Singlet oxygen quenchers and the photodynamic inactivation of E. coli ribosomes by methylene blue

$\text{E. coli}$ ribosomes are readily photoinactivated by methylene blue in the presence of air. A variety of singlet oxygen quenchers like NaN₃, 2,5-dimethylfuran, hydroquinone and ascorbic acid provide about 60% protection against this photoinactivation indicating that a major mechanism of ribosome inactivation proceeds through the formation of singlet oxygen, with small contributions (<40%) from other mechanisms. The singlet oxygen quenchers, 1,4-diazabicyclo [2.2.2] octane and triethylamine give unexpected results, in that they show no protection against photoinactivation.     

Comprehensive analysis of metabolic sensitivity of 1,4-butanediol producing Escherichia coli toward substrate and oxygen availability

Nowadays, chemical production of 1,4-butanediol is supplemented by biotechnological processes using a genetically modified Escherichia coli strain, which is an industrial showcase of successful application of metabolic engineering. However, large scale bioprocess performance can be affected by presence of physical and chemical gradients in bioreactors which are a consequence of imperfect mixing and limited oxygen transfer. Hence, upscaling comes along with local and time dependent fluctuations of cultivation conditions. This study emphasizes on scale-up related effects of microbial 1,4-butanediol production by comprehensive bioprocess characterization in lab scale. Due to metabolic network constraints 1,4-butanediol formation takes place under oxygen limited microaerobic conditions, which can be hardly realized in large scale bioreactor. The purpose of this study was to assess the extent to which substrate and oxygen availability influence the productivity. It was found, that the substrate specific product yield and the production rate are higher under substrate excess than under substrate limitation. Furthermore, the level of oxygen supply within microaerobic conditions revealed strong effects on product and by-product formation. Under strong oxygen deprivation nearly 30% of the consumed carbon is converted into 1,4-butanediol, whereas an increase in oxygen supply results in 1,4-butanediol reduction of 77%. Strikingly, increasing oxygen availability leads to strong increase of main by-product acetate as well as doubled carbon dioxide formation. The study provides clear evidence that scale-up of microaerobic bioprocesses constitute a substantial challenge. Although oxygen is strictly required for product formation, the data give clear evidence that terms of anaerobic and especially aerobic conditions strongly interfere with 1,4-butanediol production.     

Synthesis and structure of selected quaternary N-(1,4-anhydro-5-deoxy-2,3-O-isopropylidene-D,L-ribitol-5-yl)ammonium salts

The syntheses have been developed for quaternary N-(1,4-anhydro-5-deoxy-2,3-O-isopropylidene-D,L-ribitol-5-yl)ammonium salts derived from five aromatic amines, pyridine, 2-methylpyridine, 3-carbamoylpyridine, 4-(N,N-dimethylamino)pyridine, and quinoline, as well as two tertiary aliphatic amines, trimethylamine and triethylamine. Reactions of 1,4-anhydro-2,3-O-isopropylidene-5-O-tosyl-D,L-ribitol with tri-n-propylamine and tri-n-butylamine were unsuccessful. The products were identified on the basis of their 1H and 13C NMR spectra. The structure of N-(1,4-anhydro-5-deoxy-2,3-O-isopropylidene-D,L-ribitol-5-yl)trimethylammonium tosylate was additionally elucidated by X-ray diffractometry.     

