Pulse radiolysis studies of mangiferin: A C-glycosyl xanthone isolated from Mangifera indica

Pulse radiolysis technique has been employed to study the reaction of different oxidizing and reducing radicals with mangiferin. The reaction of OH radical showed the formation of transient species absorbing in 380–390 and 470–480 nm region. The reaction with specific one-electron oxidants (N₃, CCl₃O₂) also showed the formation of similar transient absorption bands and is assigned to phenoxyl radicals. The pK_a values of the transient species have been determined to be 6.3 and 11.9. One-electron oxidation potential of mangiferin at pH 9 has been found to be 0.62 V vs. NHE. The reaction of e_aq⁻ showed the formation of transient species with λ_max at 340 nm, which is assigned to the ketyl anion radical formed on addition of e_aq⁻ at carbonyl site. Reactions of one-electron oxidised mangiferin radicals with ascorbic acid have also been studied.     

Characterization,stability, and origin of natural radiation-induced defects in the biogenic calcite of Belemnitella americana from the Upper Cretaceous: An electron paramagnetic resonance study

The rostrum of Belemnitella americana (Morton) from the Marshalltown formation (Kmt, Upper Cretaceous) of the Chesapeake and Delaware Canal was investigated by electron paramagnetic resonance (EPR) spectroscopy. The rostrum composed of biogenic calcite possessed inorganic radical centers CO₂⁻, SO₂⁻, and SO₃⁻ with isotropic resonances with g values of 2.0007, 2.0057, and 2.0031, respectively. SO₃⁻ was found to also display an axially symmetric resonance typical of that seen in calcite of geologic origin with g_⊥=2.0036 and g_∥=2.0021. Mn²⁺ signals of orthorhombic symmetry and very narrow line width (∼0.1 mT) were also noted (|D|=9.3 mT (∼0.009 cm⁻¹), |E|=3.1 mT (∼0.003 cm⁻¹)). Isochronal annealing studies reveal that these inorganic radical species reside in energy traps that are significantly deeper than previously determined as revealed by their annealing temperatures: SO₂⁻ (isotropic), T^*∼340 °C; SO₃⁻ (isotropic), T^*∼230 °C; SO₃⁻ (axial), T^*∼190 °C. These data suggest that these spin centers may be used to extend the upper limit for dating purposes to times on the order of 1 Ma for SO₃⁻ (axial) and 200–300 Ma for SO₃⁻ (isotropic). Spin–spin and spin–lattice relaxation studies employing progressive microwave saturation were determined for all sulfur-based radical species and found to be consistent with the supposition of the isotropic signals existing in environments that are conducive to dynamic averaging of the g-anisotropy.     

Oxidation of naringenin by gamma-radiation

The reaction of OH with naringenin (4′,5,7-trihydroxyflavanone) in the presence of air induced the formation of the hydroxylation product eriodictyol (3′,4′,5,7-tetrahydroxyflavanone). Its yield was dependent on pH. The initial degradation yield of naringenin was G_i(-Nar)=(2.5±0.2)×10⁻⁷ mol dm⁻³ J⁻¹. For the reaction with OH, a rate constant k (OH+naringenin)=(7.2±0.7)×10⁹ M⁻¹ s⁻¹ was determined. In the presence of N₂O and NaN₃/N₂O, no eriodyctiol was formed. Apigenin (4′,5,7-trihydroxyflavon) was detected as decay product of the naringenin phenoxyl radicals. In Ar-saturated solutions, naringenin exhibited a pronounced radiation resistance, G(-naringenin) ∼0.3×10⁻⁷ mol dm⁻³ J⁻¹.     

Synthesis and properties of a redox active ligand with bispicorylamino groups and its dinuclear complex

A ligand, 4-methoxy-4′,4″-bis[N,N-bis(2-pyridylmethyl) aminomethyl]triphenyl-amine, and its palladium and copper dinuclear complexes were designed and prepared in order to examine intramolecular interactions between an organic cation radical and the metal ion. Novel NMR techniques, COSY and NOESY, were applied to the palladium complex to examine its conformation in solution. The palladium complex was found to prefer a folded conformation at ambient temperature, indicating the occurrence of intramolecular stacking interaction. CV measurements of the copper complex showed reversible Cu^I/Cu^II and TPA/TPA⁺ redox couples. The spin–spin interaction in the radical pendant copper complex generated upon one electron oxidation of the copper complex was examined by cw-ESR measurements.     

