Semidione Radical Research Articles

Study of the molecular-structural composition of alcoholic extracts of radio-induced potato tubers by mass spectrometry and EPR

This article presents data on electron paramagnetic resonance (EPR) and mass spectrometric analysis of potato tubers, irradiated with gamma rays, in order to examine and identify changes in the molecular composition of organic matter following radiation exposure. The products of the Maillard reaction were compared with the products of intramolecular radiolysis of organic constituents of potatoes. The presence of free radicals was verified using EPR. DDPM (2,3-dihydro-3,5-dihydroxy-6-methyl-4 (H) -pyran-4-one) was among the radiolysis products detected via mass spectrometry, which points to the intramolecular dehydration of potato carbohydrates. EPR signals indicate single-electron transitions of the semidione radical anionic molecular compounds. It has been shown that irradiation with gamma rays significantly destroys the carbohydrate, lipid, keto-carotene and amino acid molecules of potatoes.

ChemInform Abstract: Addition of Semidione Radicals to Arenediazonium Ions: Synthesis of 1, 1-Diacyl-2-arylhydrazines.

ChemInform Abstract: Addition of Semidione Radicals to Arenediazonium Ions: Synthesis of 1, 1-Diacyl-2-arylhydrazines.

Pulse radiolytic reduction studies of 1,4,4a,8a-tetrahydro-endo–1,4-methano-naphtha-5,8-dione (THMND): effect of tertiary structure

Radiolytic reduction of a substituted 5,8-naptha dione (THMND), synthesized in our laboratory, has been investigated by pulse radiolysis and steady-state γ-radiolysis in pure aqueous, aqueous-formate and in aqueous-2-propanol-acetone mixed solvent systems. The rate constants of formation of the semi-dione radicals were approx. 109 dm3 mol-1 s-1 in aqueous-formate and aqueous-2-propanol-acetone mixed solvent. The semi-dione radicals decay by second order kinetics with rate constants (2k) of about 109 and 108 dm3 mol-1 s-1 in the above two solvents, respectively. The pKa value of the radical was found to be 5.0 in aqueous-formate solution and 5.8 in the aqueous-2-propanol-acetone mixed solvent. The one-electron reduction potential (E1) value at pH 7, determined from the pulse-radiolysis experiment, was found to be –420 ± 20 mVvs. NHE at 298 K and was independent of solvent. Ab initio calculations on its one-electron reduction reaction suggest the formation of a radical, which is different from a semiquinone where the electron density is delocalised over the two oxygen atoms. Experimental absorption maxima of the radical in aqueous solution also agree very well with the ab initio calculated values. Steady-state γ-radiolysis of THMND produces the corresponding two-electron reduced species.

Identification of cis-ethanesemidione as the organic radical derived from glycolaldehyde in the suicide inactivation of dioldehydrase and of ethanolamine ammonia-lyase.

The hydrate of glycolaldehyde is a substrate analogue that induces the formation of cob(II)alamin and 5'-deoxyadenosine from adenosylcobalamin at the active site of dioldehydrase, and the resulting complex is inactive. The carbon atoms of glycolaldehyde hydrate remain bound to this complex, and it has been postulated that the first step or steps of the catalytic process on glycolaldehyde hydrate generate an intermediate that undergoes a destructive side reaction leading to inactivation of the enzyme [Wagner, O. W., Lee, H. A., Jr., Frey, P. A., and Abeles, R. H. (1966) J. Biol. Chem. 249, 1751-1762]. All evidence suggests that dioldehydrase reaction proceeds by a radical mechanism, and the glycolaldehyde hydrate is expected to be converted initially into a radical. Electron paramagnetic resonance (EPR) spectroscopic analysis of the inactivated complex shows that glycolaldehyde is transformed into a cis-ethanesemidione radical that is weakly spin-coupled to the cob(II)alamin in the active site of the enzyme. This radical has been identified by analysis of EPR spectra obtained from samples with (13)C- and (2)H-labeled forms of glycolaldehyde. The analysis shows that the stable radical associated with the inactive complex is symmetrical and that it contains a single solvent-exchangeable proton, consistent with a cis-ethanesemidione. Glycolaldehyde also inactivates ethanolamine ammonia-lyase (EAL). EPR studies of ethanolamine ammonia-lyase reveal that treatment with glycolaldehyde also results in formation of an ethanesemidione radical bound in the active site. The suicide inactivation in both enzymatic reactions is postulated to result from formation of this stable radical, which cannot react further to abstract a hydrogen atom from 5'-deoxyadenosine. Analysis of the electron spin-spin coupling between the semidione radicals and cob(II)alamin in both enzymes indicates that the distance between the radical and Co(2+) is approximately 11 A in each case.