Improved preparation and synthetic uses of 3-deoxy-d-arabino-hexonolactone: an efficient synthesis of Leptosphaerin

Acetylation of d-glucono-1,5-lactone and subsequent treatment with triethylamine gave 2,4,6-tri-O-acetyl-d-erythro-hex-2-enono-1,5-lactone. Hydrogenation of the latter in the presence of palladium on carbon yielded 2,4,6-tri-O-acetyl-3-deoxy-d-arabino-hexono-1,5-lactone (5) in almost quantitative yield calculated from gluconolactone. Catalytic hydrogenation of 5 with platinum on carbon in the presence of triethylamine gave 2,4,6-tri-O-acetyl-3-deoxy-d-arabino-hexopyranose in quantitative yield. Deacetylation of 5 gave 3-deoxy-d-arabino-hexono-1,4-lactone, which was converted into 3-deoxy-5,6-O-isopropylidene-2-O-methanesulfonyl-d-arabino-hexono-1,4-lactone (10). The latter was converted into 2-acetamido-2,3-dideoxy-d-erythro-hex-2-enono-1,4-lactone (Leptosphaerin). When 10 was boiled in water in the presence of acid, it gave a high yield of 2,5-anhydro-3-deoxy-d-ribo-hexonic acid.     

Laccase-induced cross-coupling of 4-aminobenzoic acid with para-dihydroxylated compounds 2,5-dihydroxy-N-(2-hydroxyethyl)-benzamide and 2,5-dihydroxybenzoic acid methyl ester

A fungal laccase from Trametes villosa (EC 1.10.3.2 p-phenoloxidase) was used to mediate the oxidation and cross-coupling of two para-dihydroxylated benzoic acid derivatives with 4-aminobenzoic acid. The incubation of 2,5-dihydroxy-N-(2-hydroxyethyl)-benzamide and 4-aminobenzoic acid with laccase under oxygen conditions resulted in the formation of 2-(4′-carboxy-anilino)-N-(2″-hydroxyethyl)-3,6-dioxo-1,4-cyclohexadien-1-carboxamide as the main product (yield > 85%). When 2,5-dihydroxybenzoic acid methyl ester was a co-substrate of 4-aminobenzoic acid, 2-(4′-carboxy-anilino)-N-(2″-hydroxyethyl)-3,6-dioxo-1,4-cyclohexadien-1-carboxy methyl ester was produced (yield > 75%). Both products were N–C coupling dimers consisting of para-quinone and benzoic acid moieties. The formation of quinone structures in the presence of T. villosa laccase may be useful in pharmaceutical synthesis. Because of high product yields and low amount of by-products laccase of T. villosa seems to be a suitable enzyme among laccases acting at pH 5 for the synthesis of heterologous dimers.     

Synthesis of four stereoisomers of 1,4-thiazane-3-carboxylic acid 1-oxide via the asymmetric transformation (combined isomerization-preferential crystallization) of 1,4-thiazane-3-carboxylic acid

In order to synthesize four stereoisomers of 1,4-thiazane-3-carboxylic acid 1-oxide (TCA SO), (S)-1,4thiazane-3-carboxylic acid [(S)-TCA], which is one of the precursors, was prepared by the asymmetric transformation (combined isomerization-preferential crystallization) of (RS)-TCA. This asymmetric transformation was used (2R, 3R)-tartaric acid [(R)-TA] as a resolving agent and salicylaldehyde as the epimerization catalyst in propanoic acid at 110 degrees C to afford a salt of (S)-TCA with (R)-TA in 100% de with a yield of over 90%. Optically pure (S)-TCA was obtained by treating the salt with triethylamine in methanol in a yield of over 80%, based on (RS)-TCA as the starting material. In addition, asymmetric transformation of (R)-TCA gave (S)-TCA in a yield of 60-70%. (S)-TCA was oxidized by hydrogen peroxide in dilute hydrochloric acid to selectively crystallize (1S, 3S)-TCA.SO. (1R, 3S)-TCA SO of 70% de from the filtrate was allowed to form a salt with (R)-TA after a treatment with triethylamine to give (1R, 3S)-TCA SO as a single diastereoisomer. (1R, 3R)- and (1S, 3R)-TCA.SO were also prepared by starting from (R)-TCA that had been synthesized from L-cysteine.     

