Steen Steenken

Reaction pathways and mechanisms of photodegradation of pesticides

The photodegradation of pesticides is reviewed, with particular reference to the studies that describe the mechanisms of the processes involved, the nature of reactive intermediates and final products. Potential use of photochemical processes in advanced oxidation methods for water treatment is also discussed. Processes considered include direct photolysis leading to homolysis or heterolysis of the pesticide, photosensitized photodegradation by singlet oxygen and a variety of metal complexes, photolysis in heterogeneous media and degradation by reaction with intermediates generated by photolytic or radiolytic means.

Reduction potentials of flavonoid and model phenoxyl radicals. Which ring in flavonoids is responsible for antioxidant activity

Model phenoxyl and more complex flavonoid radicals were generated by azide radical induced one-electron oxidation in aqueous solutions. Spectral, acid–base and redox properties of the radicals were investigated by the pulse radiolysis technique. The physicochemical characteristics of the flavonoid radicals closely match those of the ring with the lower reduction potential. In flavonoids which have a 3,5-dihydroxyanisole (catechins), or a 2,4-dihydroxyacetophenone (hesperidin, rutin, quercetin)-like A ring and a catechol- or 2-methoxyphenol-like B ring, the antioxidant active moiety is clearly the B ring [reduction potential difference between the model phenoxyls is ΔE(A–B ring models) > 0.1 V]. In galangin, where the B ring is unsubstituted phenyl, the antioxidant active moiety is the A ring. Even though the A ring is not a good electron donor, E7 > 0.8/NHE V, it can still scavenge alkyl peroxyl radicals, E7= 1.06 V, and the superoxide radical, E7 > 1.06 V. Quercetin is the best electron donor of all investigated flavonoids (measured E10.8= 0.09 V, and calculated E7= 0.33 V). The favourable electron-donating properties originate from the electron donating 0-3 hydroxy group in the C ring, which is conjugated to the catechol (B ring) radical through the 2,3-double bond. The conjugation of the A and B rings is apparently minimal, amounting to less than 2.5% of the substituent effect in either direction. Thus, neglecting the acid–base equilibria of the A ring, and using those of the B ring and the measured values of the reduction potentials at pH 3, 7 and 13.5, the pH dependence of the reduction potentials of the flavonoid radicals can be calculated. In neutral and slightly alkaline media (pH 7–9), all investigated flavonoids are inferior electron donors to ascorbate. Quercetin, E7= 0.33 V, and gallocatechins, E7= 0.43 V, can reduce vitamin E radicals (assuming the same reduction potential as Trolox C radicals, E7= 0.48 V). Since all investigated flavonoid radicals have reduction potentials lower than E, =1.06 V of alkyl peroxyl radicals, the parent flavonoids qualify as chain-breaking antioxidants in any oxidation process mediated by these radicals.

Electron-Transfer-Induced Acidity/Basicity and Reactivity Changes of Purine and Pyrimidine Bases. Consequences of Redox Processes for Dna Base Pairs

Changes in the oxidation state of the DNA bases, induced by oxidation (ionization) or by reduction (electron capture), have drastic effects on the acidity or basicity, respectively, of the molecules. Since in DNA every base is connected to its complementary base in the other strand, any change of the electric charge status of a base in one DNA strand that accompanies its oxidation or reduction may affect also the other strand via proton transfer across the hydrogen bonds in the base pairs. The free energies for electron transfer to or from a base can be drastically altered by the proton transfer processes that accompany the electron transfer reactions. Electron-transfer (ET) induced proton transfer sensitizes the base opposite to the ET-damaged base to redox damage, i.e., damage produced by separation of charge (ionization) has an increased change of being trapped in a base pair. Of the two types of base pair in DNA, A-T and C-G, the latter is more sensitive to both oxidative and reductive processes than the former. Proton transfer induced by ET does not only occur between the heteroatoms (O and N) of the base pairs (intra-pair proton transfer), but also to and from adjacent water molecules in the hydration shell of DNA (extra-pair proton transfer). These proton transfers can involve carbon and as such are likely to be irreversible. It is the A-T pair which appears to be particularly prone to such irreversible reactions.

Addition–elimination paths in electron-transfer reactions between radicals and molecules. Oxidation of organic molecules by the OH radical

Reactions between the ˙OH radical and molecules Y that ultimately lead to electron transfer from Y to ˙OH have been studied by in-situ radiolysis or photolysis, electron spin resonance and pulse radiolysis techniques with optical and conductance detection. These radical–molecule reactions proceed in aqueous solution via the intermediate formation of covalently bound adducts HO—Y˙. These radicals are able to undergo heterolysis, which may proceed by spontaneous or by catalysed paths. The heterolysis results in a one-electron oxidation of Y, and the overall reaction thus consists in a one-electron transfer from the molecule to ˙OH.[graphic omitted]In the addition step (I), a reducing radical is formed by reaction of the oxidizing ˙OH with Y, which is usually neither an oxidant nor a reductant. In the heterolysis step (II), however, the reducing HOY˙ is converted into the oxidizing Y˙+. This phenomenon, termed redox inversion, is the consequence of the change in oxidation state of Y by two units in going from HOY˙ to Y˙+. Examples of redox processes of this kind are given from the class of substituted benzenes and of N-heterocyclics, and structure–reactivity relations governing the heterolysis of HO—Y˙ are discussed.