A dimerisation reaction of benzene carbaldehydes catalysed by ion pairing studied by ESR spectroscopy

The reduction of benzene carbaldehydes with alkali metals in solvents which promote formation of ion pairs (THF and MTHF) leads to a coupling reaction which yields semidione radical anions. The results reported here show the effects of ion pairing on the chemical reactivity of the intermediate radicals.

Addition of semidione radicals to arenediazonium ions: synthesis of 1,1-diacyl-2-arylhydrazines

Addition of semidione radicals to arenediazonium ions: synthesis of 1,1-diacyl-2-arylhydrazines

Detection of spin adducts in blood after administration of carbon tetrachloride to rats

Rats were treated with CCl 4 and the spin trapping agent α-phenyl- N-t-butyl nitrone (PBN), followed by ESR investigations on samples of heparinized blood. The major signal detected was the ascorbate semidione radical, but smaller concentrations of the carbon dioxide radical anion spin adduct of PBN could also be detected. The ESR signals were more intense when experiments were conducted with plasma, rather than blood. The spin adducts detected were not associated with the red blood cells, and their apparent concentrations increased when the cells were removed by centrifugation. The addition of ascorbate oxidase to the samples markedly diminished the intensity of the ascorbate semidione radical. When plasma samples from CCl 4-treated rats were extracted into toluene, the ESR spectrum of the trichloromethyl adduct of PBN was observed in the extract. Because the spectrum of this adduct was not observed in direct ESR studies of plasma, it is possible that immobilization occurred in the presence of plasma proteins. Evidence to support this hypothesis was developed by adding bovine serum albumin (BSA) to an aqueous solution of the trichloromethyl radical adduct of PBN. As the BSA concentration was increased, the intensity of the ESR spectrum was markedly diminished, and displayed features of an immobilized adduct.

Photochemical Reaction of 3,3-Dimethyl-1,2-indanedione with Xanthene

Abstract The photochemical reaction of 3,3-dimethyl-1,2-indanedione (DMID) with xanthene was investigated in detail. The products are 1-hydroxy-3,3-dimethyl-1-(9-xanthenyl)-2-indanone (3a) (49%), 2-hydroxy-3,3-dimethyl-1-indanone (13%), 1,1′-dihydroxy-3,3,3′,3′-tetramethyl-1,1′-biindan-2,2′-dione (5a) (23%), and 9,9′-bixanthenyl (6) (49%). The reaction proceeds via hydrogen abstraction by 3(nπ)* of DMID leading to triplet radical pair composed of its semidione radical and 9-xanthenyl radical. The product 3a is a combination product from the radical pair, while 5a and 6 are escaping products from it. The hydrogen abstraction occurs at C-1 carbonyl group of DMID in a fairly efficient quantum yield (Φ=0.71). Behavior of the DMID semidione radical is rather resemble those of open-chain semidione radicals than those of sterically hindered semidione radicals.

An esr and CIDEP study of the photoreduction of furil

The transient free radicals formed when furil was photoreduced have been characterized by esr and CIDEP. Two furil derived radicals were observed: one was the protonated form of the semidione radical (FH•) while the other was an allyl radical formed by radical addition to the furil ring. Initial polarization was observed for FH• which was consistent with the triplet mechanism and these CIDEP results indicate that phenol is a better chemical quencher of the furil triplet than pentachlorophenol which is in turn more efficient than 2-propanol. A method to estimate relative T1 of radicals is also described.

