Singlet Oxygen Oxidation Research Articles

Chemistry and Reaction of Singlet Oxygen in Foods.

Singlet oxygen is a highly reactive, electrophilic, and nonradical molecule. It is different from diradical triplet oxygen in its electron arrangement. Photosensitizers can form singlet oxygen from triplet oxygen in the presence of light. The reaction rate of singlet oxygen with foods is much greater than that of triplet oxygen due to the low activation energy. Singlet oxygen oxidation produces undesirable compounds in foods during processing and storage. However, carotenoids and tocopherols in foods can minimize singlet oxygen oxidation. The in-depth scientific knowledge on the formation, reactions, quenching mechanisms, and kinetics of singlet oxygen can greatly improve the quality of foods by minimizing the oxidation during processing and storage. The single oxygen oxidation of foods has contributed to the explanation of several important chemical reactions in the reversion flavor in soybean oil, sunlight flavor in milk products, and the rapid losses of vitamin D, riboflavin, and ascorbic acid in milk under light storage.

Monoterpene hydroperoxides with trypanocidal activity from Chenopodium ambrosioides.

Four monoterpene hydroperoxides were isolated from aerial parts of Chenopodium ambrosioides along with ascaridole (1), the anthelmintic principle of this plant, as anti-trypanosomal compounds. The structures of these monoterpenes were determined to be (-)-(2S,4S)- and (-)-(2R,4S)-p-mentha-1(7),8-dien-2-hydroperoxide (2a and 3a) and (-)-(1R,4S)- and (-)-(1S,4S)-p-mentha-2,8-dien-1-hydroperoxide (4a and 5a) on the basis of spectroscopic methods and chemical correlations. In vitro trypanocidal activities of ascaridole (1) and these hydroperoxides (2a-5a) against epimastigotes of Trypanosoma cruzi were 23, 1.2, 1.6, 3.1, and 0.8 microM, respectively. Fresh leaves of C. ambrosioides also contained isomeric hydroperoxides 6a and 7a, and the content ratio of 2a-7a suggested that these hydroperoxides were formed through the singlet-oxygen oxidation of limonene.

Singlet Oxygen ( 1O 2) Disrupts Platelet Aggregates

Introduction: Important mediators of activated polymorphonuclear leukocytes (PMN) are the oxidants HOCl and chloramine, which generate the nonradical photon-emitting oxidant singlet oxygen ( 1O 2). Since 1O 2 inhibits platelet aggregation, we became interested in a possible oxidant mediated reversibility of platelet aggregation. Methods: Chloramine T (CT) is a stable 1O 2 generator that mimics the natural chloramine N-chloro-taurine. Platelet-rich plasma (PRP) was incubated with CT 0–8 min after addition of the aggregation agonist (10 μM adenosine-5′-diphosphate, ADP, or 5 μg/ml collagen) and the aggregation was monitored. Platelet function was also analyzed by the platelet function analyzer, PFA-100. Fifty microliters of 200 μmol/l ADP was added to 400 μl PRP. After 1 min at 37 °C, 50 μl of 0 or 30 mmol/l CT was added, and after an incubation for 3 min at 37 °C, 50 μl of 25% glutaraldehyde was added. The samples were analyzed in a transmission microscope at ×3000 and ×7000 magnification. Results: Chloramines inhibit platelet function in PRP: about 1 mM CT suppresses 50% of the aggregatory capacity of thrombocytes in normal PRP (effective dose 50%, ED 50=1 mM chloramine), which is identical to the ED 50 for CT in whole blood. The ADP- or collagen-induced platelet aggregation can be reversed by addition of CT: up to 2 min after the addition of ADP as the aggregation inducer, the aggregation is reversible to more than 70% by addition of a 1O 2 release-inducer (3 mM CT). In contrast, addition of CT 8 min after the addition of ADP results only in about 50% reversal of platelet aggregation. The electron microscopic images of platelets before ADP, after incubation for 4 min at 20 μmol/l ADP, after incubation for 1 min at 20 μmol/l ADP, and a further incubation for 3 min at 3 mmol/l CT demonstrate an ADP-dependent formation of platelet aggregates, which are disrupted by 1O 2 into the single platelets; a phenomenon comparable to the decomposition of a puzzle or the continental drift of the major earth plates. The morphology of oxidized and unoxidized platelets is similar. Conclusion: This study demonstrates that 1O 2 inhibits and reverses platelet aggregation. The physiologic signal action and the direct anticoagulant action of 1O 2 might be a new principle for pharmacologic intervention in atherothrombosis.

