KINETICS OF TRIPLET OXIDATION OF METALLO-PORPHYRIN COMPOUNDS TO THEIR CORRESPONDING RADICAL CATIONS

Abstract— The kinetics of photooxidation of triplets of metalloporphyrin compounds to their corresponding radical cations was investigated. Zn-tetraphenyl porphyrin (ZnTPP) and Mg-tetraphenylpor-phyrin (MgTPP) triplets were oxidized by europium salt with rate constants of 4.8 × 10⁵M^-1s^-1 and 2.1 × 10⁶M^-1s^-1, respectively. The high rate constant of oxidation of MgTPP triplet might be related to the ground state oxidation potential, being 0.54 V (SCE) for the Mg complex and 0.71 (SCE) for the Zn complex.
The rate constant of oxidation of ZnTPP excited singlet is in the order of diffusion control, i.e. ˜ 10¹⁰M ^-1 s^-1. Excitation of ferric, cupric, cobaltic, and vanadyl tetraphenylporphyrin did not result in a long-lived triplet state that would allow oxidation studies using flash photolysis.     

SENSIBILISIERTE PHOTOOXIDATION DURCH METHYLENBLAU, THIOPYRONIN UND PYRONIN — II. PHYSIKOCHEMISCHE GRUNDLAGEN DER PHOTODYNAMISCHEN WIRKUNG VON THIOPYRONIN

Abstract— The physical-chemical properties of thiopyronine which result in its outstanding photodynamic effectiveness (in comparison to methylene blue and pyronine) are: (1) High velocity of free radical generation (kinetics followed by spectroscopy after flash photolysis). (2) High velocity of reoxidation of free radical (rate constant determined by kinetic spectroscopy under aerobic conditions). (3) Fairly positive reduction potential of oxidized free radical (from polarographic analysis of redox behavior). (4) Irreversible oxidation behavior of leuco-thiopyronin in contrast to leuco-methylene blue (the structure of leuco-thiopyronin was elucidated by NMR-analysis). (5) Fairly high equilibrium constant for complex formation with DN A (determination by high sensitive pulse-polarography).
From the sedimentation behavior and T-jump relaxation of in vitro photodynamically treated DNA it is shown that not only does guanine photooxidation take place but also single- and double-strand breaks can occur. The mechanism of this phenomenon is not yet elucidated.     

ENERGY TRANSFER BETWEEN THE PRIMARY DONOR BACTERIOCHLOROPHYLL AND CAROTENOIDS IN Rhodopseudomonas sphaeroides

Abstract— The temperature dependencies of the primary donor triplet state spectra are presented for the phorosynthetic bacteria Rhodopseudomonas sphaeroides wild type. GIC and R26. The data suggest that energy transfer from the primary donor triplet state to the reaction center carotenoid is dependent on the type of carotenoid present, reversible in the case of strain GIC, and best understood by a model depicting the kinetic processes that can occur between two potential energy surfaces; one representing the state ³BChl₂*Car and the other representing BChl₂³Car*. Furthermore, it is shown that the onset of spin lattice relaxation in the primary donor triplet is most likely coupled to the same energy vibrational mode as that which promotes triplet state energy transfer from the primary donor to the reaction center carotenoid     

REACTIVITY OF ACRIDINE DYE TRIPLET STATES IN ELECTRON TRANSFER REACTIONS

Abstract—Rate constants, k _q , for the reaction of cationic and neutral acridine orange and 10-methylacridine orange triplet states (³AOH ⁺, ³AO, ³MAO⁺) with a series of electron donors have been measured. Two different protolytic forms of the semireduced dye radical are produced by acridine orange triplet quenching at various pH^M values in methanolic solution.
It is found that k ₄ decreases with increasing oxidation potential of the reducing agent for all triplet states in a manner which is expected for electron transfer reactions. The different reactivities of the cationic and neutral triplet forms can, therefore, be attributed to the difference in reduction potentials of these species. The difference in reduction potentials is related to the p K ^M values of triplet state, p K _T^M , and semireduced dye radical, p K ^M_S , by thermodynamic consideration. p K _T^M (³AOH⁺/³AO) is determined to be 11.2. From thisp K _S^M (AOH./AO;) is estimated to be 17–18. This is in striking contrast to the protolytic equilibrium of the semireduced dye radicals found to be pK^F= 4.1. We conclude that the last value represents the second protonation equilibrium (AOH⁺₂./AOH). This conclusion is confirmed by spectroscopic data.     