DFT calculations on the electrophilic reaction with water of the guanine and adenine radical cations. A model for the situation in DNAElectronic Supplementary Information available. See http://www.rsc.org/suppdata/cp/b1/b109204a/

Using density functional theory, the H2O (modeled by OH−) addition on the C8-site of the guanine and adenine radical cations (Gua˙+/Ade˙+) is calculated to be exothermic by −75.3 and −77.7 kcal mol−1, respectively. In contrast, in the absence of the N1 proton on Gua˙+, i.e., in the case of the neutral radical (Gua(–H)˙) the H2O addition is +29.4 kcal mol−1endothermic. Similarly, in the case of the neutral adenine radical (Ade(–H)˙), the N6-deprotonated radical cation, the H2O addition is endothermic by +43.7 kcal mol−1. Related to these observations is the fact that with the radical cations, Gua˙+and Ade˙+, the positive charge density on the C8-carbon is higher than with the deprotonated forms. This means that nucleophilic attack is likely to have a lower activation energy in the case of the former than the latter. The protonated radical, Gua˙+, simulates the situation in double-stranded (ds) DNA where the transfer of the N1 proton to solvent molecules is inhibited due to its base pairing with cytosine. In contrast, in single-stranded DNA and in RNA, Gua˙+ is expected to quickly lose its N1 proton to the water phase. In comparison, with Ade˙+ in ds DNA the exocyclic N6-atom is in contact with water molecules in the major groove of the DNA double helix and thus should be able to rapidly lose a proton to a water molecule, even when it is paired with thymine. This concept provides an explanation for the experimental observation of 7,8-dihydro-8-oxoguanine (8-OGua) formation only in ds DNA and negligible formation of 7,8-dihydro-8-oxoadenine (8-OAde) in any other form of DNA.

Generation of radical-cations from naphthalene and some derivatives, both by photoionization and reaction with SO4–˙: formation and reactions studied by laser flash photolysis

Radical-cations from naphthalene and some derivatives have been generated in aqueous acetonitrile both by direct photolysis (with λ 248 nm light via biphotonic ionization) and via reaction with SO4–˙. The radical-cation reacts rapidly with the parent substrate (k ca. 108 dm3 mol–1 s–1) and with nucleophiles (e.g. with N3–k= 4.2 × 109 dm3 mol–1 s–1 or with water, k 4 × 104 s–1). The radical-cation from 1-naphthylethanoic acid undergoes rapid decarboxylation (k 5 × 105 s–1). The radical cations from 4-methyl- and 4-methoxy-phenylethanoic acid also rapidly decarboxylate to yield the corresponding benzyl radicals.

Ionization of polynucleotides and DNA in aqueous solution by 193 nm pulsed laser light: identification of base-derived radicals.

Light of 193 nm wavelength ionizes polyA, polyC, polyG, and ss and ds DNA in aqueous solution in a monophotonic process giving hydrated electrons and radicals that result from the radical cations of the bases; there is spectroscopic evidence for positive charge migration in DNA from adenine to guanine moieties.

DFT studies on the pairing abilities of the one-electron reduced or oxidized adenine–thymine base pair

Using density functional theory (DFT), the hydrogen bonds making up the adenine–thymine (A–T) base pair are found to increase in total energy upon one-electron oxidation or reduction by 10.9 and 13.3 kcal mol−1, respectively. Due to unsymmetric changes in the H-bond lengths, this strengthening affects an expansion of the base pair length (N1′–N9) by ∼0.27 A. In the oxidized pair, A˙+–T, deprotonation from N6, and with the reduced pair, A˙−–T, protonation on N3 or N7 lead to base pairs which have similar base pairing energies as their parent A–T, i.e., the stabilization by the change in oxidation state is annihilated by (de)protonation. The calculated proton affinities of A˙−–T are large enough to explain its protonation by H2O, which involves heterolytic bond cleavage of a water molecule. The N1 protonated electron adduct of A is a powerful H-bond donor; it is able to mismatch with cytosine (−28.9 kcal mol−1). In DNA this could compete with the “legitimate” guanine-cytosine pairing. The pairing abilities of 2-aminopurine, an “unnatural” isomer of A, used as a fluorescent probe in DNA assemblies, are calculated to resemble those of A closely.

The complexation energy of the one-electron oxidized guanine–cytosine base pair and its parent system with small cations. A DFT-study

The complexation energy of the one-electron oxidized guanine-cytosine base pair and its parent system with small cations. A DFT-study. The complexation energy of small cations (Li+, Na+, Be2+ and Mg2+), substituted for the proton forming the central Watson-Crick hydrogen bond in the guanine-cytosine (G-C) base pair, is calculated using the density functional theory. The one-electron oxidized G-C base pair and its parent system are investigated. In all cases the open-shell systems form strong complexes in vacuum with energies between -92.1 and -430.1 kcal mol(-1). The complexation energy is dependent on the ionic radii and the atomic charge of the ions and follows the trend: Be2+ > Mg2+ > Li+ > Na+. The ions are expected to dehydrate upon complexation with the nucleotides and when the hydration energy is accounted for then Be2+ still has an impressive complexations energy whereas Li+ and Na+ demonstrate low values (similar to -2.5 kcal mol(-1)). The magnesium complex is calculated not to be thermochemically stable. Furthermore, the parent systems have higher complexation energies than their oxidized derivatives, which can be explained in terms of higher electron density on them compared to their neutral radicals, interacting with the small cations. The open-shell systems are planar but their parent counterparts are mostly non-planar. The ability of 7,8-dihydro-8-oxoguanine to form complexes closely resembles that of guanine

Antioxidant action of stobadine.

Abstract The above-mentioned physicochemical, chemical, as well as pharmacological properties including successful results of Phase I and II clinical testing of stobadine as an antianginal agent permit us to consider this compound as potential drug in prevention and/or treatment of tissue injuries caused by oxidative stress.