Semiquinone Radical Anion Research Articles

Photoreduction of 9,10-anthraquinone by triethylamine: a Fourier-transform EPR study

Abstract Using time resolved Fourier-transform EPR spectroscopy the photoreduction of anthraquinone by triethylamine in 2-propanol solution was investigated. The spin-polarized anthraquinone triplet deactivates by hydrogen transfer from the solvent and electron transfer from triethylamine generating 10-hydroxy-anthroxyl-9 radicals, ketylradicals and semiquinone anion radicals and amine radical cations, respectively. All radicals are spin-polarized by triplet mechanism and partially by radical pair mechanism. Competitive chemical kinetics of triplet deactivation, lifetimes of two different primary radical ion pairs and secondary reduction reactions of ground state anthraquinone have to be included in the kinetic depending on the lifetimes on the Coulomb-coupled radical ion pairs follow unimolecular decay or are amine catalysed depending on the number of amine molecules in the first solvation shell. Furthermore, the absolute amount of spin polarization and spin lattice relaxation rates of the radicals could be determined.

Porphyrins linked to high acceptor strength cyano quinones as models for the photosynthetic reaction center

The synthesis of biomimetic photosynthetic model compounds, composed of 10, 15, 20-tritolylporphyrins linked to different quinones by cyclohexylene bridges, is described. Cyclic voltammetry measurements show a significant change of the reduction potential of the quinone moiety through introduction of cyano groups as electron withdrawing substituents. EPR and ENDOR spectra of the semiquinone anion radical derivatives reveal a considerable spin density redistribution within the semiquinone moiety caused by the substituents. Lamp irradiation in situ of the porphyrin hydroquinones, dissolved in reversed Triton X-100 micelles, through the resonator slits of the EPR spectrometer does not only yield hyperfine resolved absorptive, but also emissive EPR spectra. This is indicative of strong electron spin polarization effects. From the polarization pattern it can be deduced that the radical/triplet pair mechanism between two photoactive species, located in one micelle, gives rise to this effect.

One-electron photoreduction of flavin mononucleotide: time-resolved resonance Raman and absorption study

Resonance Raman and absorption spectra of the lowest excited triplet state T 1, and semiquinone neutral and anion radicals of flavin mononucleotide (FMN) have been obtained by time-resolved Raman spectroscopy and laser flash photolysis. Both spectra have revealed that photoreduction of FMN is initiated by the process in which the T 1 state of FMN abstracts an electron from substrates such as EDTA to generate the semiquinone anion radical FMN −· both in acidic and basic solutions over the pH range 5–9, and when the pH is smaller than 8.4, FMN −· is further converted into the semiquinone neutral radical FMNH · by combining with H +. The electron transformation is much faster in basic solutions than in acidic solutions. In more strongly acidic solutions pH <4.4, protonation of the T 1 state 3FMN∗ occurs in less than 80 ns and the protonated T 1 state 3[FMNH +]∗ abstracts an electron from the substrate to generate FMNH ·. No indication of the abstraction of a hydrogen atom by 3FMN∗ to generate FMNH · was obtained.

Reactivity of semiquinone radical anions of hydroxyperylenequinone with oxygen

Hyprocrellin B (HB) could be regarded as a 4,9-perylenequinone derivative bearing two hydroxy group at the 3- and 10- two carbonyl groups. The product of demethylation of the 11-methoxy of HB, HB(11-OH), bears three hydroxy groups at the 3-, 10-positions peri to positions. Illumination of the HB (11-OH) in the presence of an electron donor led to accumulation of the semiquinone radical anion HB − (11-OH). The ESR hyperfine structure was recorded and the UV-visible spectum was investigated. HB − (11-OH) reacted with oxygen with generation of the conjugated mono base HB(11-O −) and hydrogen peroxide. The effect of the position of the hydroxy groups on the reactivity of the semiquinone radical anion with oxygen was investigated by redox potential, acid dissociation constant and NMR measurements. It was indicated that the acidity of the 11-hydroxy group in HB (11-OH) was stronger than that of the groups at 3- and 10-positions. The reaction of HB − (11-OH) with oxygen yields at first the original quinone HB(11-OH) and O 2 −, and then HB (11-OH) transfers the phenolic hydroxy proton at the 11-position to O 2 to yield the conjugated base HB (11-O −) and HO 2 which disproportionates rapidly to H 2O 2 and O 2.