Degradation of 1,4-naphthoquinones by Pseudomonas putida

Pseudomonas putida J1 and J2, enriched from soil with juglone, are capable of a total degradation of 1,4-naphthoquinone, 2-hydroxy-1,4-naphthoquinone, and 2-chloro-1,4-naphthoquinone. Naphthazerin and plumbagin are only converted into the hydroxyderivatives 2-hydroxynaphthazerin and 3-hydroxyplumbagin, respectively, whereas 2-amino-1,4-naphthoquinone is not attacked at all. The degradation of 1,4-naphthoquinone begins with a hydroxylation of the quinoid ring, yielding 2-hydroxy-1,4-naphthoquinone (lawsone). Lawsone is reduced to 1,2,4-trihydroxynaphthalene with consumption of NADH. The fission product of the quinol could not be detected by direct means because of its instability. However, the presence of 2-chromonecarboxylic acid, a secondary product of lawsone degradation, leads to the conclusion, that the cleavage of the quinol takes place in the meta-position. The resulting ring fission product is converted into salicylic acid by removal of the side chain, presumably as pyruvate. Further degradation of salicyclic acid leads to the formation of catechol, which is then cleaved in the ortho-position and then metabolized via the 3-oxoadipate pathway. The initial steps in the degradation of 2-chloro-1,4-naphthoquinone, namely, the hydroxylation of the quinone to 2-chloro-3-hydroxy-1,4-naphthoquinone, followed by the elimination of the chlorine substituent lead to lawsone, which is further degraded through the pathway described. The degradation steps could be verified by the accumulation products of mutant strains blocked in different steps of lawsone metabolism. Generation of mutants was carried out by chemical and by transposon mutagenesis. The regulation of the first steps of the pathway catalysed by juglone hydroxylase and lawsone reductase, was investigated by induction experiments.     

The metabolism of thymol by a Pseudomonas

1. Pseudomonas putida when grown with thymol contained a meta-fission dioxygenase, which required ferrous ions and readily cleaved the benzene nucleus of catechols between adjacent carbon atoms bearing hydroxyl and isopropyl groups. 2. 3-Hydroxythymo-1,4-quinone was excreted towards the end of exponential growth and later was slowly metabolized. This compound was oxidized by partially purified extracts only when NADH was supplied; the substrate for the dioxygenase appeared to be 3-hydroxythymo-1,4-quinol, which was readily and non-enzymically oxidized to the quinone. 3. 2-Oxobutyrate (0.9 mole) was formed from 1 mole of 3-hydroxythymo-1,4-quinone with the consumption of 1 mole of oxygen; acetate, isobutyrate and 2-hydroxybutyrate (which arose from the enzymic reduction of 2-oxobutyrate) were also formed. 4. These products, which were produced only when the catechol substrate contained a third hydroxyl group, appeared to result from the enzymic hydrolysis of the ring-fission product.     

Cytochrome aa3 in facultatively aerobic and hyperthermophilic archaeon Pyrobaculum oguniense

We partially purified and characterized the cytochrome aa3 from the facultatively aerobic and hyperthermophilic archaeon Pyrobaculum oguniense. This cytochrome aa3 showed oxygen consumption activity with N, N, N', N'-tetramethyl-1,4-phenylenediamine and ascorbate as substrates, and also displayed bovine cytochrome c oxidase activity. These enzymatic activities of cytochrome aa3 were inhibited by cyanide and azide. This cytochrome contained heme As, but not typical heme A. An analysis of trypsin-digested fragments indicated that 1 subunit of this cytochrome was identical to the gene product of subunit I of the SoxM-type heme--copper oxidase (poxC). This is the first report of a terminal oxidase in hyperthermophilic crenarchaeon belonging to the order Thermoproteales.     