SENSIBILISIERTE PHOTOOXIDATION DURCH METHYLENBLAU, THIOPYRONIN UND PYRONIN-I MITTEILUNG: FLASH-PHOTOOXIDATION VON p-DIAMINOTOLUOL

The dye (F)*sensitized photooxidation of p-diaminotoluene (RH₂) was examined by flash-spectroscopy and the mechanism was elucidated. The first chemical reaction is the generation of two free radicals from the triplet dye and the hydrogen donor. These radicals may react in several ways without oxygen: or additionally with oxygen: The connection of this mechanism with photodynamic action is discussed.     

STOCHASTIC MODELING OF LIGHT ENERGY CONVERSION IN PHOTOSYNTHESIS

Abstract— Photosynthetic quantum conversion and early electron transport is modeled as a stochastic process on a digital computer to determine what free-energy losses are a necessary consequence of specific assumptions about the reaction structure, kinetics, and thermodynamics of the participating molecules. Maximal free-energy yield requires that all dark reactions be near equilibrium, so the potentials of all half-cells on each side of the light act are nearly the same. This near equilibrium requires that all forward rate constants be at least 10² times the rate of light absorption, and that all reverse rate constants be at least the rate of light absorption. The behavior of model systems with one primary donor and one primary acceptor is comparatively independent whether there is one or an infinite number of secondary electron donors and acceptors. A system having no metastable (e.g. triplet) state of the light-activated donor can convert light energy with nearly ideal efficiency, provided that the standard electrode potentials of the primary donor and primary acceptor half-cells are precisely located with respect to one another and to the potentials of the ultimate donor and acceptor. While not necessary for near maximal free-energy yield, a metastable intermediate allows a flexibility in the choice of half-cell potentials which is not possible in the absence of such an intermediate.     

TRANSIENT PHOTOCHEMISTRY OF SAFRANIN-O

Abstract— We have characterized the spectra, acidity constants and decay kinetics of the triplet and semireduced radical species of Safranin-O. Between pH 3.0 and 10.6, there are three triplet species denoted ³DH^{2 +}₂, ³DH⁺ and ³D, the p K _as being 7.5 and 9.2. All three triplet species exhibit first order decay, the rate constant for ³DH⁺ being ca. 5-fold lower than the rate constants of ³DH⁺ and ³D. Ascorbic acid and ethylenediaminetetraacetic acid (EDTA) quench the triplet state under appropriate pH conditions and the pH dependencies of the yield of semireduced dye indicate that ³DH⁺ is more reactive than ³DH⁺ or ³D. With EDTA as the reducing agent, there is the additional requirement that at least one of the amino nitrogens be deprotonated to obtain a significant yield of semireduced dye. In these reactions, ascorbic acid is oxidized reversibly, but EDTA is oxidized irreversibly, so that with the latter reducing agent photolysis causes buildup of the leucodye, which on subsequent photolysis can reduce triplet state dye. With ascorbic acid as the reducing agent, the regeneration of the ground state dye is reversible, the decay of the semireduced radical being second order. In general, the transient photochemistry of Safranin-O resembles that of Thionine, the major difference being that the lifetimes of ³DH^{2 +}₂ and ³DH⁺ are much longer for Safranin-O than for Thionine.     

