HO2 ELIMINATION FROM α-HYDROXYALKYLPEROXYL RADICALS IN AQUEOUS SOLUTION

Abstract— In aqueous solutions α-hydroxyalkylperoxyl radicals undergo a spontaneous and a base catalysed HO₂ elimination. From kinetic deuterium isotope effects, temperature dependence, and the influence of solvent polarity it was concluded that the spontaneous reaction occurs via an HO₂ elimination followed by the dissociation of the latter into H⁺ and O₂^-. The rate constant of the spontaneous HO₂ elimination increases with increasing methyl substitution in α-position ( k (CH₂(OH)O₂) < 10s^-1 k (CH₃CH(OH)O₂) = 52s^-1 k ((CH₃)₂C(OH)O₂) = 665 s^-1). The OH^- catalysed reaction is somewhat below diffusion controlled. The mixture of peroxyl radicals derived from polyhydric alcohols eliminate HO₂ at two different rates. Possible reasons for this behaviour are discussed. The mixture of the six peroxyl radicals derived from d -glucose are observed to eliminate HO₂ with at least three different rates. The fastest rate is attributed to the HO₂ elimination from the peroxyl radical at C-l ( k > 7000s^-1). Because of the HO₂ eliminations the peroxyl radicals derived from d -glucose do not undergo a chain reaction in contrast to peroxyl radicals not containing an α-OH group. In competition with the first order elimination reactions the α-hydroxylalkylperoxyl radicals undergo a bimolecular decay. These reactions are briefly discussed.     

REEVALUATION OF THE SPECTRAL AND KINETIC PROPERTIES OF HO2 AND O2- FREE RADICALS

Abstract— The spectra and molar absorbances of the HO₂ and O₂^- free radicals have been redetermined in aqueous formate solutions by pulse and stopped-flow radiolysis as well as by ⁶⁰Co gamma-ray studies. The extinction coefficients at the corresponding maxima and 23°C are ²²⁵= 1400 ± 80 M ^-1 cm^-1 and ²²⁵= 2350 ± 120 M ^-1 cm^-1 respectively. Reevaluation of earlier published rate data in terms of the new extinction coefficients yielded the following rate constants for the spontaneous decay of HO₂ and O₂^-: K _Ho2+HO2= (8.60 ± 0.62) × 10⁵ M ^-1 s^-1; K _Ho2+O2-= (1.02 ± 0.49) × 10⁸ M ^-1 s^-1; K _Ho2+O2- < 0.35 M ^-1 s^-1. For the equilibrium HO₂→ O₂^-+ H⁺ the dissociation constant is K _Ho2= (2.05 ± 0.39) × 10^-5 M or p K _HO2= 4.69 ± 0.08. G (O₂^-) has been evaluated as a function of formate concentration.     

A MORE EXACT THEORETICAL INTERPRETATION OF RECENT EXPERIMENTAL DATA OBTAINED USING THE SEPARATED SENSITIZER AND SUBSTRATE METHOD TO INVESTIGATE O2(lδg) INTERACTIONS. POSSIBLE IMPORTANCE OF O2(1σg+)

Abstract— Recent experimental data obtained using the separated sensitizer and substrate method to investigate the interaction of O₂(¹δ_g) with various substances has been re-interpreted by means of a more complete theory. Comparison of experimental and recalculated values of the dependence of relative reaction rates on the sensitizer-substrate separation indicate general accord for experiments in which singlet oxygen acceptors in aqueous solution were used. The presumption is therefore that singlet molecular oxygen O₂(¹δ_g) is indeed the active oxidizing agent and that the theory presented and experiment are entirely in agreement.
For experiments in which bacterial targets were used a very distinct disagreement between theory and experiment is evident, the conclusion being that the kill rate does not depend linearly on the O₂(¹δ_g) concentration in the immediate proximity of the bacteria. However, the data is consistent with a quadratic dependence on the ¹δ_g concentration. A possible conclusion therefore is that the cytotoxic species is actually O₂(¹σ⁺_g), formed by an energy pooling reaction involving two O₂(¹δ_g) molecules.     