Singlet oxygen-mediated degradation of lignin: Isolation of oxidation products from steam-exploded lignin from straw

Four-hour irradiation of steam-exploded lignin from straw in the presence of singlet oxygen gave product showing high molecular weights. The only product obtained in low yield was the lactone 1. Irradiation of the same lignin sample for longer times allowed to obtain higher quantity of the same compound. The largest amount of the lactone was obtained after 24 h irradiation. This behaviour can be explained considering that lignin from straw can be easily oxidised by singlet oxygen and that 1 can be obtained from the singlet oxygen oxidation of 5–5′ units in the lignin.

Singlet oxygen inhibits agonist-induced P-selectin expression and formation of platelet aggregates.

Major mediators of activated polymorphonuclear leukocytes (PMN) are the oxidants HOCl and chloramine, which are a source for the nonradical photon-emitting oxidant singlet oxygen (1O2). We were interested in a possible platelet-modulating activity of 1O2. As a stable 1O2 source we chose the mild oxidant chloramine T (CT), which mimics the natural chloramine N-chloro-taurine. Freshly drawn native whole blood from donors (n = 5) was incubated at 0 to 3 mM CT for 1 minute at 37 degrees C. Then saline. 10 microM adenosine diphosphate (ADP), 5 microg/mL collagen, or 6.25 microM thrombin receptor activator peptide (TRAP) were added and the mixtures were allowed to incubate for 3 minutes at 37 degrees C. Aliquots of activated blood were fixed in 1% para-formaldehyde. After removal of the fixative, platelets were labeled with anti-CD61-FITC and anti-CD62P-PE antibodies and analyzed by flow cytometry. An oxidant concentration-dependent decrease in the expression of P-selectin appeared (at 3 mM CT to 39, 23, and 20% of the 100% saline control level for ADP, collagen, and TRAP, respectively). There was also an oxidant concentration-dependent decrease in the formation of platelet aggregates (at 3 mM CT to 8, 12, and 13% of the 100% saline control level for ADP, collagen, and TRAP, respectively; the 50% effective dose was 1.0 to 1.5 mM chloramine). In ADP- and TRAP-stimulated platelets, an oxidant-mediated increase in platelet fragments appeared (at 3 mM CT: three- to fourfold of the initial value). The addition to the blood of 30 mM of the oxyradical scavenger mannitol in contrast to excess methionine did not antagonize these oxidative modulations of platelet activation. The results were confirmed using equimolar concentrations of NaOCI and N-chloro-taurine. This study shows that 1O2 inhibits platelets, decreasing the expression of CD62P and the formation of platelet aggregates. Activated PMN might modulate hemostasis, shifting it into an antithrombotic state. The physiologic signal action and the direct anticoagulant action of 1O2 (released by chloramines such as vancomycin) might be a new principle for pharmacologic intervention in atherothrombosis.

Photosensitization by drugs

Abstract Certain drugs are known to elicit photosensitivity side effects. A satisfactory understanding of the involved mechanistic aspects is necessary to anticipate the photosensitizing potential. We have used tiaprofenic acid (TPA), a photosensitizing nonsteroidal antiinflammatory drug, to illustrate the methodology followed to address this problem. After studying the photophysical and photochemical properties of TPA, the attention has been directed towards the reactivity of its lowest lying p-p* triplet with biomolecules. Photosensitized lipid peroxidation occurs by a mixed type I (radicals) and type II (singlet oxygen) mechanism. In the case of proteins, the photosensitized reactions include Tyr, Trp, and His photodegradation, protein­protein photocrosslinking and drug­protein photobinding. This involves direct quenching of the drug triplet by the amino acid residues (Tyr and Trp) or by oxygen, followed by singlet oxygen oxidation (His and Trp). With DNA, the studies have included comet assay, induction of single-strand breaks in supercoiled DNA, and reaction with 2'-deoxyguanosine and thymidine. Product studies, together with time-resolved measurements, have shown that the fastest reaction occurs with purine bases, by a mechanism involving both radical and singlet oxygen processes. The employed methodology can be of general use to investigate the mechanistic aspects of photosensitization by drugs.

ChemInform Abstract: Controllable Selectivity of Photosensitized Oxidation of Olefins Included in Vesicles.

