ANTHRALIN-DERIVED TRANSIENTS—II. FORMATION OF THE RADICAL BY SPONTANEOUS FRAGMENTATION OF BOTH SINGLET AND TRIPLET STATES OF THE 10,10''-DEHYDRODIMER: RADICAL PAIR MULTIPLICITY EFFECTS

The singlet and triplet states of the anthralin (1,8-dihydroxy-9-anthrone) dehydrodimer have been produced selectively in benzene via pulsed laser excitation and pulse radiolysis respectively. The lifetime of S1 is less than or equal to 30 ps, that of T1 short but unspecified. Both states fragment spontaneously to yield a pair of anthralin radicals. The singlet radical pair predominantly undergoes geminate recombination within the solvent cage. In contrast, the corresponding triplet radical pair undergoes essentially exclusive cage escape to give the anthralin free radical (lambda max 370, 490 and 720 nm) which recombines under normal diffusive conditions. Both recombination processes lead, at least in part, to one or more species which have been assigned as tautomeric forms of the original dimer. The anthralin free radical in benzene is insensitive to the vitamin E model 6-hydroxy-2,2,5,7,8-pentamethylchroman and reacts only slowly with oxygen.     

Photochemical studies of A2-E.

Lipofuscin is thought to be involved in age-related macular degeneration as is one of its proposed components, an amphiphillic pyridinium-based bis-retinoid with a quaternary nitrogen atom, known as A2-E. We report the triplet state spectra obtained from photosensitisation using anthracene and 1-nitronaphthalene in benzene and methanol. The triplet state of A2-E has lambda(max) at 550 nm and a lifetime of approximately 30 micros, it is efficiently quenched by molecular oxygen with a second-order quenching rate constant of approximately 1 x 10(9) dm(3) mol(-1) s(-1). There is no significant triplet state formation from direct laser excitation of A2-E and hence its quantum yield of triplet state formation must be <0.01.     

TWO NEW STERICALLY HINDERED PHTHALOCYANINES: SYNTHETIC AND PHOTODYNAMIC ASPECTS

The synthesis and the properties of 1,4-dibutoxy-2,3-dicyanotriptycene, of a metal-free tetradibenzobarrelenooctabutoxyphthalocyanine, and of the corresponding zinc phthalocyanine are described. The two phthalocyanines do not aggregate when dissolved in benzene at concentrations up to 450 microM. For the metal-free and the zinc compounds, the red band maxima are at 736 and 757 nm, the triplet maxima are at 590 and 605 nm, the triplet state lifetimes are 58 and 177 microseconds, and the protoporphyrin-IX dimethyl ester-to-compound bimolecular rate constants for triplet energy transfer are 2.61 x 10(8) and 1.47 x 10(8) M-1 s-1. Triplet energy transfer from the metal-free compound to O2 is endoergonic by 1.0 kcal mol-1. The potential of the zinc compound for photodynamic therapy is touched upon.     

Type I and type II photosensitization by the antibacterial drug nalidixic acid. A laser flash photolysis study.

The 355 nm laser flash photolysis of nalidixic acid at pH 9.2 leads to the formation of the nalidixate anion triplet state (absorption lambda max = 620 nm; 5700 less than or equal to epsilon T less than or equal to 9000 M-1cm-1; 0.6 less than or equal to phi T less than or equal to 1). The first order triplet state decay (kT = 7.7 x 10(3) s-1) is accompanied by a diffusion controlled triplet-triplet annihilation. Oxygen efficiently quenches the triplet state (k = 3.2 x 10(9) M-1s-1). The nalidixate radical dianion (absorption lambda max = 650 nm; epsilon = 3000 M-1cm-1) is produced by the diffusion controlled reductive quenching of the triplet state by tryptophan and tyrosine. The superoxide anion (O2-.) is produced by diffusion controlled reaction of the radical dianion with oxygen. The O2-. is characterized by its reactions with ferricytochrome c and superoxide dismutase. The physiological form of nalidixic acid is thus a good Type I and Type II photosensitizer.     

