THE MECHANISM OF ENERGY TRANSFER FROM POLY-p-BENZOYLPHENYLACETIMIDO-BOVINE SERUM ALBUMIN TO SMALL-MOLECULE QUENCHERS

Abstract— Results of a quantitative photochemical study of poly- p -benzoylphenylacetimido-bovine serum albumin in the presence of small-molecule triplet quenchers are reported. The efficiency of quenching by organic salts containing low triplet energy chromophores is shown to be qualitatively dependent on their predicted association constants to the modified protein. In addition, quenching is inhibited by salts of organic acids which possess high binding affinities for the protein but do not contain chromophores of low triplet energy. Quantitative treatment of the quenching and inhibition data yields results which strongly support the operation of an 'affinity controlled' mechanism for triplet energy transfer from the benzophenone moieties of the modified-bovine serum albumin to quenchers such as α-naphthylacetate and trans -cinnamate.     

The first observation of the effect of dendritic structure to produce the triplet excited state of the core stilbene by dendron excitation

We report that both singlet and triplet energy transfers in stilbene-cored benzophenone dendrimers (trans-BPST) took place quite efficiently. On excitation (290 nm) of stilbene group, the intramolecular singlet energy transfer from the excited core stilbene to the benzophenone part (99.7%) was confirmed by quenching of the fluorescence from the core stilbene. The benzophenone in the excited singlet state is known to undergo intersystem crossing to give its excited triplet state quantitatively. However, the very weak phosphorescence from benzophenone part in trans-BPST was observed even at 77 K. The phosphorescence intensity of trans-BPST is only 1% of that of model compound (4-methylbenzophenone) at 77 K. During the irradiation, the absorption spectra also changed due to the trans-cis isomerization. This is probably due to the ultrafast triplet energy transfer from the benzophenone to produce the triplet state stilbene.     

Designing systems for a multiple use of light signals.

The photochemical and photophysical behavior of two dendrimers consisting of a benzophenone core and branches that contain dimethoxybenzene units has been investigated. Such dendrimers can undergo a variety of photochemical and photophysical processes, namely: 1) quenching of the fluorescence and phosphorescence of the dimethoxybenzene units by energy transfer to the benzophenone core (antenna effect), 2) direct and sensitized phosphorescence (and delayed fluorescence) of the benzophenone core, 3) hydrogen abstraction by the triplet excited state of the benzophenone core from solvent molecules, 4) intramolecular hydrogen abstraction by the triplet excited state of the benzophenone core from the dendrimer branches, 5) quenching of the phosphorescence and hydrogen abstraction reaction of the benzophenone core by energy transfer to terbium ions and dioxygen; 6) conversion of the UV light absorbed by the dendrimer branches into visible (Tb3+) or near infrared (O2) emission via the sequence of processes 1) and 5). The results obtained emphasize the great potential of suitably designed dendrimers for a multiple use of light signals.     

Photo-oxidation of polymers—VII: A re-investigation of the photo-oxidation of polystyrene based on a model compound study

Fluorescence quenching experiments indicate that energy transfer occurs from cumene excited at 254 nm to cumene hydroperoxide. Quantum yields show that the sensitized decomposition of the hydroperoxide occurs quantitatively and that 2-phenylpropanol-2 is the main photoproduct. In the presence of oxygen, this process plays a dominant role in the initiation of the photo-oxidation. When benzophenone is excited to the first triplet state by irradiation at 365 nm in the presence of cumene hydroperoxide, phosphorescence quenching experiments and laser flash photolysis suggest that an exciplex is formed. This exciplex dissociates into cumylperoxy and ketyl radicals in such a way that 80% of the excited ketone molecules are transformed into the corresponding pinacol. In the presence of oxygen, benzophenone primarily initiates the photo-oxidation of cumene by hydrogen abstraction but, as cumene hydroperoxide is formed, formation and reaction of the exciplex become progressively more and more important. The photochemical behaviour a fluorenone is quite different from that of benzophenone. The sensitized decomposition of cumene hydroperoxide occurs in the presence of that ketone. Surprisingly, fluorenone also initiates the photo-oxidation of cumene; the mechanism of that reaction is discussed. The whole set of results provides a sound basis for the interpretation of the photo-oxidation of polystyrene in various conditions.     