Mechanism of Electrode Reactions Involving Carboxymethyl and Chlorocarboxymethyl Radicals and Ion-Radicals

Mechanism of reduction and oxidation of carboxymethyl and chlorocarboxymethyl radicals and ion-radicals adsorbed on a mercury electrode is studied by a laser photoemission method in broad ranges of potentials and solution pH. Versions of reduction and oxidation depend on the solutions" acid–base properties. In acid solutions, a metastable complex ion-radical–(radical)–proton donor undergoes reduction. In neutral and weakly alkaline solutions, the electron transfer onto an ion-radical dominates. In strongly alkaline solutions, a metastable complex ion-radical–proton donor undergoes reduction. The last complex is oxidized in neutral and alkaline solutions at anodic potentials, whereas in acid solutions carboxymethyl radicals and ion-radicals are oxidized. Kinetic parameters of metastable complexes barely depend on the presence of the chlorine atom in the radical, in contradistinction to the reduction overvoltage of ion-radicals and their complexes, which discernibly diminishes following halogenation. The experimental data are interpreted within an earlier model for electrode reactions involving intermediates, which includes two parallel channels for the electron transfer: adsorbed radical (anion-radical) to electrode and metastable complex radical (anion-radical) to donor/acceptor of protons to electrode.     

N‐Alkoxyheterocycles As Irreversible Photooxidants

Irreversible photooxidation based on N–O bond fragmentation is demonstrated for N‐methoxyheterocycles in both the singlet and triplet excited state manifolds. The energetic requirements for bond fragmentation are studied in detail. Bond fragmentation in the excited singlet manifold is possible for ππ* singlet states with energies significantly larger than the N–O bond dissociation energy of ca 55 kcal mol^?1. For the nπ* triplet states, N–O bond fragmentation does not occur in the excited state for orbital overlap and energetic reasons. Irreversible photooxidation occurs in the singlet states by bond fragmentation followed by electron transfer. Irreversible photooxidation occurs in the triplet states via bimolecular electron transfer to the donor followed by bond fragmentation. Using these two sensitization schemes, donors can be irreversibly oxidized with oxidation potentials ranging from ca 1.6–2.2 V vs SCE. The corresponding N‐ethylheterocycles are characterized as conventional reversible photooxidants in their triplet states. The utility of these sensitizers is demonstrated by irreversibly generating the guanosine radical cation in buffered aqueous solution.     

Comportement polarographique et solvatation de quelques cations dans le mélange binaire diméthylsulfoxyde-carbonate de propylène

The electrochemical oxidation of 2,4,6-tri-tert-butylphenol as well as the phenoxide and phenoxy radical derived from it has been investigated in acetonitrile and ethanol+water. The ease of oxidation decreases in the order phenoxide, phenoxy radical, phenol with the separation between potentials for phenoxide oxidation and phenoxy radical oxidation being 1.2 V in acetonitrile. The phenoxide is oxidized to the stable phenoxy radical in a highly reversible reaction in acetonitrile and alkaline ethanol+water. Oxidation of the radical produces a phenoxonium ion which is attacked by water giving 2,4,6-tri-tert-butyl-4-hydroxy-2,5-cyclohexadienone. This two electron product is also formed upon oxidation of the phenol in either solvent. However, in acidic media the hydroxydienone dealkylates give 2,6-di-tert-butylhydroquinone which is oxidized to the final product 2,6-di-tert-butyl-1,4-benzoquinone. The dealkylation is quite rapid in anhydrous acetonitrile but the rate is depressed by the addition of water.A novel double potential step experiment was used to characterize the oxidation of the phenoxy radical. A step to a potential where the phenoxide is oxidized to the phenoxy radical is followed by a step to a potential where the phenoxy radical is oxidized. The current during the second step is unusually small because protons produced by the oxidation of the radical deactivate the phenoxide. The current-time curve was found to agree with that predicted by digital simulation.     