ChemInform Abstract: THE PHOTOCHEMICAL REACTION OF 1,1,4,4‐TETRAMETHYL‐2,3‐TETRALINDIONE WITH HYDROGEN DONORS

AbstractDie Photoreaktion (300 W Hg‐Hochdrucklampe) des Tetralindions (I) in Benzol (Bzl.) mit Xanthen (II) ergibt ein " 1,2‐Addukt" (V), Bixanthenyl (IV) und den reduzierten Ketoalkohol (III).

Photochemistry of some cyclopropyl conjugated 1,2-diketones. Part 2. Fragmentation reactions and reduction by aldehydes

The photochemistry of some cyclopropyl conjugated and some related 1,2-diketones in benzene as solvent in the absence and presence of an aliphatic aldehyde as hydrogen donor has been studied. In benzene no photodecomposition occurs with λ > 403 nm, but with radiation of lower wavelength (>300 nm) the 1,2-diketones, with the exception of dicyclopropylethanedione, undergo photodecomposition. The products arise from initial bond rupture between the two carbonyl groups, subsequent decarbonylation of part of the acyl radicals (and not of the relatively stable cyclopropylcarbonyl radicals), and eventual combination of the various types of radicals present. Upon irradiation of a mixture of biacetyl and dicyclopropylethanedione, acyl interchange was observed. The photoreaction of 1,2-diketones in the presence of aliphatic aldehydes led to the formation of dihydrodimers (2), αα-diacylalkanols (3), and esters (4). The mechanisms of formation of these products from the semidione radical formed from triplet excited 1,2-diketone by hydrogen abstraction from the aldehyde, are discussed.

The Photochemical Reaction of 1,1,4,4-Tetramethyl-2,3-tetralindione with Hydrogen Donors

Abstract The photochemical reaction of 1,1,4,4-tetramethyl-2,3-tetralindione, 1, with hydrogen donors, such as xanthene and aldehyde, in the liquid phase has been investigated. When xanthene was selected as the hydrogen donor, the main products were the “1,2-adduct,” photo-reduced α-keto alcohol, and 9,9′-bixanthenyl. The photolysis of 1 and an aldehyde gave the mixture of the “1,2-adduct” and the “1,4-adduct.” However, in the reaction with isobutyraldehyde another type of “1,2-adduct” was obtained, i.e., a combination product between the semidione radical and the isopropyl radical resulting from the decarbonylation of the isobutyryl radical, together with propene and carbon monoxide. The usual type of “1,2-adduct,” the “1,4-adduct,” and the photo-reduced product (α-keto alcohol) were the minor products in the reaction. It was also confirmed that the original “1,4-adduct” obtained from 1 and acetaldehyde rearranged photochemically to a different type of “1,2-adduct,” but no reverse rearrangement was observed under the present conditions. The mechanisms of the reactions were discussed on the basis of product analysis and the examination of the reaction by means of CIDNP.

Formation of semidione radicals on production of 1,2-dihydroxyalkl radicals in the presence of peroxides in basic solution. An electron spin resonance study

In basic solution containing hydrogen peroxide or organic peroxides, 1,2-dihydroxyalkyl radicals are converted into semidione radical anions in low yield.

Kinetic electron spin resonance spectroscopy. Part 5.—Effects of chemically induced dynamic electron polarization

A procedure has been developed for measuring the true radical concentration during in situ photolysis e.s.r. experiments, even when the e.s.r. signal is modified as a result of initial electron spin polarization in the photolytically generated radicals. The validity of kinetic e.s.r. studies is thereby extended to shorter times. This procedure depends on an extended kinetic model for chemical induced dynamic electron polarization (CIDEP) incorporating emissive-absorptive (E–A) polarization and initial polarization (IP). The model is tested, using data obtained from a pulse photolysis system, by a study of the behaviour of the semidione radicals of pyruvic acid and biacetyl in alcoholic solvents. For pairs of peaks with the same |∑AIMI|, the difference in peak heights represents E–A polarization while the sum incorporates initial polarization, so the two components can be separated. Further experiments have established the viscosity dependences of the two types of polarization.