A photochemical study of (E,E,E,E)-2-[9-(2-hydroxyethyl)imino-3,7-dimethyl-1,3,5,7-decatrien-1-yl]- 1,3,3-trimethylcyclohexene, a derivative of all-trans-retinal and ethanolamine

A derivative of all-trans-retinal (RAL) and ethanolamine, A2-E, is the main fluorescent component of human retinal lipofuscin. The accumulation of lipofuscin has been correlated with exposure to ambient radiation and loss of photoreceptors. A possible precursor to A2-E is the imine formed from RAL and ethanolamine. This compound, (E,E,E,E)-2-[9-(2-hydroxyethyl)imino-3,7-dimethyl-1,3,5,7- decatrien-1-yl]-1,3,3-trimethylcyclohexene (HIDD), has been synthesized and structurally characterized. The photophysical and photochemical properties of HIDD and its protonated form, HIDD-H+, have been investigated using steady-state and time-resolved methods. Both HIDD and HIDD-H+ are weakly fluorescent, and the fluorescence lifetime and quantum yield for HIDD are ca 0.6 ns and 4.0 +/- 0.5 x 10(-4), respectively. HIDD forms a triplet excited state on direct excitation that decays with kd = 3.4 x 10(4) s-1. The extinction coefficient and quantum yield of intersystem crossing for the HIDD triplet are measured as 7.6 +/- 1.3 x 10(4) M-1 cm-1 and 0.055 +/- 0.006, respectively. The triplet excited state of HIDD-H+ can be sensitized by triplet energy transfer and has a decay rate constant of 1.6 x 10(4) s-1. The lifetime of the HIDD triplet excited state is observed to decrease with increasing concentration of the well-known electron or hydrogen atom donors, 2,3,5,6-tetramethyl-1,4-phenylenediamine and 2,3,5-trimethylhydroquinone, and the bimolecular rate constants for these reactions are approximately 5.4 x 10(6) M-1 s-1 and 1.7 x 10(8) M-1 s-1, respectively. These types of reactions may model photooxidative mechanisms of damage in the retina.     

The photophysics of monomeric bacteriochlorophylls c and d and their derivatives: properties of the triplet state and singlet oxygen photogeneration and quenching

Measurements of pigment triplet-triplet absorption, pigment phosphorescence and photosensitized singlet oxygen luminescence were carried out on solutions containing monomeric bacteriochlorophylls (Bchl) c and d, isolated from green photosynthetic bacteria, and their magnesium-free and farnesyl-free analogs. The energies of the pigment triplet states fell in the range 1.29-1.34 eV. The triplet lifetimes in aerobic solutions were 200-250 ns; they increased to 280 +/- 70 microseconds after nitrogen purging in liquid solutions and to 0.7-2.1 ms in a solid matrix at ambient or liquid nitrogen temperatures. Rate constants for quenching of the pigment triplet state by oxygen were (2.0-2.5) x 10(9) M-1 s-1, which is close to 1/9 of the rate constant for diffusion-controlled reactions. This quenching was accompanied by singlet oxygen formation. The quantum yields for the triplet state formation and singlet oxygen production were 55-75% in air-saturated solutions. Singlet oxygen quenching by ground-state pigment molecules was observed. Quenching was the most efficient for magnesium-containing pigments, kq = (0.31-1.2) x 10(9) M-1 s-1. It is caused mainly by a physical process of singlet oxygen (1O2) deactivation. Thus, Bchl c and d and their derivatives, as well as chlorophyll and Bchl a, combine a high efficiency of singlet oxygen production with the ability to protect photochemical and photobiological systems against damage by singlet oxygen.     