Study of electron transfer interaction between hypocrellin and N, N-diethylaniline by UV-visible, fluorescence, electron spin resonance spectra and time-resolved transient absorption spectra

Hypocrellin A (HA) and hypocrellin B (HB) extracted from Hypocrella bambusae (B. et. Br) Sacc are perylene quinoid pigments. Quenching of HA or HB fluorescence by N, N-diethylaniline (DEA) results in Stern-Volmer plots; the quenching rate constants in CH 3CN are 2.28 × 10 10 M −1s −1 for HA and 2.34 × 10 10 M −1s −1 for HB and are in agreement with those calculated from the semiclassical Marcus theory. The values of the free energy change ΔG for electron transfer between HA ∗ or HB ∗ and DEA calculated from Weller's equation are −1.21 eV for HA and −1.24 eV for HB. Electron spin resonance (ESR) signals of semiquinone radical anions of HA or HB have been detected on illumination ( λ = 550 nm) of HA or HB and DEA in anaerobic dimethylsulphoxide and CH 3CN solutions, indicating that an electron transfer has occurred from the ground state of DEA to excited states of HA or HB. The UV-visible spectra of HA in the presence of DEA in dimethylformamide and the time-resolved transient absorption spectra of the interaction of HA and DEA in different concentrations of HA and DEA in CH 3CN are examined. The observation of transient absorption of the semiquinone radical anion of HA ( λ max = 620 nm) is also evidence of this electron transfer process. The interaction of HA and N-ethylaniline, aniline is also studied by UV-visible, ESR and fluorescence spectra.

Synthesis and electrochemical properties of intramolecular multi-electron transfer probe compounds

The absorption spectra of diquinone pentacene-5,7,12,14-tetrone (PDQ), have been studied and it was found that the long wavelength absorption of the neutral quinone and of the semiquinone anion radicals were different. For example, λ max(PDQ) = 375 nm, λ max(PDQ −) = 1350 nm, λ max(PDQ 2−) = 850 nm. PDQ can be used as an electron acceptor which can accept up to four electrons. Investigations of absorption spectra readily determines how many electrons have been transferred. We have now synthesized a series of compounds containing PDQ as acceptor and carbazole as donor, linked by a chain of variable length. The intramolecular multi-electron transfer ‘probe’ compounds prepared can be reduced chemically or electrochemically to give the corresponding anion radicals. The reduction potentials for these compounds were estimated by cyclic voltammetry at a mid point between the anodic and cathodic peak potentials. The reduction of the quinones to the semiquinone anion radicals was carried out using 0·1 M tetrabutylammonium tetrafluoroborate in CH 3CN under a N 2 atmosphere.

Electron transfer induced deprotonation of α- and β-pinene: Evidence for ring-closed vinylcyclobutane radical cations

Electron transfer from α- ( 1) or β-pinene ( 2), to 2,3,5,6-tetrachlorobenzoquinone generates radical cations, which are rapidly deprotonated by the semiquinone radical anion. (1S,5S)- 2 is converted to (1S,5S)- 1 and (1R,5R)- 1 yields a dehydrogenation product, verbenene, (1R,5R)- 9 with essentially quantitative retention of optical activity. The results support chiral, “ring-closed” radical cation structures in which the allylic-quaternary cyclobutane CC bond retains a significant degree of bonding.

