Oxygen uptake and involvement of superoxide radicals upon photolysis of ketones in air-saturated aqueous alcohol, formate, amine or ascorbic acid solutions

The photolysis of acetophenone, benzophenone, 4-carboxybenzophenone and benzil was studied in air-saturated aqueous solution in the presence of alcohols. The overall reaction is an oxidation of 2-propanol to acetone. The quantum yield of oxygen uptake (Phi(-O(2))) increases with increasing 2-propanol concentration up to 0.9. The photoreaction can also be initiated by quenching of the ketone triplet state by ascorbic acid, formate or an amine e.g. triethylamine. Subsequent reactions of the involved radicals with oxygen yield the superoxide radical and eventually hydrogen peroxide. For the ketones in the presence of 3-30 mM ascorbic acid or triethylamine Phi(-O(2)) = 0.3-0.9. The specific properties of ketones, including 4-methoxyacetophenone and 2-acetonaphthone, the radicals involved and the pH and concentration dependences of Phi(-O(2)) are discussed.     

PHOTOCHEMICAL REACTION BETWEEN PROTEIN AND NUCLEIC ACIDS: PHOTOADDITION OF TRYPTOPHAN TO PYRIMIDINE BASES*1

Abstract— The photochemical interactions between tryptophan and nucleic acid bases were studied. When aqueous solutions of tryptophan were irradiated (Λ > 260 nm) at neutral pH in the presence of each of the nucleic acid bases, pyrimidine bases but not purine bases were altered. Air was found to decrease the rate of reaction. Two classes of photoproducts were isolated by thin layer and ion-exchange chromatography and partially characterized. One was the dihydro-pyrimidine forms of the base (see Reeve and Hopkins, 1979) and the other consisted of tryptophan-pyrimidine photoadducts. Four tryptophan-uracil and two tryptophan-thymine adducts each with a 1:1 molecular stochiometry were found. Spectroscopic measurements and a positive reaction with Ehrlich's reagent indicate that the indole nucleus remained intact, but that the pyrimidine base was reduced at the 5, 6 double bond. The absence of a positive ninhydrin reaction and the effect of pH on the quantum yield of the photoadduct formation indicated that the ionized a-amino acid group of tryptophan was involved in photoadduct formation. Indole derivatives lacking an a-amino group were also found to form photoadducts with pyrimidine bases. The experimental results are consistent with a reaction mechanism involving tryptophan excitation to the first excited singlet state as the initial event. Radical scavenging experiments indicate that tryptophan free radicals, formed by electron dissociation from the excited state, react with the ground state pyrimidine.     

Photosensitized oxidation of tryptophan and tyrosine by aromatic ketones: A laser flash photolysis study

Photochemical processes of benzophenone (BP) and xanthone (XT) with tryptophan (TrpH) and tyrosine (TyrOH) were studied using the laser flash photolysis technique.It has been observed that the triplet state of BP and XT reacted with TrpH and TyrOH by hydrogen transfer with the formation of BP and XT ketyl radicals and oxidized radicals of TrpY and TyrOY.The related rate constants of these reactions were determined in this paper.The free energy changes (G) of these reactions suggested that the proposed hydrogen transfer mechanism was thermodynamically feasible.These results provide theoretical foundation for further studying structural effects on the photochemical behaviors of proteins with triplet state BP and XT.     

A comparative photomechanistic study (spin trapping, EPR spectroscopy, transient kinetics, photoproducts) of nucleoside oxidation (dG and 8-oxodG) by triplet-excited acetophenones and by the radicals generated from alpha-oxy-substituted derivatives through Norrish-type I cleavage

The photooxidation of 2'-deoxyguanosine (dG) and its derivative 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) by a series of acetophenones (AP-X) and benzophenone (BP) has been studied.The favorable absorption characteristics of the benzoyl chromophore enables time-resolved spectroscopy of the triplet ketones to assess their quenching kinetics by dG and 8-oxodG. Whereas the photolysis of acetophenone (AP), 2-acetoxyacetophenone (AP-OAc), and benzophenone (BP) does not produce radicals (group A ketones), the oxymethyl-substituted derivatives 2-hydroxyacetophenone (AP-OH) and 2-tert-butoxyacetophenone (AP-O(t)Bu) lead to carbon-centered radicals by alpha cleavage (group B ketones). For the latter ketones, this was confirmed by EPR studies with the spin trap 5,5-dimethylpyrroline N-oxide (DMPO) and by their triplet lifetimes that were shorter than those for the unsubstituted acetophenone. Both groups of ketones photooxidize dG and 8-oxodG; the oxidation products are spiroiminodihydantoin and guanidine-releasing products (GRP) in the case of dG and AP-OH also 8-oxodG. In the presence of O(2), the photooxidation by the group A ketones is efficient at high dG or 8-oxodG concentrations, whereas the group B ketones photooxidize dG and 8-oxodG also at low substrate concentrations. These results imply that peroxyl radicals are responsible for the photooxidation by the group B ketones, which are formed by alpha cleavage of the triplet ketone and subsequent O(2) trapping of the carbon-centered radicals. At higher dG concentrations, direct electron transfer from dG to the triplet ketone, as observed for the group A ketones, competes with the radical activity.     

