Aufklärung von Peroxyl-Radikalreaktionen in wäßriger Lösung mit strahlenchemischen Techniken

Oxidative Degradation of Polyvinyl Alcohol by the Photochemically Enhanced Fenton Reaction. Evidence for the Formation of Super-Macromolecules

The reaction mechanism of the oxidative degradation of polyvinyl alcohol (PVA) by the photochemically enhanced Fenton reaction has been studied. Fast and efficient degradation was observed in a batch reactor, using a medium pressure mercury arc in a Pyrex envelope and employing 80% of the stoichiometric amount of H2O2 required for the total oxidation of PVA and a concentration ratio as low as 1 mole of iron(II) sulfate per 20 moles of PVA sub-units (C2H4O) Model PVA polymers of three different molecular weights (15,000, 49,000 and 100,000 g mol−1) were found to follow identical degradation patterns. Strong experimental evidence supports the formation of super-macromolecules (MW: 1–5 × 106 g mol−1) consisting of oxidized PVA and trapped iron(III) at an early reaction stage. Low molecular weight intermediates, such as oxalic acid, formic acid or formaldehyde were not found during the PVA degradation, and we may deduce that the manifold of degradation reactions is mainly taking place within the super-macromolecules from which CO2 is directly released.

Tandem organocatalysis and photocatalysis: an anthraquinone-catalyzed indole-C3-alkylation/photooxidation/1,2-shift sequence

Quinones exhibit orthogonal ground- and excited-state reactivities and are therefore highly suitable organocatalysts for the development of sequential catalytic processes. Herein, the discovery of an anthraquinone-catalyzed thermal indole-C3-alkylation with benzylamines is described, which can be combined sequentially with a new visible-light-driven catalytic photooxidation/1,2-shift reaction. The one-flask tandem process converts indoles into 3-benzylindole intermediates, which are further transformed into new fluorescent 2,2-disubstituted indoline-3-one derivatives.

Reaction of N-acylhomoserine lactones with hydroxyl radicals: rates, products, and effects on signaling activity

Chemical communication in bacteria, sometimes called quorum sensing, is a fundamental microbial process that is based on the exchange of molecular signals between cells. The signaling molecules involved in this process are thermodynamically unstable in some environments and their degradation affects microbial communication. This work reports the oxidation of a series of substituted N-acylhomoserine lactones (AHLs, a class of quorum sensing signals) by hydroxyl radicals. The corresponding bimolecular rate constants were obtained and correlated positively with the length of the acyl side chain (C, in numbers of carbon atoms) ranging from 2.4 × 10(9) M(-1) s(-1) to 9.4 × 10(9) M(-1) s(-1) (C4- to C10-AHL), 2.4 × 10(8) M(-1) s(-1) for 3-oxo C6-AHL, and 2.94 × 10(9) M(-1) s(-1) for 3-oxo C8-AHL. Liquid chromatography-mass spectrometric techniques were applied to qualify the identity and quantify the yields of the hydroxyl radical oxidation products of C6-AHL (aldo, keto, and hydroxylated C6-analogues identified). The biological activity of C6-AHL and associated products was determined using the Vibrio harveyi bioluminescence bioassay. Oxidation resulted in a net increase in assay response indexed against the starting AHL. This result suggested that the application of HO• based technologies such as advanced oxidation processes for biofilm control may result in unintended quorum sensing responses by microbial communities.

Degradation of phenol by mechanical activation of a rutile catalyst

In the present paper a novel mechanochemical process for the elimination of organic pollutants dissolved in water is proposed. In this regard, phenol aqueous solutions (100mgL(-1)) were ball-milled for 0, 12, 18, 24, 36, 48, and 72h with and without a well-characterized (XRD, SEM, and N(2) Adsorption), rutile powder catalyst and the reaction products analyzed with UV and GC/MS. It was found that when the catalyst was not included in the process, phenol was not affected, but when it was included, phenol was decomposed. The catalyst itself did not change and the reaction follows a pseudo-first-order kinetics. Besides, intermediates which are characteristic of the ()OH radical mechanism were found in the reaction products. Then, a mechanism similar to those accepted for other advanced oxidation processes was proposed. The value measured for the pseudo-first-order reaction constant was very low, indicating that the reported process is inefficient. Nevertheless, this problem could be solved by applying catalysts consisting of particles with smaller diameters.

On the Mechanism of the Ascorbic Acid-Induced Release of Nitric Oxide fromN-Nitrosated Tryptophan Derivatives: Scavenging of NO by Ascorbyl Radicals

During the past years, there has been increasing interest in endogenous nitric oxide storage compounds. Recently, we briefly reported on the ascorbate-dependent release of nitric oxide ((.)NO) from N-nitrosotryptophan derivatives. In the present study, the underlying mechanism of (.)NO release is studied in more detail, primarily utilizing N-acetyl-N-nitrosotryptophan (NANT) as a model compound. The initial rate of the ascorbate-induced release of nitric oxide has been found to correspond to the rate of NANT decay. In this process, N-acetyltryptophan (NAT) is produced almost quantitatively. The final yield of nitrite amounted to around 90 % with respect to the applied amount of NANT. However, the total release of nitric oxide was only 60 %, as determined by using an FNOCT-4(fluorescent nitric oxide cheletropic trap number 4) assay. Besides nitric oxide, a second volatile product, dinitrogen oxide (N(2)O), has been identified by using (15)N NMR spectrometry, strongly indicating the intermediacy of nitroxyl (HNO). The formation of intermediate ascorbyl radical anions during the NANT-ascorbate reaction has been monitored by using ESR spectrometry. Unexpectedly, it was found that the primary oxidized product of vitamin C, dehydroascorbic acid (DHA), efficiently consumes nitric oxide. Since ESR spectrometry further revealed that ascorbyl radical anions are also generated during the spontaneous decay of DHA, the DHA-nitric oxide reaction is related to recombination of (.)NO with the thus formed ascorbyl radical anions. A conclusively established mechanism of the NANT-ascorbate reaction is presented, with O-nitrosoascorbate as a key intermediate, as additionally supported by CBS-QB3 calculations. The present study suggests that vitamin C and its oxidation products can chemically counterbalance endogenous nitric oxide levels.

