CHEMILUMINESCENCE AND ENERGY TRANSFER OF EXCITED ACETOPHENONE

Abstract— Further evidence is found that acetophenone in its triplet-state is formed as the luminescent species in the autoxidation of ethylbenzene. Energy transfer from this ketone to the triplet-states of various acceptors results in quenching of the already weak chemilumin-escence and follows a diffusion-controlled rate as expected. The poor resemblance between the experimentally recorded spectral distribution of this light emission and the known phosphorescence spectra of the assumed emitters is discussed.     

ELECTRON TRANSFER FROM PHOTOEXCITED SINGLET AND TRIPLET BACTERIOPHEOPHYTIN

Abstract— Nanosecond and picosecond. kinetic techniques have been used to study electron transfer from the first excited singlet state (Bph*) and the first excited triplet state (Bph ^T ) of bacteriopheophytin to p -benzoquinone. Quenching of the first excited singlet state by 40 m M p -benzoquinone results in a decrease in the lifetime of Bph* but does not lead directly to the formation of the π-cation radical (Bph†). In the presence of 8 M methyl iodide and 40 m M p -benzoquinone together, the singlet lifetime is reduced further; however, the quantum yield of Bph ^T is enhanced due to the increased rate of intersystem crossing between Bph* and Bph ^T . Electron transfer from Bph ^T to p -benzoquinone leads to the formation and detection of Bph†. The results are discussed in terms of the spin-selectivity of the reverse electron transfer process within the intermediate charge transfer complexes.     

CHEMILUMINESCENCE AND THE FORMATION OF SINGLET OXYGEN IN THE OXIDATION OF CERTAIN POLYPHENOLS AND QUINONES

Abstract— The possibility of ¹O₂ (¹Δ_g) participation in the oxidation of polyphenols and quinones has been investigated in two systems: (1) the system involving autooxidation leading to oxidative polymerization and destruction, and (2) the modified Trautz-Schorigin reaction, i.e. oxidation of polyphenols and HCHO with H₂O₂ in concentrated alkaline solutions. The red band with maximum at 635 nm observed in chemiluminescence of pyrocatechol, adrenaline, pyrogallol, gallic acid, adrenochrome and p -benzoquinone corresponds to the transition 2O₂(¹Δ_g) → 2O₂(³Σ^-_g). Emission bands in the range 475–540 nm arise from the superposition of the 2O₂(¹Δ_g) → 2O₂(³Σ^-_g) transition and radiative deactivation of excited oxidation products. In system (2) chemiluminescence has a broad band from 580 nm beyond 800 nm and much higher intensity than in system (1). Formaldehyde was found to enhance light emission in system (1) by a factor of about 30. The influence of solvents, including D₂O in which ¹O₂ has varying lifetimes, on kinetics of chemiluminescence as well as quenching effect of β-carotene, hydroquinone, cysteine, bilirubin and biliverdin strongly support the involvement of ¹O₂ in the chemiluminescence of both systems.     

INHIBITION AND ENHANCEMENT OF SINGLET OXYGEN ('Ag) DIMOL CHEMILUMINESCENCE

Abstract— Various nitrogen containing compounds have previously been shown to quench singlet oxygen (10z). When measuring the dimol ¹O₂ light emission arising from the H ₂O₂ /OCI- reaction, we found that certain cyclic diamines increase the emission of light, while other amines were inhibitory. This increase of light emission was seen with both 1, 4diazabicyclo[2.2.2]octane and N, N'-dimethylpi-perazine but not with acyclic analogues. Sodium azide inhibited both the normal and enhanced light emission. The enhanced light emission shows spectral properties characteristic of ^lO₂ dimol emission.     

