Oxidation of Carbon Monoxide by Perferrylmyoglobin

Perferrylmyoglobin is found to oxidize CO in aerobic aqueous solution to CO₂. Tryptophan hydroperoxide in the presence of tetra­(4-sulfonatophenyl)-porphyrinate-iron­(III) or simple iron­(II)/(III) salts shows similar reactivity against CO. The oxidation of CO is for tryptophan hydroperoxide concluded to depend on the formation of alkoxyl radicals by reductive cleavage by iron­(II) or on the formation of peroxyl radicals by oxidative cleavage by iron­(III). During oxidation of CO, the tryptophan peroxyl radical was depleted with a rate constant of 0.26 ± 0.01 s^–1 for CO-saturated aqueous solution of pH 7.4 at 25 °C without concomitant reduction of the iron­(IV) center. Carbon monoxide is as a natural metabolite accordingly capable of scavenging tryptophan radicals in myoglobin activated by peroxides with a second-order rate constant of (3.3 ± 0.6) × 10² L mol^–1 s^–1, a reaction that might be of importance in cellular membranes of the intestine for protection of tissue against radical damage during meat digestion

On the Reaction of Lupulones, Hops β‑Acids, with 1‑Hydroxyethyl Radical

Lupulones, hops β-acids, are one of the main constituents of the hops resin and have an important contribution to the overall bacteriostatic activity of hops during beer brewing. The use of lupulones as natural alternatives to antibiotics is increasing in the food industry and also in bioethanol production. However, lupulones are easy oxidizable and have been shown to be very reactive toward 1-hydroxyethyl radical with apparent bimolecular rate constants close to diffusion control <i>k</i> = 2.9 × 10⁸ and 2.6 × 10⁸ L mol^–1 s^–1 at 25.0 ± 0.2 °C in ethanol–water solution (10% of ethanol (v/v)) as probed by EPR and ESI-IT-MS/MS spin-trapping competitive kinetics, respectively. The free energy change for an electron-transfer mechanism is Δ<i>G</i>° = 106 kJ/mol as calculated from the oxidation peak potential experimentally determined for lupulones (1.1 V vs NHE) by cyclic voltammetry and the reported reduction potential for 1-hydroxyethyl radical. The major reaction products identified by LC-ESI-IT-MS/MS and ultrahigh-resolution accurate mass spectrometry (orbitrap FT-MS) are hydroxylated lupulone derivatives and 1-hydroxyethyl radical adducts. The lack of pH dependence for the reaction rate constant, the calculated free energy change for electron transfer, and the main reaction products strongly suggest the prenyl side chains at the hops β-acids as the reaction centers rather than the β,β′-triketone moiety

Kinetics and Thermodynamics of 1‑Hydroxyethyl Radical Reaction with Unsaturated Lipids and Prenylflavonoids

Hydroxyalkyl radicals have been reported to induce lipid oxidation as the key aspect of the pathogenesis of alcoholic fatty liver disease and are responsible for the alkylation and cleavage of DNA during the metabolism of a wide range of genotoxic compounds. However, relevant kinetic data for the oxidation of unsaturated lipids by 1-hydroxyethyl radical (HER) has not been reported. In this study, the rate constants for the reaction of unsaturated fatty acid methyl esters and sterols with HER have been determined using a competitive kinetic approach employing the spin-trap α-(4-pyridyl-1-oxide)-<i>N-tert</i>-butylnitrone (4-POBN) as the competitive substrate. Polyunsaturated fatty acid methyl ester is shown to react with HER with an apparent second-order rate constant ranging from (3.7 ± 0.1) × 10⁶ L mol^–1 s^–1 for methyl linoleate to (2.7 ± 0.2) × 10⁷ L mol^–1 s^–1 for methyl docosahexanoate at 25.0 ± 0.2 °C in ethanol. The apparent second-order rate constant for polyunsaturated fatty acid methyl ester oxidation by HER is dependent on the number of bisallylic hydrogen atoms rather than on the bond dissociation energy value for the weakest CH bond as determined by ab initio density functional theory calculations. Sterols displayed higher reactivity compared to unsaturated fatty acid methyl esters with apparent second-order rate constants of (2.7 ± 0.1) × 10⁶ and (5.2 ± 0.1) × 10⁷ L mol^–1 s^–1 at 25.0 ± 0.2 °C in ethanol for cholesterol and ergosterol, respectively. Similar experiments with prenylflavonoids as potential herbal chemopreventive agents for preventing alcoholic liver diseases yield apparent second-order rate constants close to the diffusion control with <i>k</i>_app values of (1.5 ± 0.2) and (3.6 ± 0.1) × 10⁹ L mol^–1s^–1 for 6-prenylnarigerin and xanthohumol at 25.0 ± 0.2 °C in ethanol solution, respectively. Polyunsaturated lipids were revealed to be highly reactive oxidizable substrates toward HER-induced oxidation in biological systems leading to damage of membranes and sensitive structures