BIOLUMINESCENCE FROM THE REACTION OF FMN, H2O2 AND LONG CHAIN ALDEHYDE WITH BACTERIAL LUCIFERASE

Abstract— The bioluminescent oxidation of reduced flavin mononucleotide by bacterial luciferase involves a long-lived flavoenzyme intermediate whose chromophore has been postulated to be the 4a-sub-stituted peroxy anion of reduced flavin. Reaction of long chain aldehyde with this intermediate results in light emission and formation of the corresponding acid. These experiments show that the typical aldehyde-dependent, luciferase-catalyzed bioluminescence can also be obtained starting with FMN and H₂O₂ instead of FMNH₂ and O₂. We postulate that the 4a-peroxy anion intermediate is formed directly by attack of H₂O₂ on FMN. The latter may be bound to luciferase. An enzyme bound intermediate is formed which by kinetic analysis, flavin specificity for luminescence, aldehyde dependence, and bioluminescent emission spectrum appears to be identical with the species generated by reaction of FMNH, and O₂ with luciferase. The quantum yield of the H₂O₂-_- and FMN-initiated biolumlnescence is low but can be enhanced by certain metal ions, which also stimulate a chemiluminescent reaction of oxidized flavin with H₂O₂. The peak of this chemiluminescence. however, appears to be at a shorter wavelength than that (490 nm) of the bioluminescence.     

THERMODYNAMIC FUNCTIONS FOR SINGLET MOLECULAR OXYGEN, O2(1△g) and O2 (1σ8+)

Abstract— A correction is offered to the approximate values previously given by Mendenhall (1978) for the enthalpy of formation and entropy of O₂(a₁Δ_g) and O₂(b₁₊) between 298 and 1500 K. Accurate values have been calculated for the functions together with the equilibrium constants for the formation of these species from O₂(X₃σ_g-).     

COOPERATION OF CHARGES IN PHOTOSYNTHETIC O2 EVOLUTION–I. A LINEAR FOUR STEP MECHANISM

Abstract— Using isolated chloroplasts and techniques as described by Joliot and Joliot[6] we studied the evolution of O₂ in weak light and light flashes to analyze the interactions between light induced O₂ precursors and their decay in darkness. The following observations and conclusions are reported: 1. Light flashes always produce the same number of oxidizing equivalents either as precursor or as O₂. 2. The number of unstable precursor equivalents present during steady state photosynthesis is ∼ 1.2 per photochemical trapping center. 3. The cooperation of the four photochemically formed oxidizing equivalents occurs essentially in the individual reaction centers and the final O₂ evolution step is a one quantum process. 4. The data are compatible with a linear four step mechanism in which a trapping center, or an associated catalyst, ( S ) successively accumulates four + charges. The S ⁴⁺ state produces O₂ and returns to the ground state S ₀. 5. Besides S ₀ also the first oxidized state S ⁺ is stable in the dark, the two higher states, S²⁺ and S³⁺ are not. 6. The relaxation times of some of the photooxidation steps were estimated. The fastest reaction, presumably S *₁← S ₂, has a (first) half time ≤ 200 μsec. The S *₂ state and probably also the S *₀ state are processed somewhat more slowly (˜ 300–400 μsec).     

PHOTOLYSIS OF SUBSTITUTED NAPHTHALENES ON SiO2 AND Al2O3

Abstract— Photolysis of naphthalene on the surface of SiO₂ under an atmosphere of air produces phthalic acid as the only major photoproduct, accounting for 49%o of the consumed naphthalene. Photolysis on Al₂O₃ also produces phthalic acid, in 31% yield. Photolysis of 1 -methylnaphthalene on SiO₂ proceeds under similar conditions to produce 2-acetylbenzoic acid (35%) as the major photoproduct with the production of a small amount of I-naphthaldchyde (6%). 1-Cyanonaphthalene does not photooxidize under similar conditions. The presence of oxygen is necessary for the photodecomposition of naphthalene and 1-methylnaphthalene to proceed. Superoxide formed from the photolysis of naphthalene at the SiO₂/air interface is readily observed by electron paramagnetic resonance spectroscopy. In the absence of naphthalene no superoxide is observed. A mechanism involving electron transfer from the S₁ state of the naphthalene to O₂ is proposed on the basis of these observations and related literature precedent.     

