PHEOMELANIN PHOTOCHEMISTRY. PHOTOLYSIS OF MODEL COMPOUNDS

Abstract— Phytochrome control of nitrate reductase activity has been studied in cotyledons and hypocotyls of light-grown Sinapis alba. Under polychromatic irradiation, an increase in the fluence rate of far-red light added to a constant source of photosynthetically active radiation causes a decrease in the phytochrome photoequilibrium and, in the hypocotyl, this results in decreased nitrate reductase activity. However, in the cotyledons this difference is only observed transiently. In both organs, enzyme activity is correlated with the level of the far-red light absorbing form of phytochrome, P_fr. These correlations are not altered when the fluence rate (with respect to phytochrome) is increased, suggesting that the responses are not fluence rate dependent. The results obtained are consistent with the notion that in fully de-etiolated seedlings, P_ft alone controls nitrate reductase activity.     

TRAPPING OF NITRIC OXIDE FORMED DURING PHOTOLYSIS OF SODIUM NITROPRUSSIDE IN AQUEOUS AND LIPID PHASES: AN ELECTRON SPIN RESONANCE STUDY

Abstract— Photolytic decomposition of sodium nitroprusside (SNP), a widely used nitrovasodilator, produced nitric oxide (NO), which was continuously monitored by electron spin resonance (ESR) spectroscopy. The NO present in the aqueous or the lipid phase was trapped by either a hydrophilic or a hydrophobic nitronyl nitroxide, respectively, to form the corresponding imino nitroxide. The conversion of nitronyl nitroxide to imino nitroxide was monitored by ESR spectrometry. The quantum yield for the generation of NO from SNP, measured from the rate of decay of nitronyl nitroxide, was 0.201 ± 0.007 and 0.324 ± 0.01 (¯± SD, n = 3) at 420 nm and 320 nm, respectively. The action spectrum for NO generation was found to overlap the optical absorption spectrum of SNP closely. A mechanism for the reaction between SNP and nitronyl nitroxide in the presence of light is proposed and computer-aided simulation of this mechanism using published rate constants agreed well with experimental data. The methodology described here may be used to assay NO production continuously during photoactivation of NO donors in aqueous and lipid environments. Biological implications of this methodology are discussed.     

IN SITU GENERATION OF PSORALENS BY PHOTOLYSIS OF WATER-SOLDBLE PRECURSORS

Abstract— The poor water solubility of typical photochemotherapeutic psoralens restricts their utility in aqueous solutions and commonly requires the use of organic co-solvents in photobiological studies. This paper describes the preparation of readily water-soluble "pre-psoralens", (Z)-3-[5-(4,6-dimethoxy)benzofuranyl]propenoic acid (3) and (Z)-3-[5-(6,7-dimethoxy)benzofuranyl]propenoic acid (4), and their novel photocyclization in aqueous media to give 5-methoxypsoralen (5-MOP) and 8-methoxypsoralen (8-MOP), respectively. Quantum efficiencies, measured at 308 nm for the cyclization, are 12. 1 × 10^-3 for 3 → 5-MOP and 2.7 × 10^-3 for 4 → 8-MOP. 5-Methoxyisopsoralen (5-MOiP, 5) is a side product from the photolysis of 3. Photocross-linking of calf thymus DNA is effected when the "pre-psoralens" are irradiated with 308 or 355 nm (3 only) light.     

INTRAMOLECULAR TRAPPING OF THE NITRENE GENERATED BY PHOTOLYSIS OF S,S-DIPHENYL N-ACYLSULFILIMINE

Abstract

Various N-acyldiphenylsulfilimines (1a-k) were found to undergo photolysis in argon atmosphere, to afford the corresponding isocyanates, diphenyl sulfide and diphenyl disulfide. Attempts to trap intramolecularly the acylnitrene with olefinic linkage or sulfide afforded small amounts of the trapped products, whereas photolysis of N-mesitoyldiphenyl-sulfilimine (Ia) afforded a C–H inserted product of the nitrene in a substantial yield together with the isocyanate. Benzophenone-sensitized photolysis of Ia indicates that the nitrene generated is a singlet species.     