Photochemical reactions of aromatic ketones with nucleic acids and their components. 3. Chain breakage and thymine dimerization in benzophenone photosensitized DNA

Abstract— Excitation of benzophenone in the presence of calf thymus and E. coli DNA leads to photosensitized damages to the macromolecule. Two main reactions are observed: thymine dimerization and chain break formation. Benzophenone photosensitized chain breaks are also observed in polyadenylic acid. The melting temperature of DNA decreases with the duration of irradiation. Under our experimental conditions, the ratio of the yields of dimers and single-chain breaks produced in DNA is about 1. Photosensitized damage to deoxyribose residues leading to chain breakage is shown to be similar to that produced by X or γ ray irradiation. The oxygen effect upon chain break production is studied and discussed in relation with its effect upon intermediate species. Thymine dimers are formed following energy transfer from benzophenone in its triplet state. In previous flash-photolysis studies we showed that benzophenone in its triplet state reacts with water molecules to give ketyl and OH radicals. Ketyl radicals are not involved in reactions with DNA. It is proposed that OH radicals produced in the above reaction are responsible for the production of single-chain breaks by attack on the deoxyribose residues.     

PHOTOINTERACTION OF BENZOPHENONE TRIPLET WITH LYSOZYME

The quenching of the benzophenone triplet by lysozyme and its constituent amino acids in aqueous solutions have been studied. Native lysozyme quenches the benzophenone triplet with a high rate constant, 4 x 10(9) M-1 s-1. The quenching process takes place with production of significant amounts of free ketyl radicals, phi ketyl = 0.56, but with a very low benzophenone consumption yield (0.022). The consumption yield is considerably smaller than that observed for the free amino acids. This difference can be explained in terms of a dominant back hydrogen transfer to the protein in the disproportionation of the free radicals produced. Reduced and carboxymethylated lysozyme shows a higher quenching rate (7.8 x 10(9) M-1 s-1) and a larger benzophenone consumption yield (0.07). The deactivation of the benzophenone triplet by the native protein leads to its inactivation, with a quantum yield of 0.01. Tryptophan and arginine residues are destroyed with a quantum yield of 0.01. In the modified enzyme tyrosine and methionine groups are also consumed.     

PHOTOCHEMICAL REACTIONS OF PYRIMIDINE DERIVATIVES WITH ACRYLONITRILE IN AQUEOUS SOLUTIONS

Abstract— Ultraviolet excitation of pyrimidine derivatives in aqueous solutions containing acrylonitrile can lead to polymerisation of acrylonitrile and/or photoaddition reactions of acrylonitrile and the pyrimidine. Polymerisation of acrylonitrile induced by excitation of orotic acid occurs even in aerated solutions and is very likely due to radicals derived from excited orotic acid molecuies. Using ¹⁴C-labelied orotic acid, the polymer is shown to contain the pyrimidine. With other pyrimidine derivatives, polymerisation is observed only in deaerated solutions and its low yield did not allow us to determine whether the polymerisation is induced by radicals or by photosensitisation. The role of the lowest excited singlet and triplet states of the pyrimidine derivatives in the photoaddition reaction is discussed. In the case of orotic acid, photoaddition is shown to proceed entirely by way of the triplet state.     

Kinetics and mechanism of sensitized photooxidation of tetramethylammonium salt of 2-(phenylthio)acetic acid in solution: Steady-state and flash photolysis studies

The mechanism for the photo-induced oxidation of the tetramethylammonium salt of 2-(phenylthio)acetic acid was elucidated. The photosensitizer was the benzophenone triplet in acetonitrile solutions. Time-resolved absorption spectra and kinetics were used to follow the intermediates which included the triplet of benzophenone, the ketyl radical of benzophenone, and an ion pair consisting of a radical anion of benzophenone and a tetramethylammonium cation. Rate constants for the growth and decay of the transients were determined along with the quantum yields of the transients. The intermediacy of other radicals was inferred by the products observed following steady-state photolysis. Quantum yields were also determined for photoproducts resulting from the steady-state irradiation. The mechanism was proposed that rationalized the quantitative observations. Of particular note was how the nature of the counter ion effected the secondary reactions of the radicals and the radical ions.     