Distribution of species in Na2O–CaO–B2O3 glasses as probed by FTIR

The article presents a simple method that can be used to get the concentration of various species in mixed-modifier borate glasses. By using the fraction of four coordinated boron in xCaO (30 − x)Na₂O70B₂O₃ (0 ≤ x ≤ 27.5 mol%) and xCaO(40 − x)Na₂O60B₂O₃ glasses (10 ≤ x ≤ 40 mol%), the concentration of BO₄⁻ and asymmetric BO₃ units related to each modifier oxide could be determined. CaO has a greater tendency to form asymmetric BO₃ units in the first glass series, while Na₂O has the ability to form BO₄ units to a greater extent. In xCaO(40 − x)Na₂O60B₂O₃ glasses, BO₄⁻ and asymmetric BO₃ units are formed at the same rate from Na₂O and CaO. The fraction of four coordinated boron, can be predicted by treating the studied glasses as if they are mixtures of Na₂O–B₂O₃ and CaO–B₂O₃ matrices. The change in N₄ is due to change in the relative concentration of these matrices.     

Pulse and γ-radiolysis studies on aqueous solution of 1,1′-dimethyl-2-selenourea

One-electron oxidation of 1,1′-dimethyl-2-selenourea (DMSeU) by hydroxyl radicals, one-electron-specific oxidants, was studied using pulse radiolysis technique in aqueous solution. Hydroxyl (OH) radicals and one-electron oxidants, N₃, X₂⁻ (X=Cl, Br, and I) react with DMSeU to form a transient having an absorption spectrum with λ_max at 430 nm. By following the absorbance at 430 nm as a function of solute concentration and in analogy with similar sulfur and selenium compounds, this transient is assigned to dimer radical cation. The dimer radical cations of DMSeU react with oxygen with bimolecular rate constant of 1.0±0.3×10⁸ M⁻¹ s⁻¹. Steady-state γ-radiolysis studies on aqueous solution of DMSeU under hydroxyl radical-induced oxidation condition indicated formation of elemental selenium as one of the by-products, which has been stabilized by the addition of poly vinyl alcohol (PVA), and characterized by dynamic light scattering technique.     

Iridium aminyl radical complexes as catalysts for the catalytic dehydrogenation of primary hydroxyl functions in natural products

The cationic tetra-coordinated 16 electron complex [Ir(trop₂dach)]⁺OTf⁻ (1) where (OTf⁻ = CF₃SO₃⁻) and the neutral amine amido complex [Ir(trop₂dach-1H)] (2) were isolated and structurally characterized. The NH function in 1 is easily deprotonated (pK_a^DMSO = 10.5) to yield the amino amido complex [Ir(trop₂dach-1H)] (2), which is deprotonated at pK_a^DMSO = 19.6 to the anionic di(amido) iridate [Ir(trop₂dach-2H)]⁻ (3); [(R,R)-top₂dach stands for the tetrachelating diamino diolefin ligand (R,R)-N,N′-bis(5H-dibenzo[a,d]cyclohepten-5-yl)-1,2-diaminocyclohexane; (R,R)-top₂dach-1H and (R,R)-top₂dach-2H indicate the mono and double deprotonated form]. Complex 3 is easily oxidized by 1,4-benzoquinone (BQ) to the neutral iridium aminyl radical complex [Ir(trop₂dach-2H)] (4). In combination with BQ as hydrogen acceptor and catalytic amounts of base, 4 serves as catalyst in the highly efficient dehydrogenation of functionalized primary alcohols to the corresponding aldehydes, RCH₂OH + BQ → RCHO + H₂BQ (H₂BQ = catechol). Alcohols like geraniol and retinol are rapidly converted to geranial and retinal, while the conversion of sterically hindered alcohols like lavandulol is slower and the primary product, lavandulal, isomerizes to isolavandulal in a classical base-catalyzed reaction.     

Pulse radiolysis of aqueous diphenyloxide

Pulse radiolysis of aqueous diphenyloxide (DPO) has been performed under various experimental conditions. The OH radicals react with DPO on various positions of the molecule with a rate constant, k=2.1×10¹⁰ l mol⁻¹ s⁻¹. The major reaction step appears to be a cleavage of the C–O bond of DPO resulting into C₆H₄OH (λ=285 nm) and C₆H₅O(λ=325 nm) radicals in addition to DPO–OH adducts. They disappear according to a second-order reaction. In the presence of air or in a gas mixture of N₂O:O₂=4:1 the DPO–OH adducts are scavenged by oxygen, resulting into peroxyl radicals, which are long-lived species. For the reaction of e_aq⁻ with DPO a rate constant, k=2×10¹⁰ l mol⁻¹ s⁻¹ was found.     