SPECTROSCOPIC STUDIES OF CUTANEOUS PHOTOSENSITIZING AGENTS—IX. A SPIN TRAPPING STUDY OF THE PHOTOLYSIS OF AMIODARONE AND DESETHYLAMIODARONE

The photolysis of amiodarone (AM) and its major metabolite mono-N-desethylamiodarone (DEA), has been studied by absorption spectroscopy, electron spin resonance spectroscopy (spin trapping) and oxygraph measurements. Changes in the absorption spectrum of both AM and DEA upon UV irradiation indicate that both drugs undergo deiodination. Spin trapping experiments with 2-methyl-2-nitrosopropane (MNP), α-phenyl-N- tert -butyl-nitrone (PBN) and 5.5-dimethyl-1-pyrroline-N-oxide (DMPO) suggest the formation of an aryl radical from AM during UV irradiation. Amiodarone also undergoes photoionization. Under aerobic conditions the photoelectron is scavenged by oxygen to give superoxide, which is trapped by DMPO. Oxygraph measurements further confirmed the consumption of oxygen and the generation of superoxide during the irradiation of aqueous solutions of AM. Deiodination, photoionization and superoxide formation were all observed at wavelengths as low as 335 nm, suggesting that some or all of these processes may be involved in AM-induced photosensitivity. The aryl radical derived from AM during UV irradiation abstracted a hydrogen atom from suitable donors (ethanol, glutathione, cysteine, linoleic acid). Reaction of the dienyl radical derived from linoleic acid would yield the corresponding peroxy radical thereby initiating lipid peroxidation. This would explain the deposition of lipofuscin, a pigment formed from the products of lipid peroxidation, in the skin of patients receiving AM.     

FLUOROMETRIC STUDY OF TRYPTOPHAN PHOTOLYSIS

Abstract— Measurements of fluorescence spectra and fluorescence intensity for tryptophan solutions at different pH show an effective decarboxylation and deamination of tryptophan molecules under UV irradiation. The nonexponential dose-relationship of decrease in total fluorescence of tryptophan solutions is due to the formation of the products retaining indole ring in the course of these reactions. Dose-relationships and quantum yields of indole ring photolysis, deamination and decarboxylation are determined for tryptophan at 254 nm irradiation. Indole ring destruction accounts for about 60% of the total photolysis of tryptophan. Decarboxylation of tryptophan is two times more effective than its deamination. In the absence of oxygen quantum yield of indole photolysis in tryptophan and in the products of decarboxylation and deamination is reduced by a factor of two and by approximately an order of magnitude, respectively. Tryptophan photolysis products which, when excited at 365 nm. fluoresce in the visible region are formed from an intermediate product of indole ring destruction.     

FREE RADICALS INDUCED BY PHOTOLYSIS OF FROZEN SOLUTIONS OF THYMIDINE-5'-MONOPHOSPHATE: AN ESR STUDY

Abstract— The photolysis (Δ < 220 nm) of thymidine-5'-monophosphate was studied by electron-spin resonance (ESR) in acidic and alkaline phases. In both cases, the H–addition radical at the C₆ position is detected at 77°K. At 225°K, a triplet 1:2:1 is observed, which suggests a H abstraction radical from the CH₃ group. When oxygen is present during irradiation, a peroxide–type radical is observed, which results partly from a reaction like R + O₂→ ROO and partly from an energy transfer from thymidine-5'-monophosphate to oxygen, probably in the ¹π₀ state.     