Abstract 9,10-Dicyanoanthracene (DCA) sensitized photooxidation of α-pinene, trans -stilbene and trans , trans -1,4-diphenyl-1,3-butadiene in mixed surfactant vesicles was investigated. While the oxidation in homogeneous solution yields the products derived from both the energy transfer and the electron transfer pathways, that within vesicles selectively yields either the singlet oxygen mediated or the superoxide radical anion mediated products depending on the status and location of the substrate and sensitizer molecules in the reaction medium. Upon incorporating the alkene in the bilayer membranes of one set of vesicles and DCA in another set of vesicles, the isolation of the substrate from the sensitizer prevents them from undergoing electron transfer. The singlet oxygen produced in the DCA-containing vesicles diffuses into the alkene-containing vesicles and reacts with the alkene. Thus, only the singlet oxygen oxidation products are obtained, and no product derived from the superoxide radical anion is detected. In contrast, incorporating both the sensitizer and the substrate within the same set of vesicles results in the two dissimilar molecules close to one another in the restricted space of the bilayer membranes of the vesicles. Thus, electron transfer from the alkenes to 1 DCA ∗ is enhanced, and the efficiency of the intersystem crossing from 1 DCA ∗ to 3 DCA ∗ is reduced. The photosensitized oxidation in this case only gives the products derived from the electron transfer pathway. Moreover, the reaction of singlet oxygen with DPB in vesicles results exclusively in the 1,2-cycloaddition products rather than the 1,4-cycloaddition product. This result indicates that the organized semirigid environment in vesicles prevents the olefin molecules from conformational change.

Oxaluric Acid as the Major Product of Singlet Oxygen-Mediated Oxidation of 8-Oxo-7,8-dihydroguanine in DNA

Oxidative reactions of DNA commonly result in base modifications. Among the four DNA bases, guanine is the most susceptible to oxidation, and one of its main oxidized compounds, namely 8-oxo-7,8-dihydroguanine (8-oxoGua), has been extensively studied in terms of formation, repair, and mutagenicity. However, the latter modified purine base is readily subjected to further oxidation reactions which have recently become a matter of interest. Emphasis was placed in this work on the identification of the final singlet oxygen oxidation products of 8-oxoGua in single-stranded DNA. Oxaluric acid was found to be the predominant product of the reaction. Insights in the mechanistic pattern of oxaluric acid formation were gained from isotopic labeling experiments in association with mass spectrometry measurements. It was found that oxaluric acid is formed via an oxidized guanidinohydantoin intermediate, arising from the likely degradation of a transient 5-hydroperoxide. Two subsequent hydrolytic steps that are accompa...

Effects of Singlet Oxygen on the Extracellular Matrix Protein Collagen: Oxidation of the Collagen Crosslink Histidinohydroxylysinonorleucine and Histidine

The reaction of singlet oxygen, a putative agent of skin photodamage, with the dermal collagen crosslink histidinohydroxylysinonorleucine (HHL) and its precursor histidine is reported. Reaction studies were performed with both purified HHL and bovine dermal tissue. We demonstrate that singlet oxygen can selectively oxidize HHL and histidine amino acid residues in dermal tissue and that intermediate oxidation products of histidine lead to new crosslink products. A novel mechanism for crosslink formation was proposed to involve nucleophilic addition to a transient imidazolone intermediate formed from singlet oxygen oxidation of the histidine imidazole moiety. The implication for such adduct formation and histidine oxidation in collagen proteins is the expression of aberrant collagen crosslinks, perturbation of the dermal collagen function, and hence an altered dermal state.