Photoinduced CC‐coupling Reactions of Rigid Diastereomeric Benzophenone‐Methionine Dyads

The reactions of ketone/methionine systems are widely used as efficient and selective sources of biorelevant radical species. In this study, we address intramolecular variants of this couple with respect to its photosynthetic utility and as a mechanistic model of underlying elementary reaction steps of biological importance, especially with respect to the study of photoinitiated electron transport in complex peptides. The outcomes of this study are two‐fold: (1) steady‐state irradiation of sterically constrained benzophenone/methionine dyads afforded stable photocyclization products with high yield and product selectivity. (2) Mechanistic insights into the triplet‐triggered product formation were obtained from an analysis of the flash photolysis results and the molecular structure of the stable product formed upon irradiation. Time‐resolved experiments identified (net) hydrogen‐atom transfer from the methionine as the mechanism of the triplet quenching and the resulting biradicals as the major precursor of the isolated stable product. Both the analyses of triplet quenching and stable‐product formation in the diastereomeric pairs point to effects of chiral center configuration, i.e., significant stereoselectivity is observed for all elementary steps. The underlying stereochemical restraints were quantitatively addressed by means of molecular dynamics simulations.     

PHOTOCHEMISTRY OF MODIFIED PROTEINS BENZOPHENONE-CONTAINING BOVINE SERUM ALBUMIN

Abstract— The results of exploratory and mechanistic studies of the photochemistry of poly- p -benzoyl-acetimido-bovine serum albumin, a modified protein containing photoreactive and photosensitizing groups, are reported. Specifically described are our recent findings concerning (1) the synthesis and characterization of a modified bovine serum albumin that contains benzophenone-like moieties, (2) the photochemistry of this modified protein which appears to involve photoreductive coupling of the benzophenone chromophores to the protein backbone, and (3) triplet energy transfer from modified bovine serum albumin to small molecule acceptors resulting in quenching of the photoreaction.     

PHOTOCHEMISTRY ON SURFACES: TRIPLET-TRIPLET ENERGY TRANSFER ON MICROCRYSTALLINE CELLULOSE STUDIED BY DIFFUSE REFLECTANCE TRANSIENT ABSORPTION and EMISSION SPECTROSCOPY*

Triplet-triplet energy transfer has been studied between benzophenone and an oxazine dye (2,7-bis(diethyl-amino)-phenazoxonium chloride) co-adsorbed on the surface of microcrystalline cellulose. Ground state absorption and fluorescence measurements provide evidence for dimer formation of the oxazine dye when adsorbed on cellulose in contrast to the behaviour in ethanol solution where no dimerization is observed. The equilibrium constant for dimerization, which is found to be (1.0 × 0.1) × 10⁶ mol^?1 g (2560 × 250 dm³ mol^?1) for oxazine alone on cellulose decreases in the presence of co-adsorbed benzophenone. Fluorescence is detected from excited monomeric but not from excited dimeric oxazine. The absorption spectrum of the triplet state of oxazine adsorbed on cellulose was obtained and its extinction coefficient evaluated relative to that of triplet benzophenone which was used as a sensitizer. The lifetime of adsorbed triplet oxazine is 4.3 ms which is 300 times longer than that in acetonitrile solution. The efficiency of energy transfer from triplet benzophenone to oxazine on cellulose was studied using both time resolved sensitized absorption and phosphorescence intensity measurements as a function of oxazine concentration. Lifetime measurements show that the energy transfer process involves static quenching since the benzophenone lifetime is independent of oxazine loading at the surface. A mechanism is proposed to explain the results in which one oxazine molecule is suggested as being able to quench phosphorescence from a “pool” consisting of 2 to 3 benzophenone molecules.     

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.     

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.     