ACTIVATED OXYGEN: SINGLET MOLECULAR OXYGEN AND SUPEROXIDE ANION

Abstract— Elusive processes associated with molecular oxygen in chemical and biological systems are interpreted in terms of two activated oxygen species, singlet molecular oxygen (¹Σ⁺_g/¹Δ_g) and superoxide anion (X²π_g). The generation and deactivation of singlet oxygen by interaction with organic triplet states are discussed within a comprehensive theoretical framework. Experimental results indicate the anomalous molecular oxygen enhanced luminescence from organic chromophores in polymer matrices results from the deactivation of singlet (¹Δ_g) oxygen by energy transfer to electronically excited states of the chromophore, and three types of oxygen enhanced luminescence have been identified in these systems. Properties of the superoxide anion relevant to its solution chemistry are briefly discussed. Electron transfer theory is used to theoretically examine the generation of singlet oxygen in disproportionation reactions of the superoxide anion, predicting that, depending on the number of water molecules present, the disproportionation reaction is a proficient source of singlet oxygen. A competing quenching process imposes a limit to the steady state concentration of singlet oxygen in most chemical systems. Available experimental results on the quenching of singlet oxygen by superoxide anion are in good agreement with theoretical results obtained via application of electron transfer theory.     

ELECTRON TRANSFER FROM PHOTOEXCITED SINGLET AND TRIPLET BACTERIOPHEOPHYTIN—II. THEORETICAL

Abstract— A previous paper showed that collision of the first excited singlet state of bacteriopheophytin (Bph*) and p-benzoquinone (Q) returns Bph* to the ground state; however, excited triplet (Bph⁺) and quinone on collision produce the radical ions, (Bph⁺) and (Q^?). This paer rationalizes these findings by first estimating the half cell potentials Bph⁺/Bph* and Bph⁺/Bph^T, the energy for the various collision complexes, and the energy of the charge separated ions Bph⁺+ Q? and then estimating the rates for conversion among these various states. Thus it is estimated that the complexes [Bph*Q] or [Bph^TQ], live ?5 ps before dissociating. This is long enough for electron transfer to occur, producing the singlet and triplet charge transfer complexes, [Bph⁺Q?]^S or [Bph⁺Q?]^T, either of which could separate to Bph⁺+ Q? in ?230ps. In the singlet case, quenching by reverse charge transfer [Bph⁺Q?]^S→[Bph Q] occurs more rapidly than ion separation; however, the analogous triplet process, [Bph⁺Q?]^T→ [Bph Q], is spin forbidden, so that ion separation competes successfully with quenching. Spin scrambling, [Bph⁺Q?]^S? [Bph⁺Q?]^T, is estimated to be slow, as this explanation requires. In the bacterial photosynthetic reaction center, the initial electron transfer from an excited singlet state of the bacteriochlorophyll dimer complex (BB)* to bacteriopheophytin, giving [(BB⁺)(Bph?)]^S, successfully leads to ion separated species (i) because reverse charge transfer [(BB⁺)(Bph?)]^S→ [(BB)(Bph)] is slowed by a fairly large Franck-Condon energy, ΔE? lev, which is difficult to convert from electronic to vibrational degrees of freedom and (ii) because of the rapid subsequent electron transfer from (Bph⁺) to another acceptor X.     

RED CHEMILUMINESCENCE FROM RAM SEMINAL VESICLE MICROSOMES: PITFALLS IN THE USE OF SPECTRALLY RESOLVED RED CHEMILUMINESCENCE AS A TEST FOR SINGLET OXYGEN IN BIOLOGICAL SYSTEMS

Abstract— The assignment of the red chemiluminescence bands in the ram seminal microsome system to singlet oxygen as previously reported by Cadenas and Sies (1983) Hoppe-Seyler's Z. Physiol. Chem. 364, 519-528 was re-evaluated. Measurements of 1268 nm emission demonstrated that ram seminal vesicle microsomes produced very small quantities of singlet oxygen (0.41 2 0.05 p.M) when they metabolized high concentrations of 15-hydroperoxyarachidonic acid (1 mM). The red chemilumi- nescence, however, was not due to singlet oxygen, since it failed to increase in deuterium oxide and it was five orders of magnitude larger than the predicted emission from singlet oxygen produced in this system. Quantitative measurements of the time integral of the square root of the red chemiluminescence intensity may be a useful test to confirm the assignment of red emission to singlet oxygen in other biochemical systems.     