QUENCHING MECHANISMS AND KINETICS OF CAROTENOIDS IN RIBOFLAVIN PHOTOSENSITIZED SINGLET OXYGEN OXIDATION OF VITAMIN D2

The oxidation of vitamin D2 in a solution of 12% water, 88% acetone and 15 ppm ribqflavin under light or dark was studied by measuring the headspace oxygen. Ribofiavin accelerated the oxidation of vitamin D2 by singlet oxygen under light, but did not affect vitamin D2 oxidation under dark. Quenching mechanisms and kinetics within the range of 0–20 ppm β-cawtene or fucoxanthin and with 0–80 ppm retinyl acetate or retinol on 15 ppm riboflavin photosensitized singlet oxygen oxidation of vitamin D2 were also studied. The rate of singlet oxygen formation by 15 ppm riboflavin was 1.78 umole oxygen/mL headspace-hour. The reaction rate constant of vitamin D2 with singlet oxygen was 2.2 times 107 M−1A s−1. The carotenoids minimized the oxidation of vitamin D2 by quenching singlet oxygen. The total quenching rate constants of retinol, retinyl acetate, jucoxanthin and ft-carotene were in the order of 1.22 times 108, 5.98 times 108, 1.78 times 109 and 5.00 times 109M−1. s−1 respectively, which suggests that as the number of double bonds of carotenoids increases, the quenching rate constant ofcarotenoid increases.

Effects of β‐carotene and lycopene thermal degradation products on the oxidative stability of soybean oil

AbstractThe relative oxidative stability of soybean oil samples containing either thermally degraded β‐carotene or lycopene was determined by measuring peroxide value (PV) and headspace oxygen depletion (HOD) every 4 h for 24 h. Sobyean oil samples containing 50 ppm degraded β‐carotene that were stored in the dark at 60°C displayed significantly (P<0.01) higher HOD values compared with controls. Lycopene degradation products (50 ppm) in soybean oil significantly (P<0.05) decreased HOD of samples when stored in the dark. PV and HOD values for samples containing 50 ppm of either β‐carotene or lycopene degradation products stored under lighted conditions did not differ significantly from controls (P<0.05). However, soybean oil samples containing 50 ppm of unheated, all‐trans β‐carotene or lycopene stored under light showed significantly lower PV and HOD values than controls (P<0.01). These results indicated that during autoxidation of soybean oil held in the dark, β‐carotene thermal degradation products acted as a prooxidant, while thermally degraded lycopene displayed antioxidant activity in similar soybean oil systems. In addition, β‐carotene and lycopene degradation products exposed to singlet oxygen oxidation under light did not increase or decrease the oxidative stability of their respective soybean oil samples.

Controllable Selectivity of Photosensitized Oxidation of Olefins Included in Vesicles

9,10-Dicyanoanthracene (DCA) sensitized photooxidation of α-pinene, trans-stilbene and trans, trans-1,4-diphenyl-1,3-butadiene in mixed surfactant vesicles was investigated. While the oxidation in homogeneous solution yields the products derived from both the energy transfer and the electron transfer pathways, that within vesicles selectively yields either the singlet oxygen mediated or the superoxide radical anion mediated products depending on the status and location of the substrate and sensitizer molecules in the reaction medium. Upon incorporating the alkene in the bilayer membranes of one set of vesicles and DCA in another set of vesicles, the isolation of the substrate from the sensitizer prevents them from undergoing electron transfer. The singlet oxygen produced in the DCA-containing vesicles diffuses into the alkene-containing vesicles and reacts with the alkene. Thus, only the singlet oxygen oxidation products are obtained, and no product derived from the superoxide radical anion is detected. In contrast, incorporating both the sensitizer and the substrate within the same set of vesicles results in the two dissimilar molecules close to one another in the restricted space of the bilayer membranes of the vesicles. Thus, electron transfer from the alkenes to 1DCA ∗ is enhanced, and the efficiency of the intersystem crossing from 1DCA ∗ to 3DCA ∗ is reduced. The photosensitized oxidation in this case only gives the products derived from the electron transfer pathway. Moreover, the reaction of singlet oxygen with DPB in vesicles results exclusively in the 1,2-cycloaddition products rather than the 1,4-cycloaddition product. This result indicates that the organized semirigid environment in vesicles prevents the olefin molecules from conformational change.