QUENCHING OF TRIPLET BENZOPHENONE BY VITAMINS E and C and BY SULFUR CONTAINING AMINO ACIDS and PEPTIDES

Abstract— The quenching rate of triplet benzophenone in water and/or water mixtures has been determined employing vitamin C, vitamin E, cystine, cysteine, reduced and oxidized glutathione, methionine and DL-penicillamine. In these systems, the ketyl radical quantum yield and the benzophenone photoreduction yield have also been measured. The ketyl quantum yield is 1.0 in presence of vitamin C and smaller than 0.3 in presence of glutathione, cysteine and cystine. The data imply that quenching by thiols and disulfides takes place, at least in very polar solvents, mainly by a mechanism involving charge transfer intermediates.     

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.     

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.     

Unexpected Hofmann elimination in the benzophenone-(phenylthio)acetic tetrabutylammonium salt photoredox system

The triplet state of benzophenone was quenched by the tetrabutylammonium salt of (phenylthio)acetic acid in acetonitrile solutions. The quenching event, following laser flash photolysis, resulted in the formation of a transient ion pair consisting of the benzophenone radical anion and the tetrabutylammonium cation. Subsequently this ion pair decays with the quaternary ammonium cation undergoing a Hofmann elimination to form butane-1 and tributylamine, which were detected in steady-state irradiation. This appears to be the first report of an ion pair consisting of a benzophenone radical anion and an organic cation (nonradical), in addition to the first report of a photoinduced Hofmann elimination in quaternary ammonium ions.     

Quenching of the triplet state of benzophenone by lanthanide 1,3-diketonate chelates in solutions

The phosphorescence of benzophenone in benzene and acetonitrile was quenched by several lanthanide (Sm, Eu, Gd, Tb and Dy) acetylacetonate chelates. The results ofStern-Volmer analysis including the quenching of benzophenone triplet and sensitization of lanthanide emission indicate that the quenching process occurs by the energy transfer mechanism via the excited triplet state of the ligand.

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Löschung des Triplettzustandes von Benzophenon mittels Lanthanid-1,3-Diketonat-Chelatverbindungen in Lösung

Zusammenfassung Die Phosphoreszene von Benzophenon in Benzophenon in Benzen und Acetonitril wurde durch 1,3-Diketonat-Chelatverbindungen von einigen Lanthaniden (Sm, Eu, Gd, Tb und Dy) gelöscht. Die Resultate derStern-Volmer-Analyse einschließlich der Auslöschung des Benzophenon-Tripletts und die Sensibilisierung der Lanthanid-Emission zeigen, daß der Löschprozeß mittels Energieübertragungsmechanismus via angeregtem Triplettzustand des Liganden stattfindet.

     

PHOTOCHEMICAL REACTIONS OF AROMATIC KETONES WITH NUCLEIC ACIDS AND THEIR COMPONENTS I—. PURINE AND PYRIMIDINE BASES AND NUCLEOSIDES*.

Abstract— The photochemical reactions of benzophenone and acetophenone with purine and pyrimidine derivatives in aqueous solutions have been investigated by flash photolysis and steady-state experiments. Upon excitation of these two ketones in aqueous solutions, two transient species are observed: molecules in their triplet state and ketyl radicals. The triplet state lifetimes are 65 μsec for benzophenone and 125 μsec for acetophenone. The ketyl radicals disappear by a second order reaction, controlled by diffusion. In the presence of pyrimidine derivatives, the triplet state is quenched and the ketyl radical concentration is decreased without any change in its kinetics of disappearance. Ketone molecules in their triplet state react with purine derivatives leading to an increase in the yield of ketyl radicals due to H-atom abstraction from the purines. Steady-state experiments show that benzophenone and acetophenone irradiated in aqueous solution at wavelengths longer than 290 nm undergo photochemical reactions. The rate of these photochemical reactions is increased in the presence of pyrimidine derivatives and even more in the presence of purine derivatives. Following energy transfer from the triplet state of benzophenone to diketopyrimidines, cyclobutane dimers are formed. The energy transfer rate decreases in the order orotic acid > thymine > uracil. Benzophenone molecules in their triplet state can also react chemically with pyrimidine derivatives to give addition photoproducts. All these results are discussed with respect to photosensitized reactions in nucleic acids involving ketones as sensitizers.     