Reaction mechanisms and structural characterization of the reactive intermediates observed after the photolysis of 3-(hydroxymethyl)benzophenone in acetonitrile, 2-propanol, and neutral and acidic aqueous solutions

Nanosecond time-resolved resonance Raman (ns-TR(3)) spectroscopy was employed to investigate the photoinduced reactions of 3-(hydroxymethyl)benzophenone (1) in acetonitrile, 2-propanol, and neutral and acidic aqueous solutions. Density functional theory calculations were utilized to help the interpretation of the experimental spectra. In acetonitrile, the neutral triplet state 1 [denoted here as (m-BPOH)(3)] was observed on the nanosecond to microsecond time scale. In 2-propanol this triplet state appeared to abstract a hydrogen atom from the solvent molecules to produce the aryphenyl ketyl radical of 1 (denoted here as ArPK of 1), and then this species underwent a cross-coupling reaction with the dimethylketyl radical (also formed from the hydrogen abstraction reaction) to form a long-lived light absorbing transient species that was tentatively identified to be mainly 2-(4-(hydroxy(3-(hydroxymethyl)phenyl)methylene)cyclohexa-2,5-dienyl)propan-2-ol. In 1:1 H(2)O:CH(3)CN aqueous solution at neutral pH, (m-BPOH)(3) reacted with water to produce the ArPK of 1 and then underwent further reaction to produce a long-lived light absorbing transient species. Three photochemical reactions appeared to take place after 266 nm photolysis of 1 in acidic aqueous solutions, a photoreduction reaction, an overall photohydration reaction, and a novel photoredox reaction. TR(3) experiments in 1:1 H(2)O:CH(3)CN aqueous solution at pH 2 detected a new triplet biradical species, which is associated with an unusual photoredox reaction. This reaction is observed to be the predominant reaction at pH 2 and seems to face competition from the overall photohydration reaction at pH 0.     

Magnetic field control of photoinduced silver nanoparticle formation

The micellar photoreduction of benzophenone in the presence of Ag+ leads to very rapid and efficient formation of silver nanoperticles. External magnetic fields can be used to control the rate of formation and properties of silver nanoparticles generated by reaction of ketyl radicals formed in the photoreduction of benzophenone in surfactant micelles. The effect is attributed to Zeeman splitting of the triplet sublevels of the confined radical pair that causes a reduction in the rate of geminate processes and increases the availability of ketyl radicals (through escape processes) for metal ion reduction.     

Sulfonated poly(ether ether ketone)/poly(vinyl alcohol) sensitizing system for solution photogeneration of small Ag, Au, and Cu crystallites

Illumination of air-free aqueous solutions containing sulfonated poly(ether ether ketone) and poly(vinyl alcohol) with 350 nm light results in benzophenone ketyl radicals of the polyketone. The polymer radicals form with a quantum yield 0.02 and decay with a second-order rate constant 6 orders of magnitude lower than that of typical alpha-hydroxy radicals. Evidence is presented that the polymeric benzophenone ketyl radicals reduce Ag+, Cu2+, and AuCl4- to metal particles of nanometer dimensions. Decreases in the reduction rates with increasing Ag(I), Cu(II), and Au(III) concentrations are explained using a kinetic model in which the metal ions quench the excited state of the polymeric benzophenone groups, which forms the macromolecular radicals. Quenching is fastest for Ag+, whereas Cu2+ and AuCl4- exhibit similar rate constants. Particle formation becomes more complex as the number of equivalents needed to reduce the metal ions increases; the Au(III) system is an extreme case where the radical reactions operate in parallel with secondary light-initiated and thermal reduction channels. For each metal ion, the polymer-initiated photoreactions produce crystallites possessing distinct properties, such as a very strong plasmon in the Ag case or the narrow size distribution exhibited by Au particles.     