HYDROGEN PEROXIDE-DEPENDENT GENERATION OF SINGLET MOLECULAR OXYGEN BY HUMAN SALIVA: ITS DETECTION BY CHEMILUMINESCENCE FROM A Cypridina LUCIFERIN ANALOG

By the addition of hydrogen peroxide to human saliva, chemiluminescence from Cypridina luciferin analog (CCLA) and oxygen evolution were observed. Chemiluminescence was inhibited by inhibitors of salivary peroxidase, azide and cyanide and by a singlet oxygen quencher, crocin. Deuterium oxide (99.75%) stimulated the initial increase of CCLA by15–50% and the integrated CCLA 2.1-3.6-fold. The result suggest that the generation of singlet oxygen by peroxidase in human saliva depends on hydrogen peroxide.     

SINGLET ENERGY TRANSFER BETWEEN AROMATIC AMINO ACIDS AND NUCLEIC ACID BASES. THEORETICAL CALCULATIONS

Abstract— Critical Forster distances for excitation energy transfer at the singlet level between the tyrosyl or tryptophyl residues of proteins and nucleic acid bases have been calculated. Efficient singlet energy transfer can be expected from tyrosine to nucleic acid bases both at room temperature (10 0 < 17 A) and at low temperature (20 < R₀ < 23 Å). At low temperature nucleic acid bases should be able to transfer their singlet excitation energy to the indole ring of tryptophan with a reasonable probability (9 < R₀ < 15 A).     

SINGLET AND TRIPLET ENERGY TRANSFER IN α-DIKETONE DERIVATIVES OF URACIL and THYMINE

Irradiation of 1-(3,4-dioxopentyl)uracil (UPD) and 1-(3.4-dioxopentyl)thymine (TPD) in acetonitrile solution at 25°C, at the wavelength (280 nm) where only the pyrimidine absorbs the light, sensitizes both fluorescence and phosphorescence of the diketone chromophore in the sidechain. From comparison of the intensity in the corrected excitation spectra with the absorption spectra in acetonitrile solution, it was estimated that the yield of singlet energy transfer in UPD was 0.17 and in TPD was 0.44. It was also observed that the ratio of phosphorescence to fluorescence was greater in the sensitized emission than in that from direct excitation of the diketone chromophore. The yield of triplet energy transfer thus measured corresponds to minimum values for the yields of intersystem crossing from singlet excited state to triplet excited state of 0.075 in the uracil chromophore of UPD and of 0.14 in the thymine chromophore of TPD. These are in agreement with other recent values for these quantities. The value of this type of system as an intramolecular triplet counter is discussed.     

PHOTOCHEMISTRY OF cis-POLYISOPRENE AND ITS SINGLET OXYGEN ADDUCT

Abstract— Reaction of singlet oxygen (¹Δ_g, ¹O₂) with cis -polyisoprene yields an allylic hydroperoxide with an olefinic double bond shifted in the polymer chain. The photochemical decomposition of the resultant hydro-peroxide and the subsequent polymer chain scission kinetics have been studied in the absence of oxygen. Quantum yields of hydroperoxide decomposition range from 3.1 to 8.4 in cyclohexane, depending on the initial amount of hydroperoxide in the polymer. On the other hand, the quantum yields for polymer chain scission are low, and vary with the frequency of the incident light. The ratio for number of polymer scissions per number of hydroperoxy groups decomposed is of the order of 10^-2. The polymer chain degradation is sensitized by the addition of ketones. Based on these data, a reaction mechanism for the overall photodegradation of the cis -polyisoprene initiated by singlet oxygen is proposed.     

REACTION OF RETINOL AND RETINAL WITH SINGLET OXYGEN

Abstract— The rate constants for the reactions of all- trans retinol and retinal with singlet oxygen were measured in a variety of solvents of different polarities. The rate constants increased with increasing solvent dielectric constant, which suggests that a charge transfer mechanism plays a part in the reaction. Further, the rate constant of reaction of singlet oxygen with retinal is greater than that with retinol. Since retinal has a lower ionization potential than retinol, these relative rates also support the hypothesis of charge transfer involvement in the reaction.     