Primary photophysical properties of ofloxacin

Steady-state fluorescence has been used to study the excited singlet state of ofloxacin (OFLX) in aqueous solutions. Fluorescence emission was found to be pH dependent, with a maximum quantum yield of 0.17 at pH 7. Two pKa*s of around 2 and 8.5 were obtained for the excited singlet state. Laser flash photolysis and pulse radiolysis have been used to study the excited states and free radicals of OFLX in aqueous solutions. OFLX undergoes monophotonic photoionization from the excited singlet state with a quantum yield of 0.2. The cation radical so produced absorbs maximally at 770 nm with an extinction coefficient of 5000 +/- 500 dm3 mol-1 cm-1. This is confirmed by one-electron oxidation in the pulse radiolysis experiments. The hydrated electron produced in the photoionization process reacts with ground state OFLX with a rate constant of 2.0 +/- 0.2 x 10(10) dm3 mol-1 s-1, and the anion thus produced has two absorption bands at 410 nm (extinction coefficient = 3000 +/- 300 dm3 mol-1 cm-1) and at 530 nm. Triplet-triplet absorption has a maximum at 610 nm with an extinction coefficient of 11,000 +/- 1500 dm3 mol-1 cm-1. The quantum yield of triplet formation has been determined to be 0.33 +/- 0.05. In the presence of oxygen, the triplet reacts to form both excited singlet oxygen and superoxide anion with quantum yields of 0.13 and < or = 0.2, respectively. Moreover, superoxide anion is also formed by the reaction of oxygen with the hydrated electron from photoionization. Hence the photosensitivity due to OFLX could be initiated by the oxygen radicals and/or by OFLX radicals acting as haptens.     

SINGLET AND TRIPLET REACTIVITY IN THE PHOTOREDUCTION OF OXONINE(3,7-DIAMINOPHENOXAZIN-5-IUM CHLORIDE) BY IRON (II)

The oxazine dye, oxonine (3,7-diaminophenoxazin-5-ium chloride), 1, is photoreduced by Fe (II) sulfate in dilute sulfuric acid. The reaction mechanism is analogous to that for the photo-reduction of thiazine dyes by Fe (II), the most important difference being that reduction of oxonine occurs predominantly from its excited singlet state, S1, rather than from the triplet state, T1. The latter is formed with an intersystem crossing (isc) quantum yield of ca 1.7 x 10(-3). The quenching of S1 by Fe (II) has a rate constant kSQ = 2.2 +/- 0.1 x 10(9) M-1 s-1 and affords the one electron reduced product, semioxonine (R), with a limiting quantum yield, phi SR, of 0.26 +/- 0.02. In contrast, quenching of T1, generated by bromide ion quenching of S1 or by diacetyl sensitization, occurs with KTQ approximately 1.2 x 10(6) M-1 s-1, extrapolated to zero ionic strength, and affords R with a limiting probability, phi TR = 1.1 +/- 0.2. Three possible reasons for the lower quantum yield of the more exothermic S1 reduction are discussed. These are energy transfer from S1 to Fe (II), different rates of escape of R from the encounter complex as a consequence of the different states of protonation of R as initially formed from S1 and T1, and spin allowed back electron transfer in an exciplex formed between S1 and Fe (II). Evidence is also presented for a very low probability (ca 1%) induced isc from the encounter of S1 with paramagnetic Fe (II). Rate parameters for other processes important to the overall reduction mechanism such as disproportionation of R to leucooxonine L and oxonine, k(R)DIS = 1.7 +/- 0.2 x 10(9) M-1 s-1, oxidation of R by Fe (III), k(R)OX = 1.5 +/- 0.1 x 10(5) M-1 s-1, and oxidation of L by Fe (III), kLOX = 1.1 +/- 0.1 x 10(3) M-1 s-1, have also been measured. These results are contrasted with those for the closely related thionine/Fe(II) photoredox reaction, the most well understood system for photogalvanic energy conversion.     

PHOTOPROPERTIES OF ALKOXY-SUBSTITUTED PHTHALOCYANINES WITH DEEP-RED OPTICAL ABSORBANCE

Triplet-state properties of 1,4,8,11,15,18,22,25-octa-n-butoxyphthalocyanine and its zinc derivative were determined for the first time. The T1 state of the metal-free phthalocyanine was characterized by a short lifetime (tau T = 17 microseconds) and low quantum yield (phi T = 0.095), and quenching of the triplet by O2 occurred with a bimolecular rate constant (kT sigma = 1.3 x 10(8) M-1 s-1) that is indicative of an endogonic reaction. The zinc complex (ZnPc(OBu)8) was markedly better as a triplet photosensitizer with respect to both tau T (60 microseconds) and phi T (0.5). Quenching by O2 produced singlet oxygen with nearly 100% efficiency, and kT sigma (1.7 x 10(9) M-1s-1) was close to the spin-statistical diffusion-controlled limit. Phosphorescence measurements showed the energy of the T1 state of ZnPc(OBu)8 to be 100 kJ/mol, which is 6 kJ/mol above the 1 delta g state of O2. These photoproperties, together with Q-band absorption maxima in the mid-700 nm range indicate that metal-centered 1,4,8,11,15,18,22,25-octaalkoxyphthalocyanines have excellent potential as sensitizers in photodynamic therapy.     