Generation of semiquinone radical anion and reactive oxygen ( 1O 2, O 2•− and •OH) during the photosensitization of a water-soluble perylenequinone derivative

5(or 8)-sulfonated hypocrellin B (5-SO 3H-HB) is a water-soluble perylenequinone derivative. Electron spin resonance studies in combination with chemical methods show that 5-SO 3H-HB is an active photosensitizer. Visible-light illumination of 5-SO 3H-HB solutions generates a semiquinone radical anion, singlet oxygen, a superoxide radical anion, a hydroxyl radical and hydrogen peroxide. In anaerobic solution, the semiquinone radical anion is predominately photoproduced, via self-electron transfer between the excited and ground states of 5-SO 3H-HB. In aerobic solution, singlet oxygen is generated in the photosensitization of 5-SO 3H-HB. In addition, a superoxide radical anion is also generated from 5-SO 3H-HB on illumination in aerobic solution, via the reduction of oxygen by the semiquinone radical anion. In aqueous solution, the superoxide radical anion formed can rapidly transformed to a hydroxyl radical via the Fenton reaction and via the reaction of the semiquinone radical anion with H 2O 2. Based on these results, the pathways for the photodynamic action of 5-SO 3H-HB have been proposed.

Characteristics of the reaction between semiquinone radical anion of hypocrellin A and oxygen in aprotic media

Hypocrellin A (HA), a hydroxylperylenequinone derivative, is an efficient phototherapeutic agent. The reactivity of the semiquinone radical anion of HA (HA −) with O 2 in aprotic medium has been studied. Although the standard redox potential of the HA −HA couple (−360 mV vs. a normal hydrogen electrode (NHE)) is less negative than that of the O 2 −O 2 couple (−500 mV vs. NHE), it appears that HA − can still be oxidized with oxygen. The electron transfer is accompanied by a proton transfer between one of the phenolic hydroxyl groups of HA and O 2 − yielding the conjugated base of HA (HA b −) and hydrogen peroxide. On the basis of the results obtained from spectrophotometric, electron spin resonance spin-trapping and CCl 4-inhibiting experiments, a reasonable reaction pathway was proposed for the oxidation of HA − with O 2 in aprotic media.

EPR and ENDOR characterization of semiquinone anion radicals related to photosynthesis

AbstractEPR and ENDOR experiments were performed on the radical anions of 1,4‐benzoquinone, 2,3‐dimethyl‐1,4‐benzoquinone, 2,3,5,6‐tetramethyl‐1,4‐benzoquinone, 2,3‐dimethyl‐5‐ethyl‐1,4‐benzoquinone, plastoquinone‐1, plastoquinone‐9 and ubiquinone‐10. In liquid solutions the isotropic hyperfine coupling constants could be determined. In the frozen state hyperfine tensors for α‐protons and the freely rotating methyl groups were measured whereas other β‐protons (CH2) were broadened beyond detection probably caused by several superimposed chain conformations. In the immobilized radicals proton hyperfine couplings of hydrogen bonds formed between the alcoholic solvent and the CO groups of the quinones could be determined by experiments in protonated and deuterated solvents. Two different types of strong hydrogen bonds exist in these systems: in‐plane with aiso &gt; 0 and out‐of‐plane with aiso ≈︁ 0. The latter type is observed if substituents at both carbons adjacent to the CO groups force the hydrogen bond out of the molecular plane. By use of the point‐dipole model and oxygen spin densities obtained from 17O hyperfine couplings, hydrogen bond lengths could be determined from the measured dipolar proton hyperfine tensors. These findings are very important for the binding and function of quinones in biological systems such as the photosynthetic reaction centres of plants and bacteria.

Elektronentransfer und Kontaktionen-Bildung, 42 [1,2] Cyclovoltammetrische und ESR / ENDOR-Untersuchungen der Einelektronen-Reduktion von Diphenochinonen / Electron Transfer and Contact Ion Pair Formation, 42 [1,2] Cyclovoltammetric and ESR / ENDOR Investigations of the One-Electron Reduction of Diphenoquinones