ELIMINATION OF THE EFFECT OF CONTAMINATING CO2 IN THE 18O-LABELING OF THE CO2 PRODUCED IN BIOLUMINESCENT REACTIONS

Abstract— Although the mechanism of bioluminescent reactions in various species, such as fireflies, ostracod crustaceans ( Cypridina ), sea pansies ( Renilla ), and the deep-sea shrimp Oplophorus , are thought to involve dioxetanone intermediates, studies reported in the past from different laboratories have included widely different experimental results, most likely due to various factors including the effects of contaminating CO₂. With the improved technique employed in the present study, bioluminescent reactions of the firefly and Cypridina in ¹⁸O₂ gas resulted in an incorporation of over 75% of ¹⁸O into one oxygen of the product CO₂. with a reproducibility within a few per cent. When ¹³CO₂. instead of the product CO₂ of the bioluminescent reaction, was studied in an H₂¹⁸O medium, the exchange of one oxygen of ¹³CO₂ with H₂O was 64%. and the effect of contaminant CO₂ amounted to 1418% of the total CO₂. These results suggest that every molecule of CO₂ formed in the bioluminescent reactions of the firefly and Cypridina had intially contained 1 oxygen atom derived from O₂.     

CALCULATED RATES AND EQUILIBRIA OF THERMAL INTERCONVERSION OF TRIPLET (3g-) AND SINGLET (1Δg and g+) MOLECULAR OXYGEN

Abstract— From spectroscopic data and rate constants in the literature, equilibrium constants and rates of thermal formation of singlet oxygen (¹Δ_g and ¹Σ_g⁺) were calculated for a number of conditions. For the gas phase we estimate K _eq(¹Δ_g³Σ_g^-) = 1.67 exp(-94.31 KJ/RT) and K _eq(¹Σ_g⁺/³Σ_g^-) = 0.33 exp(-157.0 KJ/RT). The calculated rate constants for the ³Σ_g⁺→¹Δ_g transition of O₂ at 25°C varied from 2.5 × 10^-11 s^-1 in water to 4.8 × 10^-16 s^-1 in air, assuming equal solvent interactions with the ground and excited states. Physical quenchers for singlet oxygen are expected to be catalysts for its thermal formation. Equations are presented which allow one to estimate whether such catalysis by quenchers will result in a pro-oxidant effect.     

PHOTOOXYGENATION OF 3-HYDROXYFLAVONE IN A 12 K O2 MATRIX

Abstract— We present an infrared study of 3-hydroxyflavone in 12 K Ar and O₂ matrices. Although it is extremely photostable in an Ar matrix, a remarkable oxygenation reaction for 3-hydroxyflavone takes place upon photoexcitation in an O₂ matrix. The primary photoproduct is concluded to be a keto-hydroperoxide. On further photolysis the keto-hydroperoxide affords the photostable secondary product 2-(benzoyloxyl)benzoic acid, as well as CO and CO₂ through two independent pathways.     

Energetics of Photoinduced Electron Transfer in the Indole-O2 Cluster in Gas Phase: Possible Consequences for Photoexcited Tryptophan in Solution

Abstract— A direct process for an activationless electron transfer from photoexcited tryptophan to molecular oxygen is proposed. By photodetachment of mass-selected indole-O_2- clusters in gas phase a neutral indole₊ O_2- charge-separated exciplex state is found at 2.25 0.2 eV above the neutral ground state. By theory also, the existence of an excited charge separated state at 3.05 0.2 eV is postulated. In gas phase both charge-separated cluster states are energetically below the first singlet states ₁L_b and ₁L_a and the lower even below the first triplet state T₁ of indole. In gas-phase clusters these energetics imply a very efficient quenching of photoexcited indole by fast electron transfer to oxygen. We discuss a similar mechanism for tryptophan-O₂ in aqueous environment and find it without activation barrier and presumably extremely fast. In the collisional tryptophan_*-O₂ complex the efficiency and the time scale of the charge transfer process should be mostly solvent independent. In polar solvent a complete charge separation and free superoxide formation are expected. We correlate this model with previous fluorescence and phosphorescence quenching data of excited tryptophan by O₂ and propose electron transfer to be the relevant process.     

ON THE GENERATION OF THE HYDROXYLATION AGENT FROM SUPEROXIDE RADICAL. CAN THE HABER–WEISS REACTION BE THE SOURCE OF OH RADICALS?