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.     

Practical use of chemical probes for reactive oxygen species produced in biological systems by γ-irradiation

Application of chemical probes, for detection of reactive oxygen species (ROS), was tested during γ-irradiation. The ethanol/α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) and 3,3′-diaminobenzidine (DAB) were structurally stable enough to detect OH and H₂O₂, increasingly generated by γ-irradiation up to 1000 Gy. Interestingly, the production rate of H₂O₂, but not OH, during γ-irradiation, was significantly different between in vitro systems of lettuce and spinach. These results suggest that 4-POBN and DAB could be utilized as a semi-quantitative probe to quantify OH and H₂O₂, produced by γ-irradiation up to 1000 Gy.     

Effect of N+ ion implantation on antioxidase activity in Blakeslea trispora

The effect of N⁺ implantation on the activities of CAT, POD, SOD, T-AOC and the capacities of scavenging O₂⁻ and OH in Blakeslea trispora (−) were studied. Results showed that N⁺ implantation caused different changes of CAT, POD, SOD, T-AOC activities and cell scavenging O₂⁻ and OH capacities. With the implantation dose increasing CAT activity was lower than the control sample, while POD, SOD activities and the scavenging O₂⁻ and OH capacities all decreased at the beginning, and then increased lately. At the dose of 6.0×10¹⁵ N⁺ cm⁻² T-AOC activity was lowest, while at the dose of 1.2×10¹⁵ N⁺ cm⁻² its activity was highest, and this change trend was same to the B. trispora (−) survival rate curve. So we speculated that the changes of these antioxidases activity of B. trispora (−) induced by low-energy N⁺ probably have some relationship with its “saddle shape” survival rate curve.     

Photo- and chemically-produced phylloquinone biradicals: EPR and ENDOR study

Phylloquinone biradical triplet species were generated by 300 nm irradiation of frozen (77 K) solutions or by treatment with AlCl₃. The shape of the (Δm_s=1) electron paramagnetic resonance (EPR) signal of the triplet is axially symmetric (E=0) with D=19±0.5 mT for photo-induced and D=11.2±0.5 mT for chemically induced radicals. A half-field signal (Δm_s=2) in the region of g≈4 was detected in both cases, confirming its assignment as a triplet. An additional line arising at the center of the (Δm_s=1) signal with g=2.0048±0.0002 was assigned to the phylloquinone radical anion (PhQ⁻). Electron nuclear double resonance (ENDOR) measurements of the triplet revealed the sign of the D parameter. For photo-generated radicals it appeared to be negative, which is the characteristic of radical dimers with well-separated partners (biradicals). Spin–spin distances of 5.3 and 6.3 Å, respectively, were estimated from the D parameter of photo-generated and chemically prepared phylloquinone biradicals.     

Reduction reactions of bovine serum albumin and lysozyme by CO- .2 radical in polyvinyl alcohol solution: a pulse radiolysis study

Pulse radiolytic reduction of bovine serum albumin (BSA) and lysozyme by CO^⨪₂ radical in presence of polyvinyl alcohol (PVA) has been studied. At pH 6.8 in presence of 2% (w/v) PVA, reduction of BSA and lysozyme (both at 1×10⁻⁴ mol dm⁻³) give an additional transient peak at 390 nm along with the usual 420 nm peak. The bimolecular rate constants for the reaction of CO^⨪₂ radical at pH 6.8±0.2 with BSA are 2.7×10⁸ and 7.13×10⁸ dm³ mol⁻¹ s⁻¹ at 420 nm and 390 nm respectively. The same for lysozyme are 3.2×10⁸ and 5.6×10⁸ dm³ mol⁻¹ s⁻¹ at 420 nm and 390 nm, respectively. Dimethyl disulfide also gives 390 nm and 420 nm peaks in this system upon reduction with CO^⨪₂ radical. The 390 nm peak is ascribed to the sulfenium radical (RSS(H)R). Studies on the variation of pH suggests the protonation of RSSR^⨪ radical (420 nm) to form RSS(H)R radical (390 nm) in this viscous media. The decay of RSS(H)R radical occurs via formation of RS radical and RS(H)S(H)R, the final product being RSH in both cases.     