RADICAL INTERMEDIATES IN THE FLUORESCEIN-AND EOSIN-PHOTOSENSITIZED AUTOXIDATION OF l-TYROSINE*

Abstract The Primary reactions of the cosin-and fluorescein-photosensitized autoxidation of L-tyrosine were studied in aqueous media (pH = 8.6) by the flash-photolysis technique. The dye molecules were quantitatively converted to their triplet states in a single flash. The triplet dye molecules were found to react with tyrosine or oxygen. Ground state or radical dye molecules were formed in these reactions. Some 40 per cent of the triplet-tyrosine reactions yielded radicals, in triplet dye-oxygen reactions the corresponding yield was less than 10 per cent. The ground state dye was regenerated from the semireduced dye in reactions with oxygen and from the semioxidized dye in reactions with tyrosine. In the absence of oxygen the radicals formed in the photoinduced electron-transfer between the triplet dye and tyrosine recombined to a large extent.     

QUENCHING OF TRIPLET STATES OF AROMATIC KETONES BY INDOLE AND 3-METHYL INDOLE IN BENZENE SOLUTION. EVIDENCE FOR HYDROGEN-ATOM ABSTRACTION, FOR QUENCHING DUE TO FAVOURABLE CHARGE TRANSFER INTERACTION AND FOR THE LACK OF ELECTRONIC ENERGY TRANSFER

Abstract— Quenching of the triplet states of the aromatic ketones (KCO), benzophenone, acetophenone and xanthone, by indole and 3-methyl indole gives rise to the neutral radicals resulting from hydrogenatom transfer with variable efficiency (40–100%). Thus in addition to the reaction,
³KCO*+ R H →KCOH + R .
some other quenching path or paths occur. There is no evidence for any triplet energy transfer even when this is energetically favourable, and it is suggested therefore that quenching to give ground state species following favourable charge-transfer interactions accounts for the proportion of quenching without reaction.
The spectra of the indole radicals, R ., were determined and the kinetics of their decay in aerated and deaerated solution were investigated and reaction schemes proposed to explain these observations.     

PHOTOIONISATION AND BIPHOTONIC PROCESSES OF NUCLEIC ACIDS DERIVATIVES IN FROZEN SOLUTIONS

Abstract— In frozen aqueous solutions at 777deg;K, U.V. irradiation (Λ>250 mμ) photoionises purine, adenosine and guanosine molecules, respectively, In alcoholic solutions, at 777deg;K, photosensitization of alcohol occurs. A biphotonic absorption process involving the lowest triplet state is suggested. The formation of radicals by these processes leads to a decrease of the phosphorescent triplet-state population of the molecules tested. Some consequences for the photochemistry of nucleic acids are discussed.     

Radiolytically generated benzene triplets as sensitizers for energy and combined electron/proton transfer processes

Pulse radiolysis technique has been employed to investigate energy and electron transfer reactions involving triplets of naphthols and hydroxybiphenyls. The transient absorption spectra obtained on pulse radiolysis of N₂-saturated solution of naphthols and hydroxybiphenyls in benzene are assigned to triplet–triplet absorption. It was found that biphenyl triplets undergo energy transfer to naphthols and hydroxybiphenyls forming the acceptor triplets. On the other hand, benzophenone triplets, favor electron transfer followed by H⁺ transfer reaction forming benzophenone ketyl radical and phenoxyl radical of the acceptor. An analogous sequence mimics with 2-naphthol triplets and using benzophenone, acetophenone or chloranil as electron acceptor.     

Photochemical reactions of ketones to synthesize gold nanorods

Photochemical synthesis of gold nanorods (NRs) in a micellar solution of hexadecyltrimethylammonium bromide (CTAB) was triggered by photoreactions of ketones. Photoexcitation of gold ions and silver bromide clusters ((AgBr)n) in the absence of ketones did not produce NRs. The initial products of the photoirradiation were probably ketyl radicals, which then initiated reactions to form NRs. NRs formed in darkness, if the reaction solutions were irradiated by UV light for a few minutes, thus indicating that the photochemical products catalyzed NR-formation.     

Triplet excited states and radical intermediates formed in electron pulse radiolysis of amino-substituted fluorenones

Electron pulse radiolysis of four differently substituted amino derivatives of fluorenone, namely, 1-amino-, 2-amino- 3-amino-, and 4-aminofluorenone, has been carried out to study the effect of structure on the spectroscopic and kinetic characteristics of the triplet excited states as well as the transient free radical intermediates formed under reducing and oxidizing conditions. The triplet states of these compounds have been generated in benzene by pulse radiolysis and in other solvents by flash photolysis technique and their spectral and kinetic properties have been investigated. Hydrated electron (e_aq⁻) has been found to react with these fluorenone derivatives to form the anion radical species with a diffusion-controlled rate constant. The spectral and kinetic properties of the transient ketyl and anion radicals have been studied by generating them in aqueous solutions of suitable pH. The pK_a values of ketylanion radical equilibria are in the range of 6.8–7.7 for these derivatives. The oxidized species have been generated by reaction with the azide radical. Hydrogen atom adducts as well as the cation radicals of these derivatives have also been generated by pulse radiolysis and characterized.     