Kinetics of manganese(III) acetate in acetic acid: generation of Mn(III) with Co(III), Ce(IV), and dibromide radicals; reactions of Mn(III) with Mn(II), Co(II), hydrogen bromide,and alkali bromides

The reaction of cobalt(III) acetate with excess manganese(II) acetate in acetic acid occurs in two stages, since the two forms Co(IIIc) and Co(IIIs) are not rapidly equilibrated and thus react independently. The rate constants at 24.5 degrees C are kc = 37.1 +/- 0.6 L mol-1 s-1 and ks = 6.8 +/- 0.2 L mol-1 s-1 at 24.5 degrees C in glacial acetic acid. The Mn(III) produced forms a dinuclear complex with the excess of Mn(II). This was studied independently and is characterized by the rate constant (3.43 +/- 0.01) x 10(2) L mol-1 s-1 at 24.5 degrees C. A similar interaction between Mn(III) and Co(II) is substantially slower, with k = (3.73 +/- 0.05) x 10(-1) L mol-1 s-1 at 24.5 degrees C. Mn(II) is also oxidized by Ce(IV), according to the rate law -d[Ce(IV)]/dt = k[Mn(II)]2[Ce(IV)], where k = (6.0 +/- 0.2) x 10(4) L2 mol-2 s-1. The reaction between Mn(II) and HBr2., believed to be involved in the mechanism by which Mn(III) oxidizes HBr, was studied by laser photolysis; the rate constant is (1.48 +/- 0.04) x 10(8) L mol-1 s-1 at approximately 23 degrees C in HOAc. Oxidation of Co(II) by HBr2. has the rate constant (3.0 +/- 0.1) x 10(7) L mol-1 s-1. The oxidation of HBr by Mn(III) is second order with respect to [HBr]; k = (4.10 +/- 0.08) x 10(5) L2 mol-2 s-1 at 4.5 degrees C in 10% aqueous HOAc. Similar reactions with alkali metal bromides were studied; their rate constants are 17-23 times smaller. This noncomplementary reaction is believed to follow that rate law so that HBr2. and not Br. (higher in Gibbs energy by 0.3 V) can serve as the intermediate. The analysis of the reaction steps then requires that the oxidation of HBr2. to Br2 by Mn(III) be diffusion controlled, which is consistent with the driving force and seemingly minor reorganization.     

Mechanisms of photoinitiated cleavage of DNA by 1,8-naphthalimide derivatives.

Using water-soluble 1,8-naphthalimide derivatives, the mechanisms of photosensitized DNA damage have been elucidated. Specifically, a comparison of rate constants for the photoinduced relaxation of supercoiled to circular DNA, as a function of dissolved halide, oxygen and naphthalimide concentration, has been carried out. The singlet excited states of the naphthalimide derivatives were quenched by chloride, bromide and iodide. In all cases the quenching products were naphthalimide triplet states, produced by induced intersystem crossing within the collision complex. Similarly, the halides were found to quench the triplet excited state of the 1,8-naphthalimide derivatives by an electron transfer mechanism. Bimolecular rate constants were < 10(5) M-1 s-1 for quenching by bromide and chloride. As expected from thermodynamic considerations quenching by iodide was 6.7 x 10(9) and 8.8 x 10(9) M-1 s-1 for the two 1,8-naphthalimide derivatives employed. At sufficiently high ground-state concentration self-quenching of the naphthalimide triplet excited state also occurs. The photosensitized conversion of supercoiled to circular DNA is fastest when self-quenching reactions are favored. The results suggest that, in the case of 1,8-naphthalimide derivatives, radicals derived from quenching of the triplet state by ground-state chromophores are more effective in cleaving DNA than reactive oxygen species or radicals derived from halogen atoms.     