Investigation of the dynamics of radiolytic formation of ZnO nanostructured materials by pulse radiolysis

ZnO nanostructures have been synthesized by radiolytic methods. A Cobalt-60 γ-source and a 7 MeV linear electron accelerator (LINAC) was used for the radiolysis experiments. Reducing agent like hydrated electron (e_aq⁻), which is produced in radiolysis of water, was used to synthesize ZnO nanostructure materials from zinc salt. 1 M tert-butanol was used to quench the primary oxidizing radical like hydroxyl radical (OH) radiolytic water solution. Doses of about 80–130 kGy were used to perform radiolysis experiments in the present investigation. Time-resolved pulse radiolysis has been used to monitor the transient species involved in the formation of ZnO nanostructures by monitoring at different wavelengths. A scheme for the formation of the ZnO nanostructured materials by the radiolytic method has been described. The formation of ZnO nanostructures was confirmed by X-ray diffraction (XRD) measurements. Dynamic light scattering (DLS) measurements indicated that the size of the nanostructures is in the range of 6–8 nm, which is in agreement with that obtained from XRD. It is interesting to note that ZnO nanostructured materials, as prepared by the radiolytic method, exhibit strong room-temperature fluorescence.     

An FT-IR study of the isomerization of 1-butoxy radicals under atmospheric conditions

Relative rate-studies of the reactions of 1-butoxy radicals have been carried out using a 47 L static reactor with detection of end products by FT-IR spectroscopy. Experiments were performed at 700 torr total pressure and over the temperature range 253–295 K. The chemistry of 1-butoxy is characterized by a competition between reaction with oxygen CH₃CH₂CH₂CH₂O + O₂ → n-C₃H₇CHO + HO₂ (R2), which yields butanal and isomerization CH₃CH₂CH₂CH₂O → CH₂CH₂CH₂CH₂OH (R3), to form a hydroxylated carbonyl-product. A reference spectrum attributed to the product of 1-butoxy isomerization was obtained and used to determine the competition between 1-butoxy isomerization versus reaction with oxygen. The results indicate that isomerization is the dominant fate of 1-butoxy radicals at ambient temperature and pressure and that its importance decreases with decreasing temperature. The rate-coefficient ratio k₃/k₂ (molecule cm⁻³) = 5.5 × 10²³ exp[(−25.1 ± 0.9 kJmol⁻¹)/RT] was obtained. This agrees with other estimates based on methods without monitoring of the isomerization product.     

Reaction of isocyanides with iminophosphorane-based carbene ligands: Synthesis of unprecedented ketenimine–ruthenium complexes

The RuC bond of the bis(iminophosphorano)methandiide-based ruthenium(II) carbene complexes [Ru(η⁶-p-cymene)(κ²-C,N-C[P{NP(O)(OR)₂}Ph₂]₂)] (R = Et (1), Ph (2)) undergoes a C–C coupling process with isocyanides to afford ketenimine derivatives [Ru(η⁶-p-cymene)(κ³-C,C,N-C(CNR′)[P{NP(O)(OR)₂}Ph₂]₂)] (R = Et, R′ = Bz (3a), 2,6-C₆H₃Me₂ (3b), Cy (3c); R = Ph, R′ = Bz (4a), 2,6-C₆H₃Me₂ (4b), Cy (4c)). Compounds 3–4a–c represent the first examples of ketenimine–ruthenium complexes reported to date. Protonation of 3–4a with HBF₄ · Et₂O takes place selectively at the ketenimine nitrogen atom yielding the cationic derivatives [Ru(η⁶-p-cymene)(κ³-C,C,N-C(CNHBz)[P{NP(O)(OR)₂}Ph₂]₂)][BF₄] (R = Et (5a), Ph (6a)).     

Observed bands in Raman and infrared spectra of 1,3-dioxolane and their assignments

We present a Raman study of liquid and polycrystalline dioxolane, together with data for a solid dioxolane/argon film. Observed bands are assigned to the bent conformer on the basis of ab initio calculated vibrations, accompanied with potential energy distribution. Two internal modes were calculated at 658 and 720 cm⁻¹ for the bent form. These were observed at 665 and 722 cm⁻¹ in liquid, and split into four bands (at 694, 697, 725, and 728 cm⁻¹) in the crystal at 10 K. In the solid dioxolane/argon film an additional band appears at 705 cm⁻¹ (between the bands at 675 and 726 cm⁻¹). A search for an additional dioxolane conformer trapped in argon was undertaken by performing molecular dynamics simulations of dioxolane/argon solid film and averaging overall molecular conformations. The resulting conformation did not correspond to an energy minimum, but indicated that changes of only a few degrees in dihedral angles could shift ring deformation and OCO bending modes for more than 10 cm⁻¹. Differential scanning calorimetric measurements gave evidence of solidification upon cooling at 146 K, and two phase changes on heating (one at 158.3 K, and a melting transition at 180.3 K). Further study is required on the hysteresis effect in the temperature behavior of dioxolane, and on the nature of the intermediate solid phase.     