Singlet oxygen (1O2) Inactivates Plasmatic Free and Complexed α2-Macroglobulin

α 2-macroglobulin (α 2M) is a broad-spectrum proteinase inhibitor and one of the major plasma proteins in humans. Activated phagocytes (especially granulocytes) generate large amounts of oxidants of the HOCl- and chloramine-type that release the mild nonradical, excited (light-emitting) oxidant singlet oxygen ( 1O 2). These oxidants have been shown to inactivate several specific serine protease inhibitors in human blood [e.g., α 1-antitrypsin or α 2-antiplasmin (plasmin inhibitor)]. The studies reported here demonstrate that nonradical oxidants also inactivate plasmatic α 2M. The effective dose for 50% inactivation (ED 50) of plasmatic α 2M is similar to that for plasmatic α 2-antiplasmin. Chloramines are about 1000-fold more effective than hydrogen peroxide (ED 50=0.75 μmol chloramine T/50 μl plasma). Serine protease–serine protease inhibitor complexes are resistant to oxidants. In contrast, here it is shown that α 2-macroglobulin, even after binding to serine proteases is sensitive to oxidation, the captured protease is released from the protease/α 2M complex. This is the first time that oxidative inactivation of a complexed (i.e., bound to a target protease) human protease inhibitor has be shown. The 1O 2 inhibitors methionine, cysteine, cystine, or ascorbate—in contrast to the oxy-radical scavengers mannitol, superoxide dismutase, or catalase—antagonize the chloramine/NaOCl-mediated inactivation of both uncomplexed and complexed α 2M. Thus, the oxidant involved here is of nonradicalic nature and has reaction characteristics of 1O 2. For the inhibitory function, critical oxidizable methionines or the internal thiol-ester might be targets for 1O 2. Consequently, α 2M can also be considered a carrier for proteases, since the α 2M-complexed proteases regain full activity in an oxidative environment. In local areas of inflammation or thrombolysis, activated phagocytes could create microenvironments of uncontrolled protease activity by generation of 1O 2.

Effects of singlet oxygen on membrane sterols in the yeast Saccharomyces cerevisiae.

Photodynamic treatment of the yeast Saccharomyces cerevisiae with the singlet oxygen sensitizer toluidine blue and visible light leads to rapid oxidation of ergosterol and accumulation of oxidized ergosterol derivatives in the plasma membrane. The predominant oxidation product accumulated was identified as 5alpha, 6alpha-epoxy-(22E)-ergosta-8,22-dien-3beta,7a lpha-diol (8-DED). 9(11)-dehydroergosterol (DHE) was identified as a minor oxidation product. In heat inactivated cells ergosterol is photooxidized to ergosterol epidioxide (EEP) and DHE. Disrupted cell preparations of S. cerevisiae convert EEP to 8-DED, and this activity is abolished in a boiled control indicating the presence of a membrane associated enzyme with an EEP isomerase activity. Yeast selectively mobilizes ergosterol from the intracellular sterol ester pool to replenish the level of free ergosterol in the plasma membrane during singlet oxygen oxidation. The following reaction pathway is proposed: singlet oxygen-mediated oxidation of ergosterol leads to mainly the formation of EEP, which is enzymatically rearranged to 8-DED. Ergosterol 7-hydroperoxide, a known minor product of the reaction of singlet oxygen with ergosterol, is formed at a much lower rate and decomposes to give DHE. Changes of physical properties of the plasma membrane are induced by depletion of ergosterol and accumulation of polar derivatives. Subsequent permeation of photosensitizer through the plasma membrane into the cell leads to events including impairment of mitochondrial function and cell inactivation.

Spongiabutenolides A – D: Minor γ-hydroxybutenolide diterpenoids from a Philippines Spongia sp.

Four novel diterpenoids, spongiabutenolides A – D ( 2–5), all of which contain a γ-hydroxybutenolide moiety, have been isolated from a Philippines marine sponge of the genus Spongia. The structures of spongiabutenolides A – D ( 2–5) and the methyl esters 6 and 7 were elucidated by interpretation of spectral data. Spongiabuteonolide A ( 2) was synthesized from spongia-13(16),14-diene-19-oic acid ( 1) by singlet oxygen oxidation.

Singlet oxygen oxidation of pyrroles. Formation of 5-substituted derivatives

The tert-butyl ester of 3-methoxy-2-pyrrolecarboxylic acid 3 undergoes reaction with singlet oxygen to form an intermediate imino hydroperoxide 4. This hydroperoxide may be trapped by a variety of nucleophiles yielding 5-substituted pyrroles 6. With strong nucleophilic substituents at the 5-position, these pyrroles may add to unreacted hydroperoxide 4 to form bipyrrole-like products 7.

Semisynthesis of 3-beta-hydroxyartemisinin

3-beta-Hydroxyartemisinin (5) was synthesized from 3-beta-hydroxydihydroartemisinic acid (7c) via singlet oxygen oxidation followed by air (triplet oxygen) oxidation. Compound 7c was synthesized in two steps from dihydroartemisinic acid, 7a.