Photochromic dibenzobarrlenes: long-lived triplet biradical intermediates

Upon exposure to UV light, the disubstituted dibenzobarrelene derivative 1a turns green in the solid phase and reverts back to its original pale-yellow color within several hours in the dark. The lifetime of the colored species in degassed benzene at room temperature is 37 +/- 2 s (Ea for decoloration is 14.5 +/- 0.7 kcal mol-1 and log A is 8.92 +/- 0.5 s-1) and highly sensitive to molecular oxygen; the Stern-Volmer quenching constant is 6.9 +/- 0.2 x 108 M-1 s-1. Similarly, the disubstituted dibenzobarrelenes 1b and 1c exhibited pink coloration when exposed to UV light in the solid phase. On the basis of combined experimental and theoretical evidence, it is proposed that upon photoexcitation the excited singlet state of 1a undergoes rapid intersystem crossing to its triplet state, followed by intramolecular delta-H abstraction, to yield the triplet biradical intermediate (3)2. Upon prolonged irradiation, 2 undergoes cyclization to the alcohol 3, which affords the enone 4 as the final photoproduct. The delta-H abstraction on the triplet-state potential energy surface, calculated at the B3LYP/6-31G* level of density functional theory (DFT), has an activation energy of 18.5 kcal/mol. Further, the absorption spectrum of the triplet biradical (3)2, obtained from time-dependent DFT calculations, displays an intense absorption maximum at 670 nm, which is in good agreement with the observed absorption peak at 700 nm. The molecular-orbital analysis of the triplet diradical (3)2 suggests that its long-wavelength absorption involves the transition of the unpaired electron from the comparatively localized benzyl-type HOMO to the extensively conjugated benzoyl-type LUMO. The present experimental and theoretical results strongly support the intervention of a long-lived triplet biradical (3)2 in the photochromism of appropriately substituted dibenzobarrelenes.     

Time-Resolved Studies of the Excited-State Dynamics of meso-Tetra(hydroxylphenyl)chlorin in Solution

Meso-tetra(hydroxyphenyl)chlorin (m-THPC) is a new photosensitizer developed for potential use in photodynamic therapy (PDT) for cancer treatment. In PDT, the accepted mechanism of tumor destruction involves the formation of excited singlet oxygen via intermolecular energy transfer from the excited triplet-state dye to the ground triplet-state oxygen. Femtosecond transient absorption measurements are reported here for the excited singlet state dynamics of m-THPC in solution. The observed early time kinetics were best fit using a triple exponential function with time constants of 350 fs, 80 ps and > or = 3.3 ns. The fastest decay (350 fs) was attributed to either internal conversion from S2 to S1 or vibrational relaxation in S2. Multichannel time-resolved absorption and emission spectroscopies were also used to characterize the excited singlet and triplet states of the dye on nanosecond to microsecond time scales at varying concentrations of oxygen. The nanosecond time-resolved absorption data were fit with a double exponential with time constants of 14 ns and 250 ns in ambient air, corresponding to lifetimes of the S1 and T1 states, respectively. The decay of the T1 state varied linearly with oxygen concentration, from which the intrinsic decay rate constant, ki, of 1.5 x 10(6) s-1 and the biomolecular collisional quenching constant, kc, of 1.7 x 10(9) M-1 s-1 were determined. The lifetime of the S1 state of 10 ns was confirmed by fluorescence measurements. It was found to be independent of oxygen concentration and longer than lifetimes of other photosensitizers.     

Ozonolysis of vinyl compounds,CH2=CH-X,in aqueous solution--the chemistries of the ensuing formyl compounds and hydroperoxides