PHOTOFRAGMENTATION OF α‐OXOCARBOXYLIC ACIDS IN AQUEOUS SOLUTION. AN EPR STUDY. FORMATION OF SEMIDIONE RADICALS BY DECARBOXYLATIVE SUBSTITUTION OF α‐OXOCARBOXYLIC ACIDS BY ACYL RADICALS‐I: GLYOXYLIC, PYRUVIC, α‐OXOBUTYRIC, α‐OXOGLUTARIC AND α‐OXOISOCAPROlC ACIDS

Abstract— By means of in situ photolysis EPR of aqueous solutions of α‐oxocarboxylic acids (RCO‐CO2H) at pH values above 5, semidione radical anions [RC(O‐)=C(O‘)R] and α‐hydroxy‐α‐carboxy alkyl radicals [RC(OH)CO2‐] were detected. C02 was identified as a reaction product. On photolysis of mixtures of α‐oxocarboxylic acids (RCOCO2H and R'COCC2H), “mixed” semidione radical anions [RC(O‐>=C(O)R’] were observed in addition to RC(O‐)=C(O‘)R, R'C(O‐)=C(O’)R‘, RC(OH)CO2‐ and R'C(OH)CO2‐. The experimental results are explained in terms of photodecarboxylation (α‐clea‐vage) of electronically excited RCOCOJ to yield RCO and CO2. The radicals RC(OH)CO2‐ are formed by reduction of RCOCO2‐ by CO2‐. The semidione radicals are produced by addition of RCO to RCOCO2‐ followed by decarboxylation of the intermediate adduct. This mechanism was confirmed by generating acyl radicals independently and reacting them with α‐oxocarboxylic acids. Selected product studies support the mechanism suggested.

Formation of cis‐ and trans‐Semidione Radical Anions by Addition of Acyl Radicals to α‐Oxo Carboxylic Acids

Formation of <i>cis</i>‐ and <i>trans</i>‐Semidione Radical Anions by Addition of Acyl Radicals to α‐Oxo Carboxylic Acids

Long-range proton hyperfine coupling in bicyclic radicals

Long-range isotropic proton hyperfine coupling in rigid bicyclic systems is investigated using a non-empirical configuration interaction study of an appropriate σ-bonded fragment in conjunction with a semi-empirical description of a particular spin label. The coupling is found to depend on the symmetry of the highest occupied molecular orbital of the spin label and upon the stereochemistry of σ-bonded moiety. Relationships are obtained linking the hyperfine coupling to the adjacent spin density of the label for both cases wherein the highest occupied molecular orbital is either symmetric or anti-symmetric relative to the reflection plane containing the C-H bond. Numerical calculations are carried out for the particular case of the bicyclo-[2.2.1]heptane semidione radical anion wherein the reflection plane contains the γ-CH2 group as well as bisecting the molecular orbitals of the spin label.

Theory of electron spin g-values for carbonyl radicals

A complete SCF calculation is performed to obtain g-values for both cis and trans isomers of the semidione radical (CHO) 2 −. Results are then generalized to all carbonyl π radicals through the equation 〈g〉=2.0026 + Γ∑ ρ0 ΔE where Δ E is an n → π ∗ excitation energy, ϱ 0 is the Λ spin density on oxygen, and Γ is a function of both total molecular symmetry and local symmetry at the carbonyl group. Theoretical and empirical expressions for Λ are obtained. The general equation can be used to determine spin densities from measured g-values and visible /uv absorption spectra.

Electron spin resonance investigation on cis and trans semidione alkali metal ion pairs

The e.s.r. spectra of the ion pairs formed by hexamethoxybibenzoyl semidione with each of the five alkali metals have been recorded in different solvents over the temperature range –80 to +40°C. Two superimposed spectra were observed which are assigned to the ion pairs of the cis and trans isomers of the semidione radical anion, on the basis of g factors, 13C splitting and alkali metal splittings. Both cis and trans isomers are found to be associated with the counter ion. The sign of the alkali hyperfine coupling constant is discussed on the basis of linewidth analysis and theoretical calculations of alkali spin densities.

Temperature variation of the proton hyperfine splittings in the E.S.R. spectrum of the biacetyl semidione radical

Temperature variation of the proton hyperfine splittings in the E.S.R. spectrum of the biacetyl semidione radical