nevertheless, this quotient is lower than the corresponding value in water when O(2)((1)Delta(g)) is generated in the aqueous pseudophase. For trpME, the k(r,app)/k(t,app)values in vesicles and in water are quite similar, confirming the fact that trpME is located in the water pseudophase. Results are discussed in terms of relative protection against O(2)((1)Delta(g)) attack in a microheterogeneous medium as compared with water.

The Rose Bengal-sensitized photooxidations of the dipeptides l-tryptophyl-l-phenylalanine (Trp-Phe), l-tryptophyl-l-tyrosine (Trp-Tyr) and l-tryptophyl-l-tryptophan (Trp-Trp) have been studied in pH 7 water solution using static photolysis and time-resolved methods. Kinetic results indicate that the tryptophan (Trp) moiety interacts with singlet molecular oxygen (O(2)((1)Delta(g))) both through chemical reaction and through physical quenching, and that the photooxidations can be compared with those of equimolecular mixtures of the corresponding free amino acids, with minimum, if any, influence of the peptide bond on the chemical reaction. This is not a common behavior in other di- and polypeptides of photooxidizable amino acids. The ratio between chemical (k(r)) and overall (k(t)) rate constants for the interaction O(2)((1)Delta(g))-dipeptide indicates that Trp-Phe and Trp-Trp are good candidates to suffer photodynamic action, with k(r)/k(t) values of 0.72 and 0.60, respectively (0.65 for free Trp). In the case of Trp-Tyr, a lower k(r)/k(t) value (0.18) has been found, likely as a result of the high component of physical deactivation of O(2)((1)Delta(g)) by the tyrosine moiety. The analysis of the photooxidation products shows that the main target for O(2)((1)Delta(g)) attack is the Trp group and suggests a much lower accumulation of kynurenine-type products, as compared with free Trp. This is possibly because of the occurrence of another accepted alternative pathway of oxidation that gives rise to 3a-oxidized hydrogenated pyrrolo[2,3-b]indoles.

The visible-light irradiation of the system Rivoflavin plus 3-hydroxyflavone or plus 7-hydroxyflavone, under aerobic conditions, produces a series of competitive processes that depend on the relative concentrations of the pigment and the flavones. The picture comprises photochemical mechanisms that potentially operate in nature. They mainly include the quenching of Rf singlet (1Rf*) and triplet (3Rf*) excited states (with bimolecular rate constants in the order of 109 M–1 s–1) and superoxide radical anion-mediated reactions. The participation of the oxidative species singlet molecular oxygen was not detected. The overall result shows chemical transformations in both Rf and 3-hydroxyflavone. No experimental evidence was found indicating any chemical reaction involving 7-hydroxyflavone. The fate of the pigment also depends on the amount of the dissolved flavonoid. At 50 mM concentrations of these compounds or higher, practically no photochemistry occurs, owing to the extensive quenching of 1Rf*. When the concentration of the flavones is in the mM range or lower, 3Rf* is photogenerated. Then, the excited triplet species can be quenched mainly by the flavones through an electron-transfer process, yielding the semireduced pigment. The latter interacts with dissolved oxygen producing O2·–, which reacts with both the pigment and 3-hydroxyflavone. In summary, 3-hydroxyflavone and 7-hydroxyflavone participate in the generation of superoxide ion in an Rf-sensitized process, and simultaneously 3-hydroxyflavone constitutes a degradable quencher of the oxidative species.Key words: flavones, riboflavin, sensitization, singlet molecular oxygen, superoxide radical anion.

Kinetic aspects of the sensitized photooxidation of alpha- and beta-chymotrypsins have been studied at pH 6 and 8. The sensitization, employing classical O2(1Deltag)-photogenerators, such as xanthene dyes, is a kinetically intricate process because of the presence of ground state dye-protein associations and to the simultaneous participation of superoxide ion and singlet molecular oxygen [O2(1Deltag)]. Both proteins, that possess the same distribution pattern of photooxidizable amino acids, suffer a pure O2(1Deltag)-mediated photodynamic attack, using the carbonylic sensitizer Perinaphthenone. Overall and reactive rate constants for the O2(1Deltag)-quenching (in the order of 108 and 107/M/s, respectively), and rates of oxygen consumption determined by time-resolved, spectroscopic and polarographic methods indicate that alpha- and beta-chymotrypsins are less photooxidizable at pH 6, as a result of an enhancement of the O2(1Deltag)-physical quenching component. In general terms, beta-chymotrypsin exhibits the greater overall proclivity to interact with O2(1Deltag), whereas structural factors, possibly evidenced by a higher exposure of the reactive tryptophan residues, impart an increased photooxidation degree to the proteins at pH 8, specially to the alpha-chymotrypsin.

Within our current studies on the natural photodegradation of representative aquatic environmental contaminants, the visible light irradiation of the model compound 4-hydroxypyridine (4-OHP), in air-equilibrated aqueous solution and in the presence of riboflavin (Rf), has been studied employing stationary photolysis, polarographic detection of oxygen uptake, stationary and time-resolved fluorescence spectroscopy, and laser flash photolysis. The results indicate that 4-OHP — a compound inert towards the attack of singlet molecular oxygen (O2(1Δg)) generated by Rose Bengal sensitization — quenches excited singlet and triplet states of Rf, with rate constants of 2.4×10 9 M −1 s −1 and 1.2×10 7 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 4-OHP: at ≥20 mM limited photodecomposition occurs due to the quenching of excited singlet Rf, while at ca. 5 mM triplet Rf is photogenerated and subsequently quenched either by oxygen, giving rise to O2(1Δg), or by 4-OHP, yielding semireduced Rf through an electron transfer process. Superoxide dismutase inhibition of the oxygen uptake and flash photolysis data indicate that superoxide anion is generated, likely by the reaction of the anion radical from Rf with dissolved oxygen, also yielding neutral, ground state Rf. The final result is that both 4-OHP and Rf are photodegraded, probably through oxidation with superoxide anion.

In the presence of the synthetic xanthenic dye rose bengal or the pigment riboflavin, the bacteriostatics sulfathiazole (STZ) and succinylsulfathiazole (SCSTZ) suffer visible-light promoted degradation in dilute aqueous and aqueous-ethanolic solutions. The events occur to a different extent, depending on the chemical structures of the thiazoles and pH of the medium. The photo-oxidation process is accompanied by a partial loss of the antibacterial activity of the thiazoles. The extent of photodegradation occurs quickly under alkaline conditions and is less aggressive in the physiological pH region, although not negligible in kinetic terms. Photo-oxidation quantum efficiencies, evaluated through singlet molecular oxygen [O2 1 Δ g ] phosphorescence detection, spectrophotometric and polarographic methods, range from 0.002 to 0.31 as the upper limit in alkaline media. With rose bengal as a sensitiser in aqueous medium, photo-oxidation essentially proceeds via singlet molecular oxygen. The results indicate the occurrence of several photoprocesses that involve a series of competitive reactions, including singlet molecular oxygen photo-oxidation, when the vitamin riboflavin was employed as a sensitiser, in a 1:1 water: ethanol medium. Their relative importance depends on the concentration of the thiazoles and riboflavin. The bacteriostatics efficiently quench both the excited singlet and triplet states of the pigment. The final result of the interactions is the phototransformation of the thiazoles and the partial photostabilization of riboflavin. The O2 1 Δ g -mediated photo-oxidation of STZ produces a decrease in its bacteriostatic properties.