Synthesis, reactive oxygen species generation and copper-mediated nuclease activity profiles of 2-aryl-3-amino-1,4-naphthoquinones

Here we report a series of 2-aryl-3-amino-1,4-naphthoquinones that generated reactive oxygen species (ROS) such as superoxide and hydrogen peroxide upon incubation in pH 7.4 under ambient aerobic conditions. ROS generation from these compounds was sensitive to structural modifications at the 3-amino position and a 2-aryl substituent promoted ROS generation. A number of these compounds were found to induce DNA damage in the presence of Cu(II) without any added reducing agent. Our data suggests that 2-aryl-3-amino-1,4-naphthoquinones' propensity to produce ROS correlated well with its DNA damage inducing ability. 2-Phenyl-3-pyrrolid-1-yl-1,4-naphthoquinone (22) was found to damage DNA at 1 μM suggesting that these compounds may have therapeutic relevance in targeting cancers which over-express Cu(II).     

The biotransformation of hyodeoxycholic acid by Pseudomonas sp. NCIB 10590 under anaerobic conditions

The bacterial degradation of hyodeoxycholic acid under anaerobic conditions was studied. The major acidic product has been identified as 6 alpha-hydroxy-3-oxochol-4-ene-24-oic acid whilst the major neutral product has been identified as 6 alpha-hydroxyandrosta-1,4-diene-3,17-dione. The minor acidic products were 3,6-dioxochola-1,4-diene-24-oic acid, 3-oxochol-5-ene-24-oic acid, 3-oxochol-4-ene-24-oic acid, 3-oxochola-1,4-diene-24-oic acid and 6 alpha-hydroxy-3-oxochola-1,4-diene-24-oic acid and the minor neutral products were androst-4-ene-3,17-dione, androst-4-ene-3,6,17-trione, androsta-1,4-diene-3,6,17-trione, androsta-1,4-diene-3,17-dione, 17 beta-hydroxyandrosta-1,4-diene-3-one and 6 alpha-hydroxyandrost-4-ene-3,17-dione. Evidence is presented which suggests that under aerobic conditions, one pathway of hyodeoxycholic acid metabolism exists whilst under anaerobic conditions an extra biotransformation pathway becomes operative involving the induction of a 6 alpha-dehydroxylase enzyme. A biochemical pathway of hyodeoxycholic acid metabolism by bacteria under anaerobic conditions is discussed incorporating a scheme involving such an enzyme.     

The action of lipoxygenase-1 on furan derivatives.

Several 2,5-disubstituted furans, which are known to react with peroxyacids, singlet oxygen and other active forms of oxygen were tested as potential inhibitors, co-oxidants, or substrates for soybean lipoxygenase. The furan, 10,13-epoxy-octadeca-10,12-dienoic acid, methyl ester (IV) was converted by lipoxygenase or singlet oxygen or peroxyacid to the acyclic product, methyl 10,13-dioxo-octadec-11-enoate. Apparently furan IV is able to interact with an active site of lipoxygenase (Km = 220 microM). 2,5-Dimethylfuran (I), 2,5-diphenylfuran (II) and 3-(5'-methyl-2'-furyl)propenoic acid (III) were neither substrates nor inhibitors of lipoxygenase activity. Lipoxygenase-catalyzed oxidation of furan (IV), which is inhibited by hydroquinone, is explained by a mechanism involving lipoxygenase-superoxide complex and furan-radical intermediates. Also described is the selective cleavage of furan rings by m-chloroperoxybenzoic acid to yield the 1,4-diketoethylene functional system.     

Laser flash photolysis study of the triplet reactivity of beta-lapachones

The photochemical reactivity of beta-lapachone (1), nor-beta-lapachone (2) and beta-lapachone 3-sulfonic acid (3) has been examined by laser flash photolysis. Excitation (lambda = 266 nm) of degassed solutions of , in acetonitrile or dichloromethane, resulted in the formation of detectable transients with absorption maxima at 300, 380 and 650 nm. These transients, with lifetimes of 5.0 micros, were quenched by beta-carotene at a diffusion-controlled rate constant and assigned to the triplet excited states of 1-3. Addition of hydrogen donors, such as 2-propanol, 1,4-cyclohexadiene, 4-methoxyphenol or indole led to the formation of new transients, which were assigned to the corresponding ketyl radicals obtained from the hydrogen abstraction reaction by the triplets 1-3 . In the presence of triethylamine it was observed the formation of the long-lived anion radical derived from , which shows absorption maxima at 300 and 380 nm. The low values observed for the hydrogen abstraction rate constants for the beta-lapachones 1-3 using 2-propanol and 1,4-cyclohexadiene as quenchers led us to conclude that their triplet excited states show pi pi* character.     