Semiquinone radical anions are prototype compounds for contact ion pair formation with metal counter cations. In order to investigate the still open question whether bulky alkyl groups can sterically interfere, diphenoquinone derivatives O=C(RC=CH)2C=C(HC=CR)2C=O with R = C(CH3)3, CH(CH3)2 and CH3 have been selected and the following ESR/ENDOR results are obtained for the alkaline metal cations: The tetrakis(tert-butyl)-substituted radical anion only adds Li⊕ and Na⊕, while K⊕ forms no ion pair. The 3,3ʹ,5,5ʹ-tetra(isopropyl)diphenoquinone radical anion is accessible to all cations Me⊕, although Rb⊕ and Cs⊕ seem to be present solvent-separated in solution. The tetramethyl-substituted radical anion unfortunately polymerizes rapidly. Additional information concerns the ESR/ENDOR proof for ion triple radical cation formation [Li⊕ M•⊖Li⊕]•⊕, or the difference in the coupling constants upon Me⊕ docking at one δ⊖O=C group, suggesting that about 87% of the spin density is located in the cation-free molecular half of the diphenoquinone radical anion. Based on the wealth of ESR/ENDOR information, crystallization of the contact ion pairs and their structural characterization should be attempted.

Photochemistry of a butylene linked porphyrin-quinone donor-acceptor system studied by time-resolved and steady-state EPR spectroscopy

Radicals generated photochemically from a covalently linked porphyrin-quinone donor-acceptor system dissolved in reversed micelles and isotropic solution have been studied by steady-state and time-resolved EPR spectroscopy. In these systems photoinducedintramolecular as well asintermolecular electron transfer processes occur which result in the formation of semiquinone radical anions and porphyrin radical cations. Disproportionation of the semiquinone leads to the formation of porphyrin-hydroquinone (and porphyrin-quinone). The porphyrin-hydroquinone is itself photoactive and reacts through the photoexcited triplet state of the porphyrin. Reduction of the porphyrin to the dihydro from — probablyvia hydrogen abstraction by the photoexcited porphyrin from the hydroquinone — appears to be the dominant reaction. Once formed the dihydrophyrin undergoes further similar photochemistry. Emissively polarized spectra are observed from these systems in steady-state EPR experiments. Timeresolved EPR indicates that this polarization is essentially due to the radical triplet pair mechanism.

Spectroscopic study on the photoreduction of hypocrellin A: generation of semiquinone radical anion and hydroquinone

Hypocrellin A (HA) is an efficient phototherapeutic agent. Illumination of HA in dimethylsulphoxide (DMSO) or DMSO-buffer (1:1 by volume, pH > 6.7) generated a strong electron paramagnetic resonance (EPR) signal. This EPR signal was intensified in the presence of reductants. The EPR spectrum obtained was assigned to the semiquinone radical anion of HA (HA .−) based on a series of experimental results. Decay of HA .−, attributed to a radical-radical reaction, follows second-order kinetics. In acidic DMSO-buffer (1:1 by volume) solution, no EPR signal of HA .− was detected in the absence or presence of reductant. This was explained by a fast disproportionation of radicals, facilitated by protonation of radicals. The spectrophotometric measurements indicated that on illumination HA was directly reduced to its two-electron reduction product, i.e. hydroquinone, in acidic solution. The absorption maximum of the hydroquinone of HA which is at 496 nm at pH 5.8 shifts bathochromically with increase in pH of the medium. In DMSO or DMSO-buffer (1:1 by volume, pH > 6.7) solutions the semiquinone radical anion of HA was also observed spectrophotometrically. The absorption maximum of HA .− is at around 628 nm. Strong intramolecular hydrogen bonding was considered to exist in the chromophore of HA .−.