PHOTOLYSIS OF MELANIN PRECURSORS: FORMATION OF SEMIQUINONE RADICALS AND THEIR COMPLEXATION WITH DIAMAGNETIC METAL IONS

Abstract— Semiquinone free radicals have been generated by the UV photolysis of aqueous solutions of melanin precursors (catechols and catecholamines) and characterized by electron spin resonance (ESR) spectroscopy. Hyperfine parameters of the semiquinones are sensitive to the nature of the substituents on the aromatic ring, reflecting (i) the ionization state of the substituent; (ii) restricted rotation of methylene protons; and (iii) the presence of a chiral carbon center. In the presence of complexing metal ions (Zn²⁺, Cd²⁺) the semiquinones form chelate complexes whose ESR spectra differ from those of the uncomplexed radicals. Satellite peaks demonstrating hyperfine coupling to metal isotopes (⁶⁷Zn, ¹¹¹Cd and ¹¹³Cd) present in natural abundance were detected in several instances. Concentrations of metal complexes are much higher than those of the uncomplexed radicals, suggesting that diamagnetic metal ions may be useful in identifying the presence of semiquinone radicals of this kind in various systems.     

PHOTOLYSIS OF AMIODARONE, AN ANTIARRHYTHMIC DRUG

Abstract— The photochemical behaviour of amiodarone was examined in vitro in order to get more insight on the chemical reactions involved in the cutaneous phototoxicity processes . Irradiation at 300 nm of amiodarone degassed in ethanol solution leads to a photodehalogenation followed by a much slower α-cleavage reaction. Desethylamiodarone, the main metabolite of AD was found to undergo the same reaction as AD. Results of photosensitization and quenching experiments together with phosphorescence spectra indicated that the reaction proceeds via the triplet excited stateof amiodarone. Radical species formed during photolysis were identified by ESR spectroscopy. CH₃CHOH, HO₂ and an unidentified radical were detected using 5,5-dimethyl-1-pyrroline-1-oxide as spin trap. In aerated solutions, photosensitization of oxygen by amiodarone was demonstrated by adding singlet oxygen scavengers such as dimethylfuran and cholesterol. Overall, these results suggest that Type I and Type II mechanisms may take place in the phototoxicity of amiodarone and its metabolite.     

氯代烃光解活泼自由基的ESR研究

本文用2,4-二硝基苯亚甲基特丁基氮氧化物(DNPBN)作自由基捕捉剂研究了氯代烃光解过程产生的自由基中间体。实验结果表明在汞灯照射下键能较低的多氯代烷烃分子中的碳-氯键极易发生均裂而产生碳中心自由基与活泼的氯原子, 并且后者可被DNPBN捕获, 形成颇为稳定的氮氧自由基加合物[Cl-DNPBN]~·。对多氯代苯,随着取代氯原子数目增多, 加合物[Cl-DNPBN]~·的ESR信号强度也明显增加。此外, DNPBN对氯原子的大量捕捉实验还证明它对氯原子是一种高效的特征捕捉剂。     

A FLASH PHOTOLYSIS STUDY OF 1-METHYLINDOLE

Abstract— The conventional flash photolysis of 1-methylindole in aqueous media was studied at Λ_excitation≥290 nm. The transients observed 20 μs after excitation consisted mainly of the radical cation (R⁺). the hydrated electron (e^-_aq) and the triplet state (T). Electron counting experiments indicate that photoionization is the only source of R⁺ with e^-_aq/R⁺= 1.07±0.09 in neutral media. Quenching of the R⁺ yield with H⁺ indicates that the fluorescent state is the precursor to 80% of the photoionization events with the remainder probably arising from a prefluorescent state. The triplet decays with a lifetime of 29 μs in deaerated neutral media. This decay is unchanged by N₂O saturation, but T reacts with acrylamide with k ≥2.8 × 10⁹ M ^-1. In 2 M Br^-, R⁺ and T yields are increased by factors of 2–3. Consideration of fluorescence quenching and T enhancement by Br-permits an estimate of φ_Isc between 0.33 and 0.49. The increased R⁺yield at high Br-concentrations cannot be accounted for by induced photoionization or triplet state reactions.     