Regioselective sulfonylation of 6,1',6'-tri-O-tritylsucrose through dibutylstannylation: synthesis of 4'-O-sulfonyl derivatives of sucrose

3-O-Mesyl-1,6-di-O-trityl-beta-D-fructofuranosyl-(2-->1)-6-O-trityl-alpha-D-glucopyranoside (3) was synthesized via stannylation of 6,1',6'-tri-O-tritylsucrose with dibutyltin oxide in benzene, followed by treatment of the crude product with methanesulfonyl chloride in the presence of triethylamine in dichloromethane at 0 degrees C. A similar treatment of the tri-tritylsucrose in toluene, instead of benzene, yielded 4-O-mesyl-1,6-di-O-trityl-beta-D-fructofuranosyl-(2-->1)-6-O-trityl-alpha-D-glucopyranoside (4) as the major product. The X-ray crystal structure of the corresponding acetyl derivative, 3-O-acetyl-4-O-mesyl-1,6-di-O-trityl-beta-D-fructofuranosyl-(2-->1)-2,3,4-tri-O-acetyl-6-O-trityl-alpha-D-glucopyranoside (5), confirms the position and stereochemistry of the methanesulfonyl group at C-4 of the fructofuranosyl ring.     

The oxidation of tetrachloro-1,4-hydroquinone by microsomes and purified cytochrome P-450b. Implications for covalent binding to protein and involvement of reactive oxygen species

The enzymatic oxidation of tetrachloro-1,4-hydroquinone (1,4-TCHQ), resulting in covalent binding to protein of tetrachloro-1,4-benzoquinone (1,4-TCBQ), was investigated, with special attention to the involvement of cytochrome P-450 and reactive oxygen species. 1,4-TCBQ itself reacted very rapidly and extensively with protein (58% of the 10 nmol added to 2 mg of protein, in a 5-min incubation). Ascorbic acid and glutathione prevented covalent binding of 1,4-TCBQ to protein, both when added directly and when formed from 1,4-TCHQ by microsomes. In microsomal incubations as well as in a reconstituted system containing purified cytochrome P-450b, 1,4-TCHQ oxidation and subsequent protein binding was shown to be completely dependent on NADPH. The reaction was to a large extent, but not completely, dependent on oxygen (83% decrease in binding under anaerobic conditions). Inhibition of cytochrome P-450 by metyrapone, which is also known to block the P-450-mediated formation of reactive oxygen species, gave a 80% decrease in binding, while the addition of superoxide dismutase prevented 75% of the covalent binding, almost the same amount as found in anerobic incubations. A large part of the conversion of 1,4-TCHQ to 1,4-TCBQ is apparently not catalyzed by cytochrome P-450 itself, but is mediated by superoxide anion formed by this enzyme. The involvement of this radical anion is also demonstrated by microsomal incubations without NADPH but including the xantine/xantine oxidase superoxide anion generating system. These incubations resulted in a 1.6-fold binding as compared to the binding in incubations with NADPH but without xantine/xantine oxidase. 1,4-TCHQ was shown to stimulate the oxidase activity of microsomal cytochrome P-450. It is thus not unlikely that 1,4-TCHQ enhances its own microsomal oxidation.     