Reactions of ozone with some vinyl compounds of the general structure CH2=CH-X were studied in aqueous solution. Rate constants (in brackets, unit: dm3 mol-1 s-1) were determined: acrylonitrile (670), vinyl acetate (1.6 x 10(5)), vinylsulfonic acid (anion, 8.3 x 10(3)), vinyl phenylsulfonate (ca. 200), vinyl diethylphosphonate (3.3 x 10(3)), vinylphosphonic acid (acid, 1 x 10(4); mono-anion, 2.7 x 10(4); di-anion, 1 x 10(5)), vinyl bromide (1 x 10(4)). The main pathway leads to the formation of HOOCH2OH and HC(O)X. As measured by stopped flow with conductometric detection, the latter one may undergo rapid hydrolysis by water, e.g. HC(O)CN (3 s-1). Other HC(O)X hydrolyse much slower, e.g. HC(O)PO3(Et)2 (7 x 10(-3) s-1) and HC(O)P(OH)O2- (too slow to be measured). The OH(-)-induced hydrolyses range from ca. 5 dm3 mol-1 s-1 [HC(O)PO(3)2-] to 3.8 x 10(5) dm3 mol-1 s-1 [HC(O)CN]. HC(O)Br mainly decomposes rapidly (too fast for the determination of the rate) into CO and Br- plus H+, and the competing hydrolysis is of minor importance (3.7%). The slow hydrolysis of HC(O)PO(3)2- at pH 10.2, where HOOCH2OH is rapidly decomposed into CH2O plus H2O2, allows an H2O2-induced decomposition (k = 260 dm3 mol-1 s-1) to take place. Formate and phosphate are the final products.     

C60-地塞米松的激光激发

利用激光光解装置检测了C60-地塞米松(C60-DE)的苯溶液在355 nm激光照射下产生的激发三重态, 3C60-DE*出现四个吸收峰, 分别位于700、440、350 和310 nm. 在330 nm处观察到了它的漂白吸收最大值, 这与其基态吸收最大值相对应. 3C60-DE*能够将能量转移给O2分子而淬灭. 与3C60*相比, 3C60-DE*的三重态鄄三重态(T-T)淬灭速率常数减小(3C60*为(5.03±1.31)×109 L·mol-1·s-1, 3C60-DE*为(3.53±0.87)×109 L·mol-1·s-1), 而寿命增加了(3C60*为(12.0±2.6) μs, 3C60-DE*为(18.0±3.3) μs), 这可能是C60分子上连接了地塞米松分子后减小了C60球之间碰撞的几率所致.     

Laser flash photolysis of 1,2-diketopyracene and a theoretical study of the phenolic hydrogen abstraction by the triplet state of cyclic alpha-diketones

Laser flash photolysis (LFP) studies, atoms in molecules (AIM) studies, and density functional theory (DFT) calculations have been performed in order to study the mechanism of the hydrogen abstraction by alpha-diketones in the presence of phenols. Laser irradiation of a degassed solution of 1,2-diketopyracene in acetonitrile resulted in the formation of a readily detectable transient with absorption at 610 nm, but with very low absorptivity. This transient decays with a lifetime of around 2 micros. The quenching rate constant for substituted phenols, kq, ranged from 1.10x10(8) L mol-1 s-1 (4-cyanophenol) to 3.87x10(9) L mol-1 s-1 (4-hydroxyphenol). The Hammett plot for the reaction of the triplet of 1,2-diketopyracene with phenols gave a reaction constant rho=-0.9. DFT calculations (UB3LYP/6-311++G**//UB3LYP/6-31G*) of the triplet complex ketone-phenol revealed that hydrogen transfer has predominantly occurred and that the reaction with alpha-diketones are generally 7 kcal/mol less endothermic than the respective reactions of the monoketones. These results together with the geometries obtained from the DFT calculations, natural bond order (NBO) analysis, and AIM results indicate that hydrogen abstraction for alpha-diketones is facilitated by the electrophilicity of the ketone, instead of neighboring group participation by the second carbonyl group.     

A meta effect in organic photochemistry? The case of SN1 reactions in methoxyphenyl derivatives

The photochemistry of isomeric methoxyphenyl chlorides and phosphates has been examined in different solvents (and in the presence of benzene) and found to involve the triplet state. With the chlorides, C-Cl bond homolysis occurs in cyclohexane and is superseded by heterolysis in polar media, while the phosphate group is detached (heterolytically) only in polar solvents. Under such conditions, the isomeric triplet methoxyphenyl cations are the first formed intermediates from both precursors, but intersystem crossing (isc) to the singlets can take place. Solvent addition (forming the acetanilide in MeCN, the ethers in alcohols, overall a SN1 solvolysis) is a diagnostic reaction for the singlet cation, as reduction and trapping by benzene are for the corresponding triplet. Solvolysis is most important with the meta isomer, for which the singlet is calculated (UB3LYP/6-31 g(d)) to be the ground state of the cation (DeltaE = 4 kcal/mol) and isc is efficient (kisc ca. 1 x 108 s-1), and occurs to some extent with the para isomer (isoenergetic spin states, kisc ca. 1.7 x 106 s-1). The triplet is the ground state with the ortho isomer, and in that case isc does not compete, although trapping by benzene is slow because of the hindering of C1 by the substituent. The position of the substituent thus determines the energetic order of the cation spin states, in particular through the selective stabilization of the singlet by the m-methoxy group, a novel case of "meta effect".     