Beer Thiol-Containing Compounds and Redox Stability: Kinetic Study of 1‑Hydroxyethyl Radical Scavenging Ability

The 1-hydroxyethyl radical is a central intermediate in oxidative reactions occurring in beer. The reactivity of thiol-containing compounds toward 1-hydroxyethyl radical was evaluated in beer model solutions using a competitive kinetic approach, employing the spin-trap 4-POBN as a probe and by using electron paramagnetic resonance to detect the generated 1-hydroxyethyl/4-POBN spin adduct. Thiol-containing compounds were highly reactive toward the 1-hydroxyethyl radical with apparent second-order rate constants close to the diffusion limit in water and ranging from 0.5 × 10⁹ L mol^–1 s^–1 for the His-Cys-Lys-Phe-Trp-Trp peptide to 6.1 × 10⁹ L mol^–1 s^–1 for the reduced lipid transfer protein 1 (LTP1) isolated from beer. The reactions gave rise to a moderate kinetic isotope effect (<i>k</i>_H/<i>k</i>_D = 2.3) suggesting that reduction of the 1-hydroxyethyl radical by thiol-containing compounds takes place by hydrogen atom abstraction from the RSH group rather than electron transfer. The content of reduced thiols in different beers was determined using a previously established method based on ThioGlo-1 as the thiol derivatization reagent and detection of the derivatized thiols by reverse-phase liquid chromatography coupled to a fluorescence detector. The total level of thiol in beer (oxidized and reduced) was determined after a reduction step employing 3,3′,3″-phosphanetriyltripropanoic acid (TCEP) as the disulfide reductant. A good correlation among total protein and total thiol content in different beers was observed. The results suggest a similar ratio between reduced thiols and disulfides in all of the tested beers, which indicates a similar redox state

Photoinduced Charge Shifts and Electron Transfer in Viologen–Tetraphenylborate Complexes: Push–Pull Character of the Exciplex

Viologen–tetraarylborate ion-pair complexes were prepared and investigated by steady-state and time-resolved spectroscopic techniques such as fluorescence and femtosecond transient absorption. The results highlight a charge transfer transition that leads to changes in the viologen structure in the excited singlet state. Femtosecond transient absorption reveals the formation of excited-state absorption and stimulated emission bands assigned to the planar (<i>k</i>_obs < 10¹² s^–1) and twisted (<i>k</i>_obs ∼ 10¹⁰ s^–1) structures between two pyridinium groups in the viologen ion. An efficient photoinduced electron transfer from the tetraphenylborate anionic moiety to the viologen dication was observed less than 1 μs after excitation. This is a consequence of the push–pull character of the electron donor twisted viologen structure, which helps formation of the borate triplet state. The borate triplet state is deactivated further via a second electron transfer process, generating viologen cation radical (V^•+)

Quinolines by Three-Component Reaction: Synthesis and Photophysical Studies

<div><p>The synthesis of five quinolines 8-octyloxy-4-[4-(octyloxy)phenyl]quinoline and 6-alkoxy- 2-(4-alkoxyphenyl)-4-[(4-octyloxy)aryl]quinolines are described by three-component coupling reaction mediated by Lewis acid FeCl3 and Yb(OTf)3. 4-n-octyloxybenzaldehyde, anisaldehyde, 4-n-octyloxyaniline p-anisidine, and 1-ethynyl-4-heptyloxybenzene, 1-ethynyl-4-octyloxybenzene and 2-ethynyl-6-heptyloxynaphthalene are the reagents in this protocol. A Yb3+ catalyst resulted in higher yields of quinolines than Fe3+. Polarizing optical microscopy (POM) revealed that none of the quinolines were liquid crystals, even the more anisotropic. UV-Vis measurements of one of the quinolines in polar solvent show two absorption bands at 280 and 350 nm related to π,π* and n,π* transitions. No changes were observed to lower-energy absorption band (ε < 104 mol L-1 cm-1) related to n,π* transition. A laser flash photolysis study for one of the quinolines relates a main transient band at 450 nm with a lifetime of 2.6 µs in ethanol, which is completely quenched in the presence of oxygen. This transient band was assigned to triplet-triplet absorption of one of the quinolines, which is semi-oxidised in the presence of phenol. Radiative rate constants have been determined along singlet and triplet excited state energies (3.39 and 3.10 eV, respectively). The chemical structure of one of the quinolines was also unequivocally confirmed by single-crystal X-ray analysis.</p></div