Abstract— In many biological systems, the role of O₂^- in hydroxylation and toxic processes was assumed to be due to the formation of OH radicals. The Haber-Weiss reaction (Haber and Weiss, 1934)—(H₂O₂+ O₂^-→ OH + OH^-+ O₂) was suggested as the origin of this activity.
In this study it is shown that this reaction pathway is too slow, and that OH is probably formed from the reaction of complexed superoxide with H₂O₂ or/and from the reduction of Fe(III), bound to biological compounds, by O₂^-; the reduced Fe(II) can then react with H₂O₂ as a Fenton reagent, to yield OH.
It is also shown that singlet oxygen cannot be formed in these biological systems neither from the dismutation of OJ nor from the reaction of O₂^- with OH. Singlet oxygen may be formed from the reduction of metal complexes by O₂^-.     

PREPARATION AND PHOTOPHYSICAL STUDIES OF PORPHYRIN-C60 DYADS

Abstract Porphyrin-C₆₀ dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels-Alder reaction. The porphyin singlet lifetimes of both the zinc (Pz_n-C₆₀) and free base (P-C₆₀) dyads, determined by time-resolved fluorescence measurements, are ≦17 ps in toluene. This substantial quenching is due to singlet-singlet energy transfer to C₆₀ The lifetime of Pzn-¹C₆₀ is -5 ps in toluene, whereas the singlet lifetime of an appropriate C₆₀ model compound is 1.2 ns. This quenching is attributed to electron transfer to yield P_zn^bull;+-C₆₀^bull;-. In toluene, P-¹C₆₀ is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 ran and a lifetime of-10 μs was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C₆₀ triplet. In the more polar benzonitrile, P-¹C₆₀ underoes photoinduced electron transfer to give P^•+-C₆₀^bull;-. The electron transfer rate constant is −2 × 10¹¹ s⁻¹.     

THE ROLE OF O2- IN THE CHEMILUMINESCENCE OF LUMINOL*

Abstract— The chemiluminescence of luminol in buffered aqueous solutions is inhibited by superoxide dismutase. This occurs whether the luminescence is induced by ferricyanide, persulfate, hypochlorite, or by the action of xanthine oxidase on xanthine. Since superoxide dismutase inhibits reactions which involve O₂^-, we conclude that this radical is a constant factor in the chemiluminescence of luminol in aqueous solutions. The kinetics of light production are discussed in terms of hypothetical mechanisms that fit the available data. The strong luminescence of luminol in aprotic solvents or in aqueous systems containing relatively high concentrations of H₂O₂ could not be explored in this way, because superoxide dismutase is inactive under such conditions.     

PHOTOCHEMISTRY OF trans-[Rh(BIS(DIPHENYLPHOSPHINO)ETHANE)2X2][PF6]: TRANSIENT ABSORBANCE KINETIC STUDIES OF METAL-HALOGEN BOND HOMOLYSIS

Abstract— Laser flash photolysis of trans -[Rh(dppe)₂X₂][PF₆] (X=Br and I; dppe=bis(diphenylphosphino)ethane) in CH₂Cl₂ or CH₃CN produces the d⁷ Rh(II) radicals, [Rh(dppe)₂X]⁺, and halogen atoms. The kinetics of the disappearance of [Rh(dppe)₂X]⁺ radicals in CH₂Cl₂ or CH₃CN were mixed order: H-atom abstraction from solvent to produce the rhodium hydrides, [RhH(dppe)₂X][PF₆], and Rh/X recombination. In the poor H-atom donor solvent, benzonitrile, Rh/Br recombination was observed to be uncomplicated by competing H-atom abstraction. The hydride complexes [RhH(dppe)²X][PF₆], formed by H-atom abstraction were completely characterized by ³¹P{¹H}-NMR, ¹H-NMR, and mass specrometry. Cyclohexene was used as an effective trap for photogenerated Br atoms and yielded bromocyclohexane and 3-bromocyclohexene in a relative yield, 1:9. The photochemical mechanism is discussed in light of the transient absorbance and trapping studies.     