Mimicking primary processes in photosynthesis—covalently linked porphyrin quinones

Porphyrin quinones (P-Qs), covalently linked via different aliphatic bridges, have been synthesized and studies in their (porphyrin) cationic and (semiquinone) anionic radical states by EPR, ENDOR and TRIPLE resonance techniques. Electron transfer (ET) from the porphyrin donor to the quinone acceptor could be observed by time-resolved picosecond fluorescence spectroscopy (singlet ET) and by time-resolved EPR spectroscopy (triplet ET) in isotropic fluid solution and in anisotropic media (liquid crystals and reversed micelles). Steady-state in situ photoexcitation of P-Qs in CTAB cationic reversed micelles yielded the corresponding semiquinone radical anions. In TRITON X-100 reversed micelles both the radical cation of the porphyrin and the radical anion of the semiquinone could be detected, which occured in complete emission. In covalently linked porphyrin flavins ET from the photoexcited porphyrin fragment to the flavin and, in addition, energy transfer from the photoexcited flavin to the porphyrin could be observed.

Some Mechanistic Insights into a Model Bleaching Process of Quinones by Bisulfite and Dithionite: An ESR-CIDEP Study

Abstract The free radical mechanisms of a model bleaching system involving quinones and bisulfite and dithionite in solutions were investigated by time-resolved ESR techniques. The major role of the radical anion, SO2 ·−, is established with a number of different parent ions, depending upon the environment. The reduced colorless products are mainly the hydroquinone dianions which can effectively be photoionized into semiquinone radical anions (thus the color reversal) and solvated electrons. The strongly polarized semiquinone radical anion and the solvated electron confirm the mechanisms of the photoionization. Addition of calcium carbonate to the system helps to stabilize the hydroquinone dianions by the formation of the Ca++/Q2− ion pairs and thus reduces the effects of photoreversion.

Photosensitization with anticancer agents 20—EPR studies on the photodynamic action of phleichrome: Formation of semiquinone radical and activated oxygen species on illumination with visible light

When phleichrome was illuminated with visible light, the semiquinone radical, singlet oxygen, and superoxide anion radical were detected. The formation of the semiquinone radical and activated oxygen species and the transformations and competitions between them depend on quinone and oxygen concentration, duration and intensity of illumination, and the nature of the substrate. In anaerobic solution, the semiquinone radical was predominantly photoproduced via the self-electron transfer between the excited and ground species. In contrast, in aerobic solution, singlet oxygen is the principal product in the photosensitization of phleichrome. In addition to singlet oxygen, superoxide anion radical is also generated by the quinones upon illumination in aerobic solution, but to a lesser extent. The generation of the superoxide anion is significantly enhanced by the presence of electron donors.

ESR evidence for the generation of reactive oxygen species from the copper-mediated oxidation of the benzene metabolite, hydroquinone: role in DNA damage

In previous studies, we observed that Cu(II) strongly induces the oxidation of hydroquinone (HQ), producing benzoquinone and H 2O 2 through a Cu(II) Cu(I) redox cycle mechanism. The oxidation of HQ by Cu(II) also results in plasmid DNA cleavage. In this study, using ESR spectroscopy we have investigated whether this chemical-metal redox system can generate reactive oxygen species which induce DNA damage. In order to set the stage for the ESR experiments and the inhibitors to be used in these experiments, some preliminary O 2 consumption and plasmid DNA cleavage experiments were performed. Mixing 100 μM HQ with 10 μM Cu(II) in phosphate-buffered saline (PBS) resulted in a marked consumption of O 2 and the concomitant generation of H 2O 2, and extensive DNA degradation in φX-174 RF I DNA. The presence of Superoxide dismutase (SOD) or mannitol did not affect either the O 2 consumption, H 2O 2 generation or DNA damage. In contrast, the Cu(I) chelators, bathocuproinedisulfonic acid (BCS) and glutathione (GSH), extensively inhibited the HQ/Cu(II)-mediated O 2 consumption and DNA damage. The presence of catalase also prevented the DNA damage. Although the HQ/Cu(II)-mediated O 2 consumption increased in the presence of azide, azide markedly inhibited the HQ/Cu(II)-induced DNA degradation, resulting in primarily open circles. Using ESR spectroscopy, it was observed that Cu(II) strongly mediated the formation of semiquinone anion radicals from HQ in PBS, which could be blocked by BCS. α-(4-Pyridyl-1-oxide)- N- tert-butylnitrone (4-POBN)-spin trapping experiments showed that the interaction of HQ with Cu(II) produced 4-POBN-CH 3 and 4-POBN-CH(OH)CH 3 adducts in the presence of dimethyl sulfoxide (DMSO) and ethanol, respectively, suggesting that hydroxyl radical or an equivalent reactive intermediate is generated from the HQ/Cu(II) system. The presence of catalase, BCS or GSH but not SOD completely prevented the formation of 4-POBN-CH 3 adduct from the HQ/Cu(II) plus 4-POBN/DMSO system. This indicates that both H 2O 2 and Cu(I) are critical for the formation of reactive oxygen from the HQ/Cu(II) system. Anaerobic conditions induced an ∼ 85% decrease in the formation of 4-POBN-CH 3 adduct. Reactive oxygen scavenger experiments showed that the formation of the 4-POBN-CH 3 adduct was significantly inhibited by azide but not by mannitol. Overall, the above results indicate that through a copper-redox cycling mechanism the copper-mediated oxidation of HQ generates reactive oxygen species which may participate in DNA damage.

