Involvement of type I and type II mechanisms in the linoleic acid peroxidation photosensitized by tiaprofenic acid

The Effect of Cosmetic Ingredients of Phenol Type on Immediate Pigment Darkening and Their (Photo)Protective Action in Association with Melanin Pigmentation: A Model In Vitro Study

Immediate pigment darkening, the first response of skin to solar exposure leading to undesired irregular pigmentation and dark spots, is the rapid onset of melanin pigmentation resulting from oxidation of the melanogenic indoles, namely 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) available in epidermal melanocytes. The search for effective sunscreen formulations is nowadays focused on UVA/B filters and additional ingredients that may scavenge the reactive oxygen species generated in these processes. In this work the effects of phenolic cosmetic ingredients (CIs), paradol-6, a ginger CO2 extract, and phenylethyl resorcinol on photosensitized DHI and DHICA oxidation were investigated showing a decrease of their consumption and melanin formation (25–30% decrease with phenylethyl resorcinol). The photoprotective role of CIs was also evaluated in model systems. Paradol-6 and ginger CO2 extract can halve linoleic acid peroxidation in the riboflavin-sensitized reaction, while dienes generation reduction (30% of control) was observed in the Rose-Bengal-sensitized photooxidation with paradol-6. The presence of DHI/DHICA melanin exerted a synergistic effect. The decay of thymine free or as a DNA base was almost completely inhibited by CIs. These results open new perspectives in the design of skin care formulations for ameliorating skin spots and contrasting ageing processes associated with sun exposure.

Investigating the Role of Antioxidant Compounds in Riboflavin-Mediated Photo-Oxidation of Methionine: A 1H-NMR Approach

Riboflavin (RF) is a well-known photosensitizer, responsible for the light-induced oxidation of methionine (Met) leading to the spoilage of wine. An NMR approach was used to investigate the role of gallic acid (GA) and sulfur dioxide (SO₂) in the RF-mediated photo-oxidation of Met. Water solutions of RF and Met, with and without GA or SO₂, were exposed to visible light for increasing time in both air and nitrogen atmospheres. Upon light exposure, a new signal appeared at 2.64 ppm that was assigned to the S(O)CH₃ moiety of methionine sulfoxide. Its formation rate was lower in a nitrogen atmosphere and even lower in the presence of GA, supporting the ability of this compound in quenching the singlet oxygen. In contrast, SO₂ caused relevant oxidation of Met, moderately observed even in the dark, making Met less available in donating electrons to RF. The competition of GA versus Met photo-oxidation was revealed, indicating effectiveness of this antioxidant against the light-dependent spoilage of wine. A pro-oxidant effect of SO₂ toward Met was found as a possible consequence of radical pathways involving oxygen.

Thiol–ene chemistry of vegetable oils and their derivatives under UV and air: a model study by using infrared spectroscopy and mass spectrometry

This model study highlights that oxidation of fatty unsaturated esters is beneficial to prepare coatings by photoinitiated thiol–ene process.