PHOTOCHEMISTRY OF BENZOTHIAZOLE MODELS OF PHEOMELANIN

Abstract— The photochemistry of several 4-hydroxy- and 4-methoxybenzothiazoles has been investigated by laser flash photolysis. In aqueous solutions of pH3–12, the 4-hydroxybenzothiazole chromophore undergoes monophotonic photoionization to afford e^-_aq with quantum yields on the order of 0.06; no evidence for triplet species was obtained. The spectra and stability of the resultant free radicals were determined using pulse radiolysis. In contrast, triplet transients with life-times on the order of 8 mUs are readily observable upon irradiation of the 4-methoxybenzothiazole analog. Triplet sensitization experiments with the water-soluble carotenoid crocetin were employed to obtain the triplet extinction coefficients and subsequently the triplet quantum yields. The significance of these differences in photochemical behavior is discussed in relationship to the photochemistry and photobiology of the epidermal melanin pigment pheomelanin.     

取代苯基重氮盐的直接光解和敏化光解的研究

本工作合成了一系列带取代基的苯基重氨盐化合物。对它们的直接光解研究表明:带推电子基的重氮盐化合物具有较高的光解反应速度,但在敏化光解研究中发现:带有拉电子基的重氟盐化合物,不论是它的敏化光解或是猝灭敏化剂荧光的能力都较带推电子基的重氮盐为强,这清晰地表明,此敏化过程是通过电子转移反应而实现的。工作中还发现,在基态条件下,重氮盐和N,N-二甲基苯胺间可生成电荷转移络合物(CTC),经Benesi-Hildebrand公式处理表明:可形成1:1的CTC。     

二甲基硫醚的光解特性

本文对二甲基硫醚(DMS)在有氧、无氧及有H₂O₂存在的情况下紫外光辐照。然后在20m长光程池中藉富里叶红外光谱仪测定光解产物的吸收光谱。实验表明在无氧的情况下DMS的C─S键断裂,并生成CH₃SSCH₃及C₂H₆,在有氧存在下生成HCHO及CH₃SO₃H,在有H₂O₂存在下生成CH₃SSCH₃.本文讨论了光解的可能机理,合理地解释光解产物的生成。     

SPECTROSCOPIC STUDIES OF CUTANEOUS PHOTOSENSITIZING AGENTS–XII. SPIN TRAPPING STUDY OF THE FREE RADICALS GENERATED DURING THE PHOTOLYSIS OF PHOTO ALLERGENS BITHIONOL and FENTICHLOR

Free radicals were trapped and observed by ESR when photoallergens bithionol and fentichlor were irradiated in the presence of spin traps N- t -butyl-α-phenylnitrone (PBN) and 5,5-dimethyl-pyrroline-N-oxide (DMPO). In the absence of air, both PBN and DMPO trapped a carbon-centered radical. The carbon-centered radical, which was capable of abstracting a hydrogen atom from cysteine, glutathione, ethanol and formate, was identified as an aryl radical derived from the homolytic cleavage of the carbon-chlorine bond. In the presence of air, both carbon-centered radicals and hydroxyl radicals were trapped by DMPO. Under similar conditions, the yield of the hydroxyl radicals was greater from bithionol than from fentichlor. The presence of the hydroxyl radical was confirmed by kinetic experiments employing hydroxyl radical scavengers (ethanol, formate). Superoxide and H₂O₂ were not involved. Experiments with oxygen-¹⁷O indicated that the hydroxyl radicals came exclusively from dissolved oxygen. The precursor of the hydroxyl radical is postulated to be a peroxy intermediate (ArOO*) derived from the reaction of an aryl radical (Ar*) with molecular oxygen. Both bithionol and fentichlor photoionized only when excited in the UVC (<270 nm) region. Free radicals have long been postulated in the photodechlorination of bithionol and fentichlor and the present study provides supporting evidence for such a mechanism. Aryl and hydroxyl radicals are reactive chemical species which may trigger a series of events that culminate in photoallergy.     

A FLASH PHOTOLYSIS STUDY OF 5-METHOXYINDOLE

Abstract— The