5435991 Use of persistent heterocyclic free-radicals in magnetic resonance imaging

The present invention provides the use of a persistent π-system free radical for the manufacture of a contrast medium for use in magnetic resonance imaging, wherein the electron delocalising π-system of said radical comprises at least one homo or heterocyclic ring, said radical being other than the chloranil semiquinone anion radical or a trityl radical. Also provided are magnetic resonance imaging contrast media containing and methods using such radicals.

Oxidation of 4‐methylcatechol by dioxygen studied by ESR spectroscopy. The different regioselectivity of OH− and MeO− nucleophilic attack and kinetic deuterium isotope effects

AbstractThe oxidation of 4‐methylcatechol by dioxygen and the subsequent reactions of the two nucleophiles OH− and MeO− with the oxidation products were studied by electron spin resonance (ESR) spectroscopy. These reactions are models for those of skin proteins with substituted 1,2‐dihydroxybenzenes (catechols) which produce the severe allergic contact dermatitis associated with plants such as poison ivy, poison oakd and the Japanese lac tree. The rates of formation of the primary and secondary semiquinone radical anions show a large kinetic deuterium isotope effect (ca 10) at 20°C, which is ascribed to a slow deprotonation of the hydrogen bridged 4‐methylcatechol monoanion. There is a marked difference in regioselectivity for OH− and MeO− substitution of the 4‐methylcatechol monoanione; OH− reacts exclusively at the 3‐position whereas MeO− attacks at the 5‐position. This dissimilarity between two normally similar nucleophiles is also ascribed to the occurrence of hydrogen bridging between the substituting OH− at the 3‐position and the adjacent quinone oxygen at the 2‐position. The expected attack at the more electropositive 5‐position occurs with MeO− because no such hydrogen bonding can occur. Disubstitution occurs for OH− but not for MeO−, probably reflecting the greater nucleophilicity of the former.

Towards the characterization of radicals formed in rhizomes of Iris germanica

When Iris germanica, Hydrocotyle verticillata, Glyceria maxima and Juncus effusus plants were treated anoxically and subsequently returned to air, long-lived free radicals were rapidly generated in the plant tissues and were detected by EPR spectroscopy. The free radicals in I. germanica rhizomes were extracted into aqueous alkali; the EPR spectrum of the aqueous solution showed fine structure which indicated the presence of at least two oxygen-centred radicals. The EPR data, and the radical lifetimes, suggested that they were probably semiquinone radical anions. Product analysis of the alkaline extract indicated that the main components were a set of flavonoids including quercetin, irisolone, selenone and derivatives of irigenin. The main flavonoids had more reduced and/or more alkylated structures than those from I. germanica grown under normal aerobic conditions. Semiquinone radical anions can readily be generated from quercetin, and similar flavonoids with 1,2-dihydroxy-, 1,4-dihydroxy- or trihydroxy-substitution patterns, by interaction with superoxide. However, the EPR-detected radicals were probably derived from flavonoids more heavily substituted than quercetin.