INTERACTIONS BETWEEN KETONES AND AMINES IN PHOTOINITIATING SYSTEMS USED FOR RADICAL POLYMERIZATION

Time resolved laser spectroscopy and GC/MS were used to investigate the primary processes and the side reactions occurring in benzophenone and chlorothioxanthone/amine systems used as photoinitiators of radical polymerization. The discrepancy observed between the experimental data and the expected behaviour is accounted for by detrimental chemical reaction involving the ketone triplet state, the ketyl radical and the amine-derived radical.     

Quinone sensitized electron transfer photooxidation of nucleic acids: chemistry of thymine and thymidine radical cations in aqueous solution

The 2-methyl-1,4-naphthoquinone (MQ) sensitized photooxidation of nucleic acid derivatives has been studied by laser flash photolysis and steady state methods. Thymine and thymidine, as well as other DNA model compounds, quench triplet MQ by electron transfer to give MQ radical anions and pyrimidine or purine radical cations. Although the pyrimidine radical cations cannot be directly observed by flash photolysis, the addition of N,N,N',N'-tetramethyl-1,4-phenylenediamine (TMPD) results in the formation of the TMPD radical cation via scavenging of the pyrimidine radical cation. The photooxidation products for thymine and thymidine are shown to result from subsequent chemical reactions of the radical cations in oxygenated aqueous solution. The quantum yield for substrate loss at limiting substrate concentrations is 0.38 for thymine and 0.66 for thymidine. The chemistry of the radical cations involves hydration by water leading to C(6)-OH adduct radicals of the pyrimidine and deprotonation from the N(1) position in thymine and the C(5) methyl group for thymidine. Superoxide ions produced via quenching of the quinone radical anion with oxygen appear to be involved in the formation of thymine and thymidine hydroperoxides and in the reaction with N(1)-thyminyl radicals to regenerate thymine. The effects of pH were examined in the range pH 5-8 in both the presence and absence of superoxide dismutase. Initial C(6)-OH thymine adducts are suggested to dehydrate to give N(1)-thyminyl radicals.     

Photochemistry and photopolymerization activity of novel perester derivatives of fluorenone: a conventional and laser flash photolysis study

The photochemistry of three novel t-butylperester derivatives of fluorenone was examined and compared with unsubstituted fluorenone and a mono-t-butylperester of benzophenone using both conventional microsecond and nanosecond laser flash photolysis. On conventional microsecond flash photolysis in 2-propanol, all four fluorenone compounds gave transient absorption in the region 300–400 nm due to a ketyl radical formed from the abstraction of a hydrogen atom from the solvent by the upper excited triplet n—π* state of the fluorenone chromophore. This assignment was confirmed by a pH-dependent study on the transient absorption spectra. The nitro-t-butylperester derivative of fluorenone gave additional absorption above 400 nm due to species associated with the nitro group. No evidence for benzoyloxy radical formation could be found in non-hydrogen-atom-donating solvents with microsecond flash photolysis which is associated with homolysis of the perester groups. On nanosecond laser flash photolysis of the fluorenone compounds at 355 nm excitation in acetonitrile and hexa-fluorobenzene, transient absorptions were observed in the region 320–640 nm due to the corresponding triplet states. All the t-butylperester derivatives showed residual absorbances at longer time delays which were tentatively assigned to the corresponding benzoyloxy radicals produced by homolysis of the perester groups. In contrast, the mono-t-butylperester of benzophenone, included for comparison only, showed very weak transient absorption in the region 320–640 nm compared with that of the strong triplet of benzophenone under the same excitation conditions. The triplet absorptions and lifetimes of the fluorenone compounds were correlated with their photopolymerization activities in bulk methylmethacrylate monomer. In oxygenated solutions, the triplet absorptions of fluorenone and benzophenone were effectively quenched; however, long-lived transient growths were observed for all the t-butylperester derivatives. The intensities of these novel transient absorptions appear to correlate with the total number of t-butylperester groups in the fluorenone molecule and tentative assignments are discussed.