Effect of flavonols on radiation-induced reactions with the participation of α-hydroxyl-containing organic radicals

The effects of flavonols and structurally related model compounds on the formation of the final radiolysis products of ethanol and aqueous ethylene glycol and 2-methoxyethanol solutions were studied. It was found that the test compounds actively interact with the -hydroxyl-containing radicals of parent substances by several mechanisms to change the direction of radiolytic processes. The applicability of flavonols to the inhibition of radiation-induced free-radical fragmentation of organic compounds was demonstrated for the first time. Mechanisms were proposed to explain the observed effects.__________Translated from Khimiya Vysokikh Energii, Vol. 39, No. 3, 2005, pp. 171–175.Original Russian Text Copyright © 2005 by Grintsevich, Shadyro.     

Pulse radiolysis studies of etoposide and 4′-demethylepipodophyllotoxin in aqueous solution

The reactions of etoposide (VP 16, 4′-demethyl-epipodophyllotoxin ethylidene-β-d-glucoside) and 4′-demethylepipodophyllotoxin (DMEP) with the primary radiolytic products of water, such as e⁻_aq, H and OH/O⁻ radicals, and the secondary radical (SO⁻₄) in aqueous solution were studied by use of the techniques of pulse radiolysis, respectively. The absorption spectra of reaction products with e⁻_aq, H and OH/O⁻ and SO⁻₄ radicals were observed, and the rate constants of them were determined by following the build-up kinetics of radicals produced or the decay of hydrated electron observed at 600 nm, respectively.     

Reactivity of OH radicals with chlorobenzoic acids—A pulse radiolysis and steady-state radiolysis study

The reactions of OH radicals with 2-, 3-, 4-chlorobenzoic acids (ClBzA) and chlorobenzene (ClBz), k(OH+substrates)=(4.5?6.2)×10⁹ dm³ mol^?1 s^?1, have been studied by pulse radiolysis in N₂O saturated solutions. The absorption maxima of the OH-adducts were in the range of 320?340 nm. Their decay was according to a second-order reaction, 2k=(1?9)×10⁸ dm³ mol^?1 s^?1. In the presence of N₂O/O₂ the formation of peroxyl radicals was detectable for 2-, 4-ClBzA and ClBz, k(OH-adduct+O₂)=(2?4)×10⁷ dm³ mol^?1 s^?1, while this reaction for 3-ClBzA was too slow to be registered. In the presence of N₂O the degradation rates induced by gamma radiation were very similar for all chlorobenzoic acids, yet the chloride formation was distinctly higher for 3-ClBzA. In the presence of oxygen the initial degradation of 2-and 4-ClBzA equaled the OH-radical concentration, whereas in case of 3-ClBzA only ～60% of OH led to degradation. The order for the efficiency of dehalogenation was 4->2->3-ClBzA. Several primary radiolytic products could be detected by HPLC. To evaluate the toxicity of final products a bacterial bioluminescence test was carried out.     

A radiation-chemical and quantum-chemical study of ascorbic acid interaction with α-hydroxyethyl radicals

The effect of ascorbic acid and its analogue 5,6-O-isopropylidenyl-2,3-O-dimethylascorbic acid (I), which has been synthesized for the purpose and does not contain mobile hydrogen atoms, on the formation of the products of continuous radiolysis of deaerated ethanol and its aqueous solutions has been studied. The ionization potentials, the molecular orbital energies, the enthalpies of homolytic dissociation of C-H and O-H bonds, and the enthalpies of H atom addition to the C=O group of the test compounds have been calculated by ab initio methods. The array of the experimental and calculated theoretical data suggests that both ascorbic acid in the undissociated form and compound I can oxidize α-hydroxyethyl radicals, whereas the monoanion of ascorbic acid acts as a reducing agent in the reactions with these transient radicals. The reduction of α-hydroxyethyl radicals in aqueous solutions by the ascorbic acid monoanion can follow both the hydrogen transfer and electron transfer mechanisms.     