Synthesis of Oligonucleotides Containing the (4R) and (4S) Diastereoisomers of 4,8-Dihydro-4-hydroxy-8-oxo-2′-deoxyguanosine

The insertion of the (4R) and (4S) diastereoisomers of 4,8-dihydro-4-hydroxy-8-oxo-2′-deoxyguanosine, the two main singlet oxygen oxidation products of 2′-deoxyguanosine, (4R)-4-OH-8-oxodGuo (3a) and (4S)-4-OH-8-oxodGuo (3b), into oligonucleotides has been achieved by means of phosphoramidite chemistry using the solid-phase synthesis approach. The synthesis of the phosphoramidite synthons 8a and 8b required 6 steps from 2′-deoxyguanosine and involved the simultaneous protection of the additional tertiary hydroxy (4-OH) and N2-amino groups of the modified base by acetylation. The modified phosphoramidites 8a, b were efficiently incorporated into several oligonucleotides (3-mer, 9-mer, 14-mer, and 22-mer). The presence and the integrity of the 3a and 3b in the synthetic oligomers was confirmed by electrospray ionization mass spectrometry, together with HPLC and MALDI-TOF mass-spectrometric analyses of enzymatic digestions. The use of various enzymes, including endonucleases (nuclease P1) and exonucleases (snake venom phosphodiesterase and calf spleen phosphodiesterase), clearly shows that the enzymatic hydrolysis of phosphodiester bonds between 3a,b and normal 2′-deoxyribonucleosides is inhibited.

Thermal degradation of single methyl oleate hydroperoxides obtained by Photosensitized oxidation

AbstractA peroxidation mixture containing methyl 9‐ and 10‐hydroperoxy‐trans‐octadecenoates (MOHP) was obtained by singlet oxygen oxidation of methyl oleate. The two hydroperoxides were collected by solid phase extraction and purified separately by high‐performance liquid chromatography. Identification and single‐isomer purity evaluations were carried out by comparing the chromatographic and gas chromatography‐mass spectrometry parameters of the corresponding reduced hydroxy derivatives. Each purified MOHP was thermally degraded and new reaction mechanisms were proposed from the identification of the degradation products. Thermal rearrangement of each hydroperoxide isomer involved an allylic 3‐carbon intermediate before further degradation steps. The two MOHP isomers obtained from singlet oxygen oxidation produced all eight hydroperoxide isomers by thermal degradation in the condensed phase at high temperature (200°C). This result supports the assumption of singlet oxygen as a promoter of the first steps of oxidation of food lipids and also reconsiders the Khan mechanism.

Article

3,5-Di-tert-butyl-ortho-quinone, 6, and 1-(3,4-dimethoxyphenyl-2-(2-methoxyphenoxy)-1-propanone, 7, models for oxidized lignin and for lignin, were used as sensitizers of photo-oxidation. Product studies by HPLC from oxidation of methyl linoleate in solution sensitized by 6 or 7, and in sodium dodecyl sulfate (SDS) sensitized by 6, showed a product distribution of six hydroperoxides, the four conjugated 9- and 13-hydroperoxides of the geometrical isomers: trans-10, cis-12 (2), cis-9, trans-11 (3), trans-10, trans-12 (4), and trans-9, trans-11 (5)-octadecadienoates plus two nonconjugated hydroperoxides. The higher cis/trans to trans/trans (ct/tt) of geometrical isomers (2 + 3//4 + 5) compared to ct/tt from known thermal free-radical peroxidations (Type 1) indicate that singlet oxygen (Type 2) oxidation occurs in reactions sensitized by 6 or 7. Kinetic studies by oxygen uptake are reported on oxidations of hydrocarbons 1-phenyl-2-methylpropene,8, and trans-stilbene,9, sensitized by the quinone, 6, or by a dye, Rose Bengal. Quenching studies imply singlet oxygen reactions. Milled wood lignin undergoes self-initiated photo-oxidation in water, and oxygen uptake was quenched by sodium azide. Cellobiose, a cellulose model, undergoes sensitized photo-oxidation using model quinone, 6, in a mixture of tert-butyl alcohol and water or using the sensitizers benzophenone or the lignin model, 7, delivered on a solid support, silica gel, and these oxidations were quenched with sodium azide. These results implicate singlet oxygen in the photo-yellowing of high lignin content wood pulps.Key words: lignin models, ortho-quinone, photo-oxidation, singlet oxygen, lignin, cellobiose.