The isomerization of dinitrogen tetroxide: O2N-NO2 --> ONO-NO2

The N2O4 isomerization in gas phase has an energy barrier of 31 kcal mol-1 at 298 K. This energy barrier may be reduced due to the interaction of the N2O4 isomers with water or nitric acid clusters adsorbed on surfaces. The Gibbs free energy barrier for this reaction in water medium is estimated to be reduced to 21.1 kcal mol-1 by using the ab initio calculations and the polarizable continuum model (PCM). By using the transition state theory (TST), this model estimates that the N2O4 isomerization may be as fast as 2.0 x 10(-3) s-1 in aqueous phase at room temperature, which confirms the Finlayson-Pitts model for the heterogeneous hydrolysis of NO2 on surfaces. The activation energy of the N2O4 isomerization is about 21 kcal mol-1. The rate coefficient for this reaction is considerably fast, 1.2 x 10(-2) s-1, in aqueous phase at T = 373 K.     

Absolute rate constants for the quenching of reactive excited states by melanin and related 5,6-dihydroxyindole metabolites: implications for their antioxidant activity

The triplet-excited state of benzophenone and the singlet-excited state of 2,3-diazabicyclo[2.2.2]oct-2-ene (Fluorazophore-P) have been employed as kinetic probes to obtain information on the antioxidant activity of the skin and eye pigment melanin and its biogenetic precursors 5,6-dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA). The excited states were generated by the laser-flash photolysis technique and their reaction kinetics was examined by time-resolved transient absorption or fluorescence spectroscopy, respectively. The reaction between triplet benzophenone and DHI produced with unit efficiency the corresponding 6O-centered semiquinone radical, which was characterized by its characteristic transient absorption. The quenching rate constants for DHI (3.1-8.4 x 10(9) M-1 s-1) and DHICA (3.3-5.5 x 10(9) M-1 s-1) were near the diffusion-controlled limit, indicating excellent antioxidant properties. Kinetic solvent effects were observed. The reactivity of synthetic melanin, assessed through the quenching rate constant of Fluorazophore-P and normalized to the number of monomer units, was more than one order of magnitude lower (2.7 x 10(8) M-1 s-1) than that of its precursors. The trend of the quenching rate constants, i.e. DHI > DHICA approximately alpha-tocopherol > melanin, along with the preferential solubility of DHICA in aqueous environments, serves to account for several experimental results from biochemical studies on the inhibition of lipid peroxidation by these natural antioxidants.     

Singlet Oxygen-mediated Photobleaching of the Prosthetic Group in Hemoglobins and C-Phycocyanin

Proteins bearing colored prosthetic groups, such as the heme group in hemoglobin or the bilin group in c-phycocyanin, quench singlet oxygen by interactions at the apoprotein and the prosthetic group levels. In both proteins, chemical modification of the chromophore constitutes only a minor reaction pathway. While total deactivation of singlet oxygen takes place with rate constants of 4.0 x 10(9) and 4.2 x 10(8) M-1 s-1 for hemoglobin and phycocyanin, respectively, the bleaching of the chromophore takes place with rate constants of 3.2 x 10(6) and approximately 1 x 10(7) M-1 s-1. Irradiation of phycocyanin with red light bleaches the chromophore with low yields (approximately 0.8 x 10(-4)). Part of this bleaching is mediated by singlet oxygen produced by the irradiation of the bilin group. The low relevance of the singlet oxygen pathway is compatible with a low quantum yield (approximately 10(-3)) of free singlet oxygen production after irradiation of the protein.