THE ABSOLUTE VALUE OF THE REACTION RATE CONSTANT OF BILIRUBIN WITH SINGLET OXYGEN IN D2O

Abstract— The chemical reaction rate constant of bilirubin with singlet oxygen in basic aqueous solution has been redetermined to be 3.5 × 10⁸ M^-1 s^-1 by a competitive technique using a 1,3-diphenylisobenzofuran in sodium dodecyl sulfate micelles. Bilirubin also physically quenches a singlet oxygen with a rate constant of 9 × 10⁸ M ^-1 s^-1. The lifetime of singlet oxygen in D₂O solution has been determined to be 35 μ s . The absorption cross-section for the molecular oxygen ³∑_g-→¹δ _g + 1 v electronic transition at 1.06μn in aqueous solution is unexpectedly larger than the gas paase cross-section.     

PHOTOINDUCED LUMINESCENCE AND EPR SIGNALS OF POLYPHENOL AND QUINONE POLYMERS

Abstract— Aqueous basic solutions, pH 9.0 of humic acids and melanin-like, synthetic polymers, obtained with adrenochrome, hydroquinone and purpurogallin, were illuminated with visible light under N₂ or O₂ atmospheres. It has been found that light enhances a singlet electron-paramagnetic-resonance (EPR) signal of polymers both under N₂ and O₂, and induces ultra-weak luminescence in the presence of O₂. Degradative oxidation of polymers, accelerated by light, leads to a decrease of EPR signal intensity and generates weak chemiluminescence.     

Illumination with Ultraviolet or Visible Light Induces Chemical Changes in the Water-soluble Manganese Complex, [Mn4O6(bpea)4]Br4

We measured the photosensitivity of an artificial tetranuclear oxo–Mn(IV) complex, [Mn₄O₆(bpea)₄]Br₄, which has an adamantane-shaped {Mn₄O₆}⁴⁺ core. Illumination caused changes in the absorption spectrum of the compound consistent with a one-electron reduction in the compound. Bromide appears to be the most probable electron donor in the reaction system. Chemical modification of the cluster appears to destabilize it, predisposing it to reductive degradation. UV light was more efficient than visible light in causing the changes. The data support the suggestion that the natural oxygen-evolving Mn complex is photosensitive and can oxidize components of the oxygen-evolving complex in its excited state causing photoinhibition, and that photostability is an important issue in designing Mn complexes for artificial photosynthesis. Furthermore, light-induced oxidation of bromide by [Mn₄O₆(bpea)₄]⁴⁺ may suggest that oxidation of chloride is involved in natural water splitting or has been involved during the evolution of the water-splitting enzyme.     

RATE CONSTANTS FOR INTERACTION OF 1O21Δg) WITH AZIDE ION IN WATER

Abstract— The rate constant for quenching of ¹O₂ by azide ion in water was determined to be (5.0 ± 0.4) × 10⁸ M ⁻¹ s⁻¹ using a variety of sensitizers (including humic acids) and ¹O₂ acceptors. The apparent second-order rate constant decreases with pH below pH 5.5 in accordance with the protonation of azide ion to form hydrazoic acid (p K _a= 4.6). Quenching by hydrazoic acid is at least 2 orders of magnitude slower than by azide ion. Greater than 99% of all interactions between ¹O₂ and azide ion involve physical quenching rather than chemical reaction. Humic acid triplets are not significantly quenched by azide ion at concentrations less than 2 m M , allowing azide ion quenching to be used as a diagnostic test for the intermediacy of ¹O₂ in photosensitized oxidations in natural surface waters.     

PHOTOCHEMICAL HYDROXYLATION OF 7,8-DIMETHYLALLOXAZINE (LUMICHROME)-A FLASH PHOTOLYSIS STUDY

Abstract— Flash photolysis experiments on the hydroxylation of lumichrome (L) in aqueous 0.5 M H₂SO₄ solution in the presence of O₂ or Ni²⁺ as triplet quenchers and quantum yield measurements confirm the assignment of the photoreactive species to the protonated form of the excited singlet state. A mechamism concerning the photochemical step is proposed, accounting for the formation of protonated 9-hydroxy-5,10-dihydrolumichrome (LOH₃⁺). This primary stable photoproduct was characterized by spectral and kinetic data. The dark reactions originating from LOH₃⁺ were investigated, and data regarding the successive steps are presented. The reaction LOH₃⁺ L→ LO + LH₃⁺ is demonstrated to be a two-electron reduction. The rate constant for the reaction of LH₂⁺ with O₂ is much larger than that for the oxidation of LH₃⁺ by oxygen.