The interaction of radiation-generated radicals with myoglobin in aqueous solution—VI: The reduction of ferrimyoglobin by α-hydroxyalkyl radicals

G-values of the formation of ferromyoglobin (Mb^II) have been determined for the continuous γ-radiolysis of N₂O-saturated neutral aqueous solutions containing ferrimyoglobin (Mb^III) and a series of aliphatic alcohols (RH) under conditions such that the competition for the primary ^.OH radicals favors RH. A comparison of the efficiencies of reduction of Mb^III by the secondary organic radicals formed via H-abstraction reactions with estimates from the literature of the fraction of ^.OH attack at the C-atom α to the —OH group indicates that α-hydroxyalkyl radicals are primarily responsible for the observed reduction. Significantly lower reduction yields are observed when RH = 1,2-diol, compared with the expected yields of strongly reducing 1,2-dihydroxyalkyl radicals; the initial reducing radicals convert into inactive β-alkanonyl analogues so that the reduction of Mb^III is not kinetically competitive with the β-elimination process. Mb^III is useful as a probe of the occurence of slow conversion processes involving radiolytically-generated radicals.     

Kinetics of the reactions of catechins with hypochlorite,peroxynitrite, and amino acid–derived peroxyl- radicals

Catechins, one of the class of flavonoids, are known as very efficient antioxidants. Here we investigated the kinetics of the reactions of three catechins, namely, catechin, epigallocatechin, and epigallocatechin gallate (EGCG) with some oxidants, which are formed in vivo under oxidative stress, hypochlorite, peroxynitrite, and amino acid peroxyl radicals. Stopped-flow spectrophotometry and pulse radiolysis technique with absorption detection were used to observe the formation of intermediate products of oxidized catechins. We found that catechins react with hypochlorite with the rate constant of the order of 10⁵–10⁶ M⁻¹ s⁻¹ at pH 7.4. Experimental kinetic traces of the reaction of EGCG with valine peroxyl radicals were fitted using chemical simulation, and the rate constant of this reaction was found to be 5 × 10⁵ M⁻¹ s⁻¹. The rate constants of the formation of unstable catechin quinones in the reaction with peroxynitrite were comparable to that of spontaneous peroxynitrite isomerization, which indicates that catechins are oxidized indirectly by peroxynitrite. Biological consequences of these reactions are discussed.     

Quantum chemical studies on addition reactions of carbene with carbon monoxide and formaldehyde

MNDO method has been employed to study the reaction paths and to optimize the structures of the reactants, products and transition states of the addition reactions of carbene with carbon monoxide and formaldehyde available. Mechanisms have been obtained.     

Asymmetric syntheses of α-methyl γ-amino acid derivatives

This project was undertaken to demonstrate the potential of asymmetric hydrogenations mediated by the chiral, carbene-oxazoline analogue of Crabtree's catalyst "cat" in asymmetric hydrogenations of allylic amine derivatives of amino acids. Peripheral features of the substrates (protecting groups, functional groups related by redox processes, and alkene geometries) were varied to optimize the stereochemical vectors exerted by the substrate and align them with the catalyst vector. N-Acetyl-protected, O-TBDPS-protected allylic substrates 9a-e emerged as the best for this reaction; syn-products were formed from the E-alkenes, while the Z-isomers gave anti-target materials, both with high diastereoselectivities. This study featured asymmetric catalysis to elaborate optically active substrates into more stereochemically complex chirons; we suggest that the approach used, optimization of stereocontrol by varying peripheral aspects of the substrate, tends to be easier than de novo catalyst design for each substrate type. In other words, optimization of the substrate vector is likely to be more facile than enhancement of the catalyst vector via ligand modifications.     

Pulse radiolysis studies of vitaminK1 in an aqueous–organic mixed solvent

Pulse radiolysis studies of VK₁ were carried out in a nitrogen-saturated aqueous–isopropanol–acetone mixed solvent. We studied the spectroscopic characteristics of semiquinone radicals of VK₁ and determined the kinetic parameters of formation and decay of the species. It was found that the decay process of semiquinone radicals of VK₁ was according to a first-order reaction, this was different from the case in ethanol solution, possible reasons were also analyzed.     

Study on the α-ray radiolysis product of hydroxysemicarbazide

Journal of Radioanalytical and Nuclear Chemistry - Hydroxysemicarbazide (HSC) as a novel salt-free organic reductant has the ability to accomplish the reduction of Pu(IV) fast in the nuclear fuel...     