Photoredox reactions of Cr(III) mixed-ligand complexes

Irradiation of chromium(III) complexes with oxalate and pyridinedicarboxylate ligands (pda = 2,3-, 2,4-, or 2,5-dicarboxylate) leads to diverse behaviors, dictated by light energy, presence of oxygen and the ligand nature. Irradiation within the MC bands is unaffected by O₂ and results in ligand substitution. The LMCT excitation is effective only when oxalate is coordinated to Cr(III); then electron transfer from oxalate to central ion generates an intermediate, consisted of a Cr(II)species and the C₂O₄^? radicals. The species undergo fast redox reactions dependent on the presence of O₂ and the pda ligand.(1) In anoxic medium the fast outersphere electron transfer from Cr(II) to solvent, generates hydrated electrons and re-oxidizes the chromium centre to Cr^III. Then geminate recombination regenerates substrate, whereas competitive release of the C₂O₄^? radical leads to substitution of one oxalate ligand by two water molecules (aquation induced by the LMCT excitation). In the presence of the pda ligand the outersphere electron transfer is accompanied by the innersphere CT, generating Cr(III) coordinated to two radical ligands: C₂O₄^? and pda^3?; the intermediate releases also e_aq^?, but this reaction is slower than that of the homoleptic oxalate complex. Hydrated electrons are scavenged also by the released radicals. All these processes are completed within microseconds and in consequence, the Cr(III) complexes irradiated in deoxygenated solutions are insensitive to subsequent oxygenation.(2) When UV-irradiation is carried out in oxygenated medium reaction of Cr(II) species with molecular oxygen competes with the outer- or inner electron transfer observed in anoxic medium. Both these pathways result in generation of chromate(VI). Quantum yield of the Cr(VI) production is sensitive to the presence and structure of pda ligand, decreasing within the series 2,3-pda > 2,4-pda > 2,5-pda.     

Kinetic behavior of the reaction between hydroxyl radical and the SV40 minichromosome

Aqueous solutions containing the minichromosomal form of the virus SV40 and the radical scavenger DMSO were subjected to γ-irradiation, and the resulting formation of single-strand breaks (SSB) was quantified. Under the irradiation conditions, most SSBs were produced as a consequence of hydroxyl radical (OH) reactions. By controlling the competition between DMSO and the viral DNA substrate for OH, we are able to estimate the rate coefficient for the reaction of OH with the SV40 minichromosome. The results cannot be described adequately by homogeneous competition kinetics, but it is possible to describe the rate coefficient for the reaction as a function of the scavenging capacity of the solution. The experimentally determined rate coefficient lies in the range 1×10⁹–2×10⁹ L mol⁻¹ s⁻¹ at 10⁷ s⁻¹, and increases with increasing scavenging capacity.     

Molecular structure of caffeine as determined by gas electron diffraction aided by theoretical calculations

The molecular structure of caffeine (3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione) was determined by means of gas electron diffraction. The nozzle temperature was 185 °C. The results of MP2 and B3LYP calculations with the 6-31G⁷ basis set were used as supporting information. These calculations predicted that caffeine has only one conformer and some of the methyl groups perform low frequency internal rotation. The electron diffraction data were analyzed on this basis. The determined structural parameters (r_g and ∠_α) of caffeine are as follows: <r(NC)_ring> = 1.382(3) Å; r(CC) = 1.382(←) Å; r(CC) = 1.446(18) Å; r(CN) = 1.297(11) Å; <r(NC_methyl)> = 1.459(13) Å; <r(CO)> = 1.206(5) Å; <r(CH)> = 1.085(11) Å; ∠N₁C₂N₃ = 116.5(11)°; ∠N₃C₄C₅ = 121. 5(13)°; ∠C₄C₅C₆ = 122.9(10)°; ∠C₄C₅N₇ = 104.7(14)°; ∠N₉–C₄=C₅ = 111.6(10)°; <∠NCH_methyl> = 108.5(28)°. Angle brackets denote average values; parenthesized values are the estimated limits of error (3σ) referring to the last significant digit; left arrow in parentheses means that this parameter is bound to the preceding one.