Mono‐ and Bis(tetrathiafulvalene)‐1,3,5‐Triazines as Covalently Linked Donor–Acceptor Systems: Structural,Spectroscopic, and Theoretical Investigations

Reaction of 2,4,6‐trichloro‐1,3,5‐triazine with lithiated tetrathiafulvalene (TTF) in stoichiometric conditions, followed by treatment with sodium methanolate, provides mono‐ and bis(TTF)–triazines as new covalently linked (multi)donor–acceptor systems. Single‐crystal X‐ray analyses reveal planar structures for both compounds, with formation of peculiar segregated donor and acceptor stacks for the mono(TTF)–triazine compound, while mixed TTF–triazine stacks establish in the case of the bis(TTF) derivative. Cyclic voltammetry measurements show reversible oxidation of the TTF units, at rather low potential, with no splitting of the oxidation waves in the case of the dimeric TTF, whereas irreversible reduction of the triazine core is observed. Intramolecular charge transfer is experimentally evidenced through solution electronic absorption spectroscopy. Time‐dependent DFT calculations allow the assignment of the charge transfer band to singlet transitions from the HOMO of the donor(s) to the LUMO of the acceptor. Solution EPR measurements correlated with theoretical calculations were performed in order to characterize the oxidized species. In both cases the spectra show very stable radical species and contain a triplet of doublet pattern, in agreement with the coupling of the unpaired electron with the three TTF protons. The dication of the bis(TTF)–triazine is paramagnetic, but no spin–spin exchange interaction could be detected.     

Photosensitized xanthone-based oxidation of guanine and its repair: a laser flash photolysis study

The photosensitized oxidation of guanine (G) by the triplet state of xanthone (XT) and the repair for photo-damaged G(-H)(·) by ferulic acid (FCA) were investigated using the laser flash photolysis technique. The rate constants of the reaction of triplet state of XT with G and with FCA were determined as 4.5×10(9) and 8.0×10(9) L mol(-1) s(-1), respectively. Laser exposure was performed on the N(2)-saturated acetonitrile/water (v/v, 1:1) solution containing G, XT and FCA. The transient absorption spectra indicated that the triplet state of XT first reacted with G predominantly to form the oxidized radical G(-H)(·). The radical G(-H)(·) was rapidly repaired by FCA, and the rate constant for the repair reaction was determined as 1.1×10(9) L mol(-1) s(-1). These results demonstrated that non-enzymatic repair is a feasible method for repairing photosensitized DNA bases oxidation.     

LASER PHOTOLYSIS STUDIES OF QUINONE REDUCTION BY PHEOPHYTIN a IN ALCOHOL SOLUTION

Abstract- Upon laser photolysis of pheophytin-benzoquinone solutions in ethanol, transients due to the pheophytin triplet state (P_t), an exciplex (Pδ⁺ Qδ^-), the pheophytin cation radical (P⁺) and the semiquinone radical (Q^-) can be observed. Kinetic analysis indicates that the evolution of these transients at times longer than one microsecond is due to the decay of the exciplex with the concomitant formation of P⁺ and Q^-, reverse electron transfer to form P and Q, solvent oxidation by P⁺, and Q^- disproportionation. In support of the suggested solvent oxidation reaction, a large deuterium isotope effect is observed upon changing the solvent from methanol to its fully-deuterated counterpart. Comparisons are made between these results and those obtained with chlorophyll as described in the preceding paper.     

COMPARAISON ENTRE L''EMISSION D''OXYGENE ET L''EMISSION DE LUMINESCENCE A LA SUITE D''UNE SERIE D''ECLAIRS SATURANTS

Abstract— Luminescence of Chlorella cells has been measured after illumination by a series of short saturating flashes.
The intensity of luminescence is strongly dependent upon the number of flashes; luminescence is minimum after a single flash and maximum after a double flash illumination.
If l /_n is the intensity of luminescence measured 0.24 sec after the nth flash, the series l _n shows oscillations as a function of n. The series l _n is very similar to the series y _n, in which y is the amount of oxygen evolved by the nth flash, the term l _n corresponding to the term y _n+1. To interpret the oscillations of the series y _n, different models of system II photochemical centers have been proposed, one by ourselves and the other one by KOK et al. In these models the electron donor Z of photosystem 11 is supposed to have 2 or 4 levels of oxidation, the more oxidized state being the precursor of oxygen. These different oxidation states of the donor Z allow storage of the photic energy in the system. The correlation between the series y _n and l _n shows that at least one of the oxidized forms of the donor Z is a substrate for the luminescence reaction.     