Interaction of hydroxyaryl aldehydes with α-hydroxyethyl radicals

The effect of some hydroxylated aromatic aldehydes on the radiation-chemical transformations of deaerated ethanol during continuous radiolysis has been studied. The data obtained show that these compounds effectively inhibit radiation-induced processes involving α-hydroxyethyl radicals (α-HER). Benzaldehyde and its hydroxylated derivatives (II, III) predominantly oxidize, and compounds containing the cinnamic moiety, (IV–VI), add α-HER to a carbonyl group or -C=C bond.     

Effect of temperature on O⊘ reactions and equilibria: A pulse radiolysis study

The pK_a of the hydroxyl radical was measured over the 20–80°C temperature range. At 20°C, the pK_a was 11.84 and fell to 10.81 at 80°C. The dissociation constant for the ozonide anion (O⊘₃⇌O₂+O⊘) was found to be 5.5 × 10^-7 mol dm^-3 at 20°C and 46.2 × 10^-7 mol dm^-3 at 70°C. The rate constants and activation energies for the reaction of O⊘ and OH with 2-propanol, methanol and 3-hexene-1,6-dicarboxylate ions have also been measured.     

Kinetic and mechanism of reactions of L-α-Glutamic acid and L-Glutamine with pyridoxal

The kinetics and mechanisms of condensation of pyridoxal with L-α-glutamic acid and L-glutamine were studied by UV spectroscopy and polarimetry. L-α-Glutamic acid reacts with pyridoxal to form a Schiff base whose subsequent hydrolysis gives rise to pyridoxamine and α-ketoglutaric acid. The reaction of Lglutamine with pyridoxal involves the Γ-NH₂ group and affords a Schiff base whose subsequent hydrolysis gives rise to pyridoxamine and L-α-glutamic acid.     

Pulse radiolysis studies on aqueous systems PtCl42−Cl− and PtCl42−Br−

Pulse irradiation of PtCl₄²⁻ in aqueous solutions containing Cl⁻ or Br⁻ indicates that oxidation of Pt(II) by OH may be wholly or partially replaced by oxidation by Cl₂⁻ or Br₂⁻, depending on the efficiency of OH scavenging by the particular halide ion. The Pt(III) species produced by the latter reactants absorb respectively at 290 and 310 nm and differ from that produced by OH. Contrary to the similar PdCl₄²⁻ no evidence of Pt(II) co-ordinating more than four Cl⁻ ions has been obtained. The rate constants of the processes and extinction coefficients of the transient species observed are reported.     

LFER and CoMFA studies on optical resolution of α-alkyl α-aryloxy acetic acid methyl esters on DACH-DNB chiral stationary phase

Summary The HPLC resolution of a series of racemic -substituted -aryloxy acetic acid methyl esters I on a -acid chiral stationary phase containing N,N-(3,5-dinitrobenzoyl)-trans-1,2-diaminocyclohexane as chiral selector was modelled by linear free energy-related (LFER) equations and comparative molecular field analysis (CoMFA). Our results indicate that the retention process mainly depends on solute lipophilicity and steric properties, whereas enantioselectivity is primarily influenced by electrostatic and steric interactions. CoMFA provided additional information with respect to the LFER study, allowed the mixing of different subsets of I and led to a quantitative 3D model of steric and electrostatic factors responsible for chiral recognition.     

Inclusion complexes of α- andβ-cyclodextrin with α-lipoic acid

A UV spectroscopic study has been performed in neutral aqueous solution to give the complex stability constants. Data analyses assuming 1:1 stoichiometry were successfully applied to both of the host-guest combinations employed, where 1:1 host-guest complex formations were observed at lower concentration of cyclodextrins (CDs). X-ray powder diffraction and IR spectroscopy measurements also demonstrated that inclusion complexes were formed in the solid state. Furthermore, thermogravimetry and DTA were used to investigated the thermal properties of these complexes. The differential thermal analysis, as well as temperature variation experiments below 100°C, indicated that after complexing the 1,2-thiolane moiety of -lipoic acid (LP) penetrated into the cavity of the CD and the S-S linkage was protected against heat.     

Effect of nitrous acid on γ-ray radiolysis and radiolytic products of N,N-dimethylhydroxylamine

Journal of Radioanalytical and Nuclear Chemistry - N, N-dimethylhydroxylamine (DMHA) is a new salt-free reductant used in spent fuel reprocessing, this paper reports the effect of HNO2 on...