单线态氧的检测及分析应用研究进展

综述了单线态氧（^1O2）的测定方法。介绍了单线态氧的ESR、磷光光度、分光光度、化学发光和荧光法的基本原理、方法研究进展和分析应用情况,并重点介绍了化学发光法和荧光法,同时比较了这几种方法的特点。     

CHEMILUMINESCENCE INDUCED BY OXIDATION OF TRYPTOPHAN BY SINGLET OXYGEN AND BY HYPOCHLOROUS ACID. IMPLICATIONS IN THE LUMINESCENCE EMITTED IN PHAGOCYTOSIS

Abstract— The mechanism of the chemiluminescence emitted during phagocytosis of opsonized zymosan was investigated. The use of an artificial system generating singlet oxygen permitted the distinction between the respective roles of O₂ and HOCl in the light production via the reactions with opsonized zymosan, amino-acids and peptides. The reaction of free hypochlorous acid, produced by the myeloperoxidase H₂ O₂-Cl- system, with free tryptophan appears to be a likely path for the production of chemiluminescence. Implications for the mechanism of in vivo chemiluminescence during phagocytosis are considered.     

INTRAMOLECULAR DONOR-ACCEPTOR SEPARATIONS IN METHIONINE- AND LEUCINE-ENKEPHALIN ESTIMATED BY LONG-RANGE RADIATIONLESS TRANSFER OF SINGLET EXCITATION ENERGY*

The Förster theory of radiationless electronic excitation energy transfer was used to determine the intramolecular separation between the tyrosine and phenylalanine residues in leucine- and methionine-enkephalin. The spectral overlap integrals and the Förster critical distances were evaluated. Transfer efficiencies were determined by observing sensitized fluorescence of the acceptor and quenching of the donor fluorescence. The results were compared with theoretical estimations, better agreement being obtained with the first method.     

SPECTRAL OBSERVATION OF SINGLET MOLECULAR OXYGEN FROM AROMATIC ENDOPEROXIDES IN SOLUTION

Abstract— The characteristic near-infrared emission band of O₂ (¹Δ_g) at 1.28 μ m has been recorded from carbon tetrachloride solutions of the 1, 4-endoperoxides of 1,4-dimethyInaphthalene and 1, 4-dimethoxy-9.10-diphenylanthracene undergoing thermal decomposition.     

REACTIVITY OF SINGLET OXYGEN TOWARD AMINO ACIDS AND PEPTIDES

Quenching of singlet oxygen (¹O₂) in D₂O-ethanol by the amino acids tryptophan, tyrosine, histidine, methionine, cysteine and their derivatives was measured by exciting the sensitizers rose bengal or meso-tetra (N-methyl-4-pyridyl)porphyrin tetratosylate in the presence of oxygen and the above quenchers in solution. In our polar solvent, containing 75% D₂O on a molar basis it was found that (1) substitution of the aromatic ring in indole, phenol and imidazole by the electron-donating methyl group increases the total (i.e. nonreactive and reactive) quenching rate constant by a factor of five to eight. Free or blocked amino and carboxyl groups removed by two methylene groups from the ring counteract the above increase in the rate constant. The reactive quenching of singlet oxygen, which leads to oxidative destruction of the aromatic ring, correlates with the above substitution effects. It has been proposed that the quenching process takes place by formation of an exciplex between ¹O₂ and the quencher. Thus our results indicate that the better an electron donor the amino acid residue is the more pronounced is the charge transfer contribution in the exciplex formed with ¹O₂ and the more likely it is to lead to charge separation and hence to a chemical reaction. (2) Oligopeptides in solution or peptide bonds linked to the amino acid residue have only a minor effect on singlet oxygen. It can therefore be expected that the polypeptide chains per se in the protein network will not interact significantly with the single oxygen molecules present. The quenching of the latter should, to a first approximation, depend only on the presence of the above reactive amino acid residues and to their accessibility to ¹O₂ as well as on the effective dielectric constant within the protein structure.     