Oxygen uptake after electron transfer from amines, amino acids and ascorbic acid to triplet flavins in air-saturated aqueous solution

The photolysis of lumichrome, riboflavin, flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) was studied in air-saturated aqueous solution at room temperature in the presence of appropriate electron donors: ascorbic acid, aromatic amino acids or amines, e.g. ethylenediaminetetraacetate (EDTA). The overall reaction is conversion of oxygen via the hydroperoxyl/superoxide radical into hydrogen peroxide. The quantum yield of oxygen uptake increases with the donor concentration, e.g. up to 0.3 for riboflavin, FMN or FAD in the presence of EDTA or ascorbic acid (0.3-10mM). The formation of H(2)O(2) is initiated by quenching of the acceptor triplet state by the electron donor and subsequent reaction of the semiquinone radical with oxygen. Specific properties of flavins are discussed including the radicals involved and the pH and concentration dependences. The quantum yield of photodegradation is low under air, but substantial under argon, where the major product absorbing in the visible spectral range is the corresponding hydroquinone.     

LUMIFLAVIN-SENSITIZED PHOTOOXYGENATION OF INDOLE

Abstract— The lumiflavin-sensitized photooxygenation of indole in aqueous solutions has been investigated by means of steady light photolysis and flash photolysis. The semiquinone of lumiflavin and the half-oxidized radical of indole were formed by the reaction between triplet lumiflavin and indole (3.7 times 10⁹ M ^-1 s^-1). The semiquinone anion radical of lumiflavin reacted with oxygen to form superoxide radical. The triplet state of lumiflavin also reacted with oxygen forming singlet oxygen, ¹O₂. But the reaction between ¹O₂ and indole (7 times 10⁷ M^_l s^_1; estimated from steady light photolysis using Rose Bengal as a sensitizer) was far less efficient than the reaction between indole and triplet lumiflavin. The quantum yield of the lumiflavin-sensitized photooxygenation of dilute indole via radical processes was much higher than that via ¹O₂ processes, though appreciable ¹O₂ was formed.     

A SURVEY OF EPR INVESTIGATIONS OF BACTERIAL PHOTOSYNTHESIS

Abstract— Investigations in which EPR has been used as a probe of the mechanism of the primary quantum conversion reaction and/or electron transport reactions in bacterial photosynthesis are surveyed. These investigations include studies of whole cell organisms and simpler sub-cellular preparations, chromatophores and bacteriochlorophyll-protein complexes. Electron paramagnetic resonance studies have successfully demonstrated that the primary donor of the photosynthetic, photochemical reaction involves a dimer of bacteriochlorophyll, generally referred to as P870. P870 is photochemically oxidized to a cation radical which exhibits a g= 2.0025 EPR signal. Comparative studies of the time behaviour of this signal in whole cells and in sub-cellular preparations show that electron flow in the whole cells is substantially different than in the cell-free systems. The primary acceptor molecule of the photochemical reaction has not been conclusively identified. When it is photochemically reduced, it exhibits a broad EPR absorbance centered at g= 1.8 observable only at low temperatures. This signal involves an iron atom and a quinone molecule. Two possible identifications of the species responsible for the g= 1.8 signal are an iron-quinone complex and an iron-sulfur protein. The latter identification would require that one primary acceptor function for two primary donors and that the removal of a tightly held quinone alter the integrity of the system so as to inhibit the photochemistry. When the primary acceptor is chemically reduced, a photo-induced, polarized triplet EPR spectrum is detected. Both absorption and emission lines are, observed as if only one substate (m= 0) of the triplet manifold is populated. The zero field parameters of the triplet spectrum suggest that the triplet is formed through a decay of a biradical, not through an optical singlet to triplet transition. Low temperature EPR studies of photosynthetic preparations which had been poised at room temperature by subjecting the preparations to different redox potentials and/or dark adaptation and illumination show the presence of a number of light-influenced, EPR active components. The spectral characteristics of these components are indicative of iron-heme (both low and high-spin forms) and iron-sulfur (both oxidized and reduced) proteins and at least one other organic free radical distinct from P870⁺. The spectra varied for different strains of bacteria. Also, some of the signals detected in the whole cell organism were not detected in cell free preparations and the kinetics of the light influenced signal amplitudes were different in the whole cells than in chromatophores.     