Preparation and photochemical properties of poly[2-(4-benzoylphenyl)-2-(4-propenoylphenyl)-propane] and its copolymer with styrene

2-(4-Benzoylphenyl)-2-phenyl propane ( 4 ) was prepared by benzoylation of 2,2-diphenylpropane ( 2 ). Acylation of ( 4 ) with 3-chloropropanoic chloride gave 2-(4-benzoylphenyl)-2-(4-propenoylphenyl)propane ( 5 ). A monomer 2-(4-benzoylphenyl)-2-(4-propenoylphenyl)propane ( 6 ) was prepared through dehydrochlorination of ( 5 ). The homopolymer of 6 (P6) and the copolymer with styrene ( P6 / S) were prepared by radical polymerization. Laser flash photolysis was employed to determine the absorption and emission spectra of transients, their lifetimes (τ) and the rate constant (k_q) of triplet quenching in benzene at laboratory temperature for 4 , P6 , and P6 / S. P6 exhibits a transient absorption maximum in a different spectral region than do the model 4 and copolymer P6/S . The products of k_q and τ determined by laser flash photolysis for these transients are higher than th Stern–Volmer constants based on inhibition of degradation. Degradation leading to formation of quenchers is the likely cause of this difference although crosslinking may also contribute. Irradiation of polymers ( P6 and P6/S ) at 366 nm leads to main chain scission with aquantum yield of 0.13 under N₂ for P6 and 0.03 for P6/S . In this bichromophoric structural unit, the benzophenone residue absorbs about 80–90% of the incident energy. Its triplet energy is about 5 kJ mol^?1 lower than that of the 1-(4-alkylphenyl)-2-propene-1-one chromophore. Different possible pathways of degradation are discussed namely the Norrish Type II reaction of the alkyl aryl ketone and direct reaction of triplet benzophenone with the main chain. In the mechanism favored the benzophenone triplet is proposed to be in equilibrium with the upper acetophenone-like chromophore from which the Norrish Type II reaction leading to chain fragmentation takes place.     

T → S Energy Transfer in Some Molecular Complexes

Calculations on the bimolecular complexes of acetophenone or benzophenone with anthracene and its substituted derivatives were carried out using a standard quantum-chemical approach to molecular systems. The deactivation pathways for lower triplet excited states of acetophenone and benzophenone were established. The probability of energy transfer from the energy donor to acceptor in the complexes was considered. The analysis of calculation results showed that the T S transfer of electronic excitation energy in these complexes is feasible only in the case of a small distance between the molecules, and the efficiency of this transfer is higher in the acetophenone rather than benzophenone complexes.     

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.     

Characterization of the triplet state of tris(8-hydroxyquinoline)aluminium(III) in benzene solution

The lowest triplet state of tris(8-hydroxyquinoline)aluminium(III) (Alq3) has been prepared by pulse radiolysis/energy transfer from appropriate donors in benzene solutions and has an absorption maximum around 510 nm with a lifetime of about 50 mus. It is quenched by molecular oxygen, leading to singlet oxygen formation. From flash photolysis and singlet oxygen formation measurements, a quantum yield of triplet formation of 0.24 was determined for direct photolysis of the complex. A value of 2.10 +/- 0.10 eV was determined for the energy of the lowest triplet state by energy transfer studies and was confirmed by phosphorescence measurements on Alq3, either in the heavy atom solvent ethyl iodide or photosensitized by benzophenone in benzene. Dexter (exchange) energy transfer was observed from triplet Alq3 to platinum(II) octaethylporphyrin.     

Tuning Excited‐State Reactivity by Proton‐Coupled Electron Transfer

The reactivity, and even reaction pathway, of excited states can be tuned by proton‐coupled electron transfer (PCET). The triplet state of benzophenone functionalized with a Brønsted acid (³*BP‐COOH) showed a more powerful oxidation capability over the simple triplet state of benzophenone (³*BP). ³*BP‐COOH could remove an electron from benzene at the rate of 8.0×10⁵ m ^?1 s^?1, in contrast to the reactivity of ³*BP which was inactive towards benzene oxidation. The origin of this great enhancement on the ability of the excited states to remove electrons from substrates is attributed to the intramolecular Brønsted acid, which enables the reductive quenching of ³*BP by concerted electron–proton transfer.