QUENCHING OF SINGLET MOLECULAR OXYGEN BY PHTHALOCYANINES AND NAPHTHALOCYANINES

Using the direct measurement of the photosensitized luminescence of singlet molecular oxygen (1O2) the rate constants (kq) have been determined for 1O2 quenching by the monomeric molecules of the following phthalocyanines and naphthalocyanines in chloroform: tetra-(4-tert-butyl) phthalocyanine (I); octa-(3,6-butoxy) phthalocyanine (II), tetra-(6-tert-butyl)-2,3 naphthalocyanine (III), aluminium tetra-(1-tert-phenyl)-2,3 naphthalocyanine (IV), tri-(n-hexyl-siloxy) derivatives of silicon- (V), tin- (VI), aluminium- (VII) and gallium- (VIII) 2,3 naphthalocyanine. The following kq values were obtained (kq x 10(-8) M-1 s-1): 2.9 (I), 59 (II), 100 (III), 20 (IV), 3.9 (V), 53 (VI), 33 (VII), 110 (VIII). As most of the quenchers have the low-lying triplet levels, a contribution of the quenching mechanism based on the energy transfer from 1O2 to these levels has been analysed. A formula is proposed describing the relation between kq values caused by this mechanism, and photophysical constants of the quencher triplet state. This formula was applied to phthalocyanines, naphthalocyanines, beta-carotene and bacterochlorophyll a. The data suggest that the energy transfer can fully explain the activity of V and strongly contributes into the activities of II, III and VI-VIII. A charge transfer interaction might be an additional mechanism involved in 1O2 quenching by compounds studied. As some phthalocyanines and naphthalocyanines are strong physical quenchers of singlet oxygen they can be used as efficient inhibitors for photodestructive processes in photochemical systems.     

COMPETITION BETWEEN THE SINGLET OXYGEN AND ELECTRON TRANSFER MECHANISMS IN THE PORPHYRIN-SENSITIZED PHOTOOXIDATION OF l-TRYPTOPHAN AND TRYPTAMINE IN AQUEOUS MICELLAR DISPERSIONS

Abstract— Kinetic studies of the hematoporphyrin–sensitized photooxidation of l -tryptophan and tryptamine at pH 10 in either homogeneous aqueous solutions or in aqueous dispersions of Triton X–100 and cetyltrimethylammonium bromide micelles indicate that the indole substrates are attacked via a mixed type I (electron transfer from triplet dye)/type II (¹O₂-involving) mechanism. Both reactive intermediates, generated by micelle-solubilized hematoporphyrin, can diffuse to attack substrate molecules located in either the bulk aqueous phase or a different micelle. In particular, incorporation of the substrate into a micelle has only minor effects on its reactivity toward¹O₂, although the ¹O₂—indole interaction appears to be more efficient in cationic micelles owing to a favourable orientation of the target with respect to the attacking species. On the other hand, the electron transfer from triplet porphyrin to a micellized substrate is virtually non-operative when the latter is located in an anionic micelle, whereas in neutral or cationic micelles, the efficiency of the process is again controlled by the substrate orientation. Studies of tryptamine photooxidation sensitized by meso-tetra-(4-sulfonato-phenyl) porphine in the presence of sodium dodecylsulphate micelles lend further support to the abovementioned conclusions.     

DITHIOPHENES AS SINGLET OXYGEN SENSITIZERS

Abstract— The nematicidal compounds, 5-methyl-2,2'dithienyl, 5-phenyl-2,2'-dithienyl and 5,5'-dichloro-2,2'-dithienyl, generate singlet oxygen upon photoactivation with near-UV light. Two chloro-substituted dithienylethenes have similar properties that are farless efficient. The nematode Aphelenchus avenae was rapidly killed by the photoactivated dithienyls under aerobic but not under anaerobic conditions,indicating that nematicidal activity of these compounds is related to their sensitizing properties. Furthermore,these nematodes did not expire in the presence of these compounds maintained in the dark. Sensitizing properties were studied by following the inhibition ofglucose–6-phosphate dehydrogenase by the photoactivated compounds and protection of enzyme activity by various additions. Only singlet oxygen quenchers were effective. The enzyme was inactivated 3 to 4 times faster in D₂0 compared to H₂0.Bleaching of p-nitrosodimethylaniline in the presence of imidazole and the dithiophenes and a specific reaction with the olefin adamantylideneadamantane were additional evidence of generation of singlet oxygen by the photoactivated compounds.