Pseudo cage effect in double energy transfer

In some systems, the donor of a triplet—triplet energy transfer can be sensitized in its singlet state through a singlet—singlet energy transfer (Dexter mechanism), where the donor is the acceptor of the triplet transfer itself. As a consequence an extra acceptor molecule in the triplet energy transfer is present in the vicinity of the donor, thus enhancing the efficiency of the transfer process. Experiments show clearly this effect and a diffusional model gives semi-quantitative agreement with the experimental data.     

TRIPLET FORMATION AND TRIPLET DECAY IN REACTION CENTERS FROM THE PHOTOSYNTHETIC BACTERIUM Rhodopseudomonas sphaeroides

Abstract— In the reaction center of photosynthetic bacteria, with the primary ubiquinone reduced, the triplet state P^R of the primary electron donor (a pair of bacteriochlorophylls named P) is PO ulated with a takes place in a few ns. We measured by flash absorption spectroscopy the influence of temperature on formation and decay kinetics of P^R and ³Car in the reaction center of several strains of R. sphaeroides . The rate of triplet energy transfer, measured as the decay of P^R after a flash, decreases when the temperature is lowered. Between 60 and 30 K the half-time of energy transfer becomes longer than the ³Car half-time decay (about 6 μs) and below 20 K the transfer is slower than the internal decay of P^R (about 100 μs). In several cases it is clear that P^R and ³Car decay independently and are not in thermal equilibrium. The singlet energy transfer from carotenoid to P occurs with a high efficiency at all temperatures.
The data can be accounted for on the basis of estimated energy levels of P^R and ³Car, in the context of the equilibrium ³P ←³D where ³P is the localized triplet state of P-870 and ³D is another triplet state. A reasonable kinetic scheme leads us to estimate that ³D is 0.0025 ± 0.005 eV above ³P. ³D may thus be the state observed by Shuvalov and Parson (1981). We propose that both triplet and singlet energy transfer between P and the carotenoid occur via a bacteriochlorophyll, to which the carotenoid should be tightly coupled via exchange interaction.     

PRIMARY PROCESSES IN THE PHOTOCHEMISTRY OF AQUEOUS SULPHACETAMIDE: A LASER FLASH PHOTOLYSIS and PULSE RADIOLYSIS STUDY

Abstract— The spectra have been measured of the transient species formed in the nanosecond flash photolysis of aqueous solutions of sulphacetamide under a variety of conditions. In addition to the excited triplet state, the cation radical and the solvated electron were observed. The ionisation of aqueous sulphacetamide was found to occur by a biphotonic process. The extinction coefficient of the cation radical of sulphacetamide was determined by both laser flash photolysis and pulse radiolysis techniques, a value of 1.9 times 10₃ dm₃mol_-1cm_-1 being obtained. The rate of electron reaction with sulphacetamide and the anion radical spectrum were also determined by the two techniques, good agreement being obtained. The spectrum of the product of the reaction of the superoxide anion radical and the corresponding rate constant have also been determined. A possible mechanism of photosensitized skin reaction due to sulphacetamide is discussed.