Photochemistry of N-isopropoxy-substituted 2(1H)-pyridone and 4-p-tolylthiazole-2(3H)-thione: alkoxyl-radical release (spin-trapping, EPR, and transient spectroscopy) and its significance in the photooxidative induction of DNA strand breaks

UVA-irradiation of the photo-Fenton reagents N-isopropoxypyridone 2b and N-isopropoxythiazole-2(3H)-thione 3b releases radicals which induce strand breaks. Transient spectroscopy establishes N-O bond scission [Phi(N)(-)(O) = (75 +/- 8)% for 2b and (65 +/- 7)% 3b] as the dominating primary photochemical process to afford the DNA-damaging radicals. Product studies and laser-flash experiments reveal that the thiazolethione 3b leads primarily to the disulfide 5, from which through C-S bond breakage, the bithiazyl 6, the thiazole 7, and the isothiocyanate 8 are derived. Upon irradiation of pyridone 2b (300 nm) in aqueous media, a mixture of isopropoxyl and 2-hydroxyprop-2-yl radicals is formed, as confirmed by trapping with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and EPR spectroscopy. In contrast, the photolysis of the thiazolethione 3b (350 nm) affords exclusively the DMPO adducts of the isopropoxyl radicals. Control experiments disclose that the thiazolethione-derived photoproduct disulfide 5, or the intermediary thiyl radicals B, scavenge the carbon-centered 2-hydroxyprop-2-yl radicals, which are generated from the isopropoxyl radicals by hydrogen shift. With supercoiled pBR 322 DNA in a 60:40 mixture of H(2)O-MeCN, the pyridone 2b and the thiazolethione 3b display moderate strand-break activity (17% open-circular DNA for 2b and 12% for 3b). In pure water, however, the pyridone 2b photoinduces substantially more DNA cleavage (32% open-circular DNA), which is attributed to the peroxyl radicals generated from the 2-hydroxyprop-2-yl radicals by oxygen trapping. The lower strand-break activity of the thiazolethione 3b derives presumably from isopropoxyl radicals, because only these are detected in the photolysis of this photo-Fenton reagent.

The importance of mesomerism in the termination of alpha-carboxymethyl radicals from aqueous malonic and acetic acids

In the dioxygen-free aqueous malonic and acetic acid systems. alpha-carboxymethyl radicals are produced through hydrogen abstraction from the parent compound by radiolytically generated OH radicals. H abstraction from the CO2-/CO2H group followed by decarboxylation is a process of small importance (< or = 5%). The alpha-carboxymethyl radicals terminate by recombination. Two types of recombination product are observed which are characterised by the formation of a C-C linkage or a C-O linkage. alpha-Carboxymethyl radicals are mesomeric systems. Their mesomeric state depends on the state of protonation and determines the proportion of the C-C- versus C-O-linked dehydrodimers they produce.

The 7-nitroindole nucleoside as a photochemical precursor of 2'-deoxyribonolactone: access to DNA fragments containing this oxidative abasic lesion

On the basis of molecular modeling studies, the 7-nitroindole nucleoside 1 was selected as a suitable photochemical precursor for photochemical generation of the C1' deoxyribosyl radical under irradiation, which led to 2'-deoxyribonolactone. The nitro-indole nucleoside derivatives 1a and 1b were prepared and their conformation was determined by X-ray crystallography and NMR spectroscopy. The photoreaction of these nucleosides gave the corresponding deoxyribonolactone derivatives efficiently, with release of 7-nitrosoindole. This reaction was successfully applied to synthesis of oligonucleotides containing the deoxyribonolactone lesion.

Differences in the reactivity of phthalic hydrazide and luminol with hydroxyl radicals

The reactivity of 5-amino-2,3-dihydro-phthalazine-1,4-dione (luminol) and phthalic hydrazide with hydroxyl radicals was studied. HO*-radicals were generated by the Fenton reaction as well as by water radiolysis. Both luminol and phthalic hydrazide react with hydroxyl radicals under intense chemiluminescence (CL) emission. However, exclusively the CL arising from phthalic hydrazide oxidation can be quenched by competition (e.g. by the addition of carbohydrates), whereas luminol CL is enhanced. The reactivities of both compounds with HO*-radicals were further studied by time-resolved spectroscopy (pulse radiolysis), competition methods, NMR spectroscopy and mass spectrometry. Whereas only slight differences were detectable by pulse radiolysis, the analysis of competition kinetics in the presence of p-nitroso-dimethylaniline (NDMA) gave a two-fold-enhanced reactivity for luminol (4.8 x 10(9) l mol(-1) s(-1)) in comparison to phthalic hydrazide (2.0 x 10(9) l mol(-1) s(-1)). NMR and mass spectrometric analyses revealed significant differences in the reactivity of HO*-radicals: whereas in luminol solutions hydroxylation of the aromatic ring system predominated, hydroxylated products were not detectable upon irradiation of phthalic hydrazide. A hypothetical mechanism is proposed which may explain the observed differences.