Biradical vs singlet oxygen photogeneration in suprofen–cholesterol systems

Cholesterol (Ch) is an important lipidic building block and a target for oxidative degradation, which can be induced via free radicals or singlet oxygen (1O2). Suprofen (SP) is a nonsteroidal anti-inflammatory drug that contains the 2-benzoylthiophene (BZT) chromophore and has a π,π* lowest triplet excited state. In the present work, dyads (S)- and (R)-SP-α-Ch (1 and 2), as well as (S)-SP-β-Ch (3) have been prepared from β- or α-Ch and SP to investigate the possible competition between photogeneration of biradicals and 1O2, the key mechanistic steps in Ch photooxidation. Steady-state irradiation of 1 and 2 was performed in dichloromethane, under nitrogen, through Pyrex, using a 400 W medium pressure mercury lamp. The spectral analysis of the separated fractions revealed formation of two photoproducts 4 and 5, respectively. By contrast, under the same conditions, 3 did not give rise to any isolable Ch-derived product. These results point to an intramolecular hydrogen abstraction in 1 and 2 from the C7 position of Ch and subsequent C–C coupling of the generated biradicals. Interestingly, 2 was significantly more photoreactive than 1 indicating a clear stereodifferentiation in the photochemical behavior. Transient absorption spectra obtained for 1–3 were very similar and matched that described for the SP triplet excited state (typical bands with maxima at ca. 350 nm and 600 nm). Direct kinetic analysis of the decay traces at 620 nm led to determination of triplet lifetimes that were ca. 4.1 μs for 1 and 2 and 5.8 μs for 3. From these data, the intramolecular quenching rate constants in 1 and 2 were determined as 0.78 × 105 s−1. The capability of dyads 1–3 to photosensitize the production of singlet oxygen was assessed by time-resolved near infrared emission studies in dichloromethane using perinaphthenone as standard. The quantum yields (ΦΔ) were 0.52 for 1 and 2 and 0.56 for 3. In conclusion, SP-α-Ch dyads are unique in the sense that they can be used to photogenerate both biradicals and singlet oxygen, thus being able to initiate Ch oxidation from their triplet excited states following either of the two competing mechanistic pathways.

Soybean phosphatidylcholine liposomes as model membranes to study lipid peroxidation photoinduced by pterin

Oxidized pterins, efficient photosensitizers under UVA irradiation, accumulate in the skin of patients suffering from vitiligo, a chronic depigmentation disorder. Soybean phosphatidylcholine (SoyPC) liposomes were employed as model membranes to investigate if pterin (Ptr), the parent compound of oxidized pterins, is able to photoinduced lipid peroxidation. Size exclusion chromatography and dialysis experiments showed that Ptr is not encapsulated inside the liposomes and the lipid membrane is permeable to this compound. The formation of conjugated dienes and trienes, upon UVA irradiation, was followed by absorption at 234 and 270 nm, respectively. The photoproducts were characterized by mass spectrometry and oxygenation of SoyPC was demonstrated. In addition, analysis of MS/MS spectra suggested the formation hydroperoxides. Finally, the biological implications of the findings are discussed.

Oxidative effects induced by dediazoniation of the p-hydroxybenzenediazonium ion in a neutral aqueous medium

The toxicity of arenediazonium ions is believed to result from the appearance of very reactive compounds during the dediazoniation process. In the case of the p-hydroxybenzenediazonium ion (PDQ), radical species generated during dediazoniation could potentially initiate lipid peroxidation. The data obtained in spectrophotometric experiments suggest that an interaction between PDQ and linoleic acid (LA) gives rise to the characteristic absorption of oxidized products deriving from LA, both in the presence and absence of a mixed micellar medium containing the surfactant Tween 20 (Tw20). Spectroscopic evidence also clearly points to the interference of these processes in the dediazoniation of PDQ. Analysis by reverse-phase, high-pressure liquid chromatography (HPLC) confirms that the decomposition of PDQ in a mixed micellar medium induces the peroxidation of both LA and methyl linoleate (MEL), thus causing the appearance of peaks characteristic of dienic conjugated hydroperoxides. The same products are observed after interaction between LA and the water-soluble 2,2'-azobis (2-amidinopropane), a frequently used initiator of lipid peroxidation. The proportion of isomers produced during the peroxidation process agrees well with that reported for reactions mediated by free radicals. A further chromatographic analysis of the decomposition of PDQ in the presence of 2-methylcyclohexa-2,5-diene-1-carboxylic acid (CHD) shows that phenol and quinone are the main products of the reaction. These results are discussed on the understanding that aryl and peroxyl radicals abstract a hydrogen atom from CHD, in accordance with our general scheme for PDQ dediazoniation described in a previous publication.