Novel type of heme-dependent oxygenase catalyzes oxidative cleavage of rubber (poly-cis-1,4-isoprene)

An extracellular protein with strong absorption at 406 nm was purified from cell-free culture fluid of latex-grown Xanthomonas sp. strain 35Y. This protein was identical to the gene product of a recently characterized gene cloned from Xanthomonas sp., as revealed by determination of m/z values and sequencing of selected isolated peptides obtained after trypsin fingerprint analysis. The purified protein degraded both natural rubber latex and chemosynthetic poly(cis-1,4-isoprene) in vitro by oxidative cleavage of the double bonds of poly(cis-1,4-isoprene). 12-oxo-4,8-dimethyltrideca-4,8-diene-1-al (m/z 236) was identified and unequivocally characterized as the major cleavage product, and there was a homologous series of minor metabolites that differed from the major degradation product only in the number of repetitive isoprene units between terminal functions, CHO-CH2--and--H2-COCH3. An in vitro enzyme assay for oxidative rubber degradation was developed based on high-performance liquid chromatography analysis and spectroscopic detection of product carbonyl functions after derivatization with dinitrophenylhydrazone. Enzymatic cleavage of rubber by the purified protein was strictly dependent on the presence of oxygen; it did not require addition of any soluble cofactors or metal ions and was optimal around pH 7.0 at 40 degrees C. Carbon monoxide and cyanide inhibited the reaction; addition of catalase had no effect, and peroxidase activity could not be detected. The purified protein was specific for natural rubber latex and chemosynthetic poly(cis-1,4-isoprene). Analysis of the amino acid sequence deduced from the cloned gene (roxA [rubber oxygenase]) revealed the presence of two heme-binding motifs (CXXCH) for covalent attachment of heme to the protein. Spectroscopic analysis confirmed the presence of heme, and approximately 2 mol of heme per mol of RoxA was found.     

Amino acid supplementation, controlled oxygen limitation and sequential double induction improves heterologous xylanase production by Pichia stipitis

Heterologous endo-beta-1,4-xylanase was produced by Pichia stipitis under control of the hypoxia-inducible PsADH2-promoter in a high-cell-density culture. After promoter induction by a shift to oxygen limitation, different aeration rates (oxygen transfer rates) were applied while maintaining oxygen-limitation. Initially, enzyme production was higher in oxygen-limited cultures with high rates of oxygen transfer, although the maximum xylanase activity was not significantly influenced. Amino acid supplementation increased the production of the heterologous endo-beta-1,4-xylanase significantly in highly aerated oxygen-limited cultures, until glucose was depleted. A slight second induction of the promoter was observed in all cultures after the glucose had been consumed. The second induction was most obvious in amino acid-supplemented cultures with higher oxygen transfer rates during oxygen limitation. When such oxygen-limited cultures were shifted back to fully aerobic conditions, a significant re-induction of heterologous endo-beta-1,4-xylanase production was observed. Re-induction was accompanied by ethanol consumption. A similar protein production pattern was observed when cultures were first grown on ethanol as sole carbon source and subsequently glucose and oxygen limitation were applied. Thus, we present the first expression system in yeast with a sequential double-inducible promoter.     

Observation by 13C NMR of the EPSP synthase tetrahedral intermediate bound to the enzyme active site