Singlet-oxygen reactions sensitized on solid surfaces of lignin or titanium dioxide: Product studies from hindered secondary amines and from lipid peroxidation

Product analyses and kinetic methods were used to determine the role of singlet oxygen in lignin-catalyzed oxidations of organic substrates. Method A used the ESR analysis of nitroxide radicals formed by singlet oxygen (Type II) on 2,2,6,6-tetramethylpiperidine, 1, or tetramethylpiperidone, 2. Method B used HPLC analysis of the 9- and 13-linoleate chain hydroperoxides formed on oxidation of methyl linoleate to distinguish free-radical peroxidation (Type I) from singlet-oxygen oxidation (Type II) on the basis of different cis,trans (kinetic) to trans,trans (thermodynamic) product ratios. Applications of method A to solid dispersions of lignin or titanium dioxide (TiO2, a known singlet-oxygen sensitizer) indicated singlet-oxygen reactions. In addition to the nitroxide triplet, irradiation of lignin produces a persistent broad signal in the solid attributed to phenoxyl radicals. Benzophenone and 3,5-di-tert-butyl-ortho-benzoquinone, 5, coated on silica gel were used as models to compare the effects of irradiating such compounds on the products and kinetics of methyl linoleate oxidation. Benzophenone acted as an initiator, giving free-radical peroxidation, whereas 5 or lignin coated with methyl linoleate acted as singlet-oxygen sensitizers, according to both product studies (method B) and the kinetic order in oxygen consumption during UV photolysis. Photolysis of phase-separated sensitizer (TiO2 or lignin) and substrate (methyl linoleate) resulted in typical singlet-oxygen products. These results indicate that singlet oxygen plays a significant role in the photo-yellowing of high-lignin-content wood pulps. Key words: lignin, singlet oxygen, mechanism, peroxidation, products.

Photoreactivity of the nonsteroidal anti-inflammatory 2-arylpropionic acids with photosensitizing side effects

The photoreactivity of the nonsteroidal anti-inflammatory 2-arylpropionic acids benoxaprofen, carprofen, naproxen, ketoprofen, tiaprofenic acid, and suprofen is reviewed with special emphasis on fundamental photophysical and photochemical properties. The absorption and emission properties of the excited states of these drugs as well as their main photodegradation routes are summarized. The photochemical mechanisms are discussed on the basis of product studies and detection of short-lived intermediates by means of laser flash photolysis. After dealing with the unimolecular processes, attention is focused on the photosensitized reactions of key biomolecules, such as lipids, proteins or nucleic acids. Finally, a short section on the photobiological effects on simple biological models is also included. Although some earlier citations are included, the literature coverage is in general limited to the last decade.

Mechanism of lipid peroxidation photosensitized by tiaprofenic acid: product studies using linoleic acid and 1,4-cyclohexadienes as model substrates

A careful study of the linoleic acid hydroperoxide (LOOH) profile obtained upon peroxidation of linoleic acid (LA) photosensitized by tiaprofenic acid (TPA) and analogous ketones has been undertaken to distinguish between type-I and type-II photoperoxidation mechanisms. 1,4-Cyclohexadiene and 1,2-dimethylcyclohexa-2,5-dienecarboxylic acid (CHDCA) have also been used as models for LA since they also have double allylic systems. Coirradiation of LA with TPA and decarboxytiaprofenic acid (DTPA) in acetonitrile and micellar media produced significant amounts of conjugated dienic LOOH. The cis,trans to trans,trans ratio depended on the irradiation time; thus, this parameter is an ambiguous tool for mechanistic assignment. An interesting finding was the decrease of the LOOH level after long irradiation times in mixtures photooxidized by DTPA, which is attributed to quenching of the DTPA triplet by the generated dienic LOOH. High-performance liquid chromatography analyses confirmed that the main pathway operating in photodynamic lipid peroxidation sensitized by (D)TPA is a type-I mechanism. However, product studies using CHDCA have clearly shown that a type-II mechanism is also operating and might contribute to the overall photooxidation process in a significant way.