Direct observation of the tetrahedral intermediate in the EPSP synthase reaction pathway was provided by 13C NMR by examining the species bound to the enzyme active site under internal equilibrium conditions and using [2-13C]PEP as a spectroscopic probe. The tetrahedral center of the intermediate bound to the enzyme gave a unique signal appearing at 104 ppm. Separate signals were observed for free EPSP (152 ppm) and EPSP bound to the enzyme in a ternary complex with phosphate (161 ppm). These peak assignments account for our quantitation of the species bound to the enzyme and liberated upon quenching with either triethylamine or base. A comparison of quenching with acid, base, or triethylamine was conducted; the intermediate could be isolated by quenching with either triethylamine or 0.2 N KOH, allowing direct quantitation of the species bound to the enzyme. After long times of incubation during the NMR measurement, a signal at 107 ppm appeared. The compound giving rise to this resonance was isolated and identified as an EPSP ketal [Leo et al. (1990) J. Am. Chem. Soc. (in press)]. The rate of formation of the EPSP ketal was very slow, 3.3 X 10(-5) s-1, establishing that it is a side product of the normal enzymatic reaction, probably arising as a breakdown product of the tetrahedral intermediate. A slow formation of pyruvate was also observed and is attributable to the enzymatic hydrolysis of EPSP, with 5% of the enzyme sites occupied by EPSP and hydrolyzing EPSP at a rate of 4.7 X 10(-4) s-1. To look for additional signals that might arise from a covalent adduct which has been postulated to arise from reaction of enzyme with PEP, an NMR experiment was performed with an analogue of S3P lacking the 4- and 5-hydroxyl groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Acetals and thioacetals from thiodiglycolaldehyde: Some oxidation products

Thiodiglycolaldehyde (2,2′-thiobisacetaldehyde, 1a) reacted severally with methanol, ethanol, and 2-propanol to give mixtures variously of thiodiglycoaldehyde bis(dialkyl acetals) (3a,b), cis-2,6-dialkoxy-1,4-oxathianes (5b-d), and trans-2,6-dialkoxy-1,4-oxathianes (7a-c). Thiodiglycolaldehyde bis(di-isopropyl acetal) (3c) was not formed in the reaction of 1a and 2-propanol, but 3c was obtained after bromoacetaldehyde di-isopropyl acetal was treated with sodium sulfide. The stereoisomers corresponding to 2,6-dimethoxy-1,4-oxathiane (5b, 7a) were obtained from the acyclic dimethyl acetal 3a. The reaction between 1a and thiols in acid media have been studied. With ethanethiol, thiodiglycolaldehyde bis(diethyl dithioacetal) was the only product, but a mixture of thiodiglycolaldehyde bis(di-tert-butyl dithioacetal), cis-2,6-bis(tert-butylthio)-1,4-dithiane, and trans-2,6-bis(tert-butylthio)-1,4-dithiane was obtained from 2-methyl-2-propanethiol. On oxidation to sulfones of the stereoisomers of 2,6-dialkoxy-1,4-oxathiane and 2,6-bis(alkylthio)-1,4-dioxane with hydrogen peroxide, the configurations were retained, but the stereoisomers of 2,6-bis(tert-butylthio)-1,4-dithiane were transformed into the same oxidation product.     

Photo-oxidation of di-n-butylsulfide by various electron transfer sensitizers in oxygenated acetonitrile.

The selective activation of different photosensitizers has been carried out under comparable conditions and their efficiency towards di-n-butylsulfide oxidation in oxygenated acetonitrile compared from the product distribution after 150 minutes of irradiation. As expected, the best selectivity towards sulfoxide is obtained with a conventional energy transfer sensitizer such as Rose Bengal (RB), but also with a quinone with a low-lying triplet state, 2,3,5,6-tetrachloro-1,4-benzoquinone (chloranil or CHLO) and with 9,10-dicyanoanthracene (DCA). More significant yields in sulfonic and sulfuric acids are obtained under sensitization with 9,10-anthraquinone (ANT) or a derivative of benzophenone, 4-benzoyl benzoic acid (4-BB), with which additional experiments were carried out in order to discuss the involvement of either singlet oxygen or superoxide radical anion. Triphenyl pyrylium tetrafluoroborate (TPT+) is inefficient under the selected conditions and sulfide photo-oxidation can only be achieved with higher TPT+ concentrations with simultaneous total TPT+ bleaching. With TPT+, 1,2,4,5-tetracyanobenzene (TCNB) and TiO2, the product distribution and the low selectivity as well as the formation of numerous common by-products are indicative of radical mechanisms. All these results are discussed according to the possible formation of activated oxygen species, such as singlet oxygen, superoxide radical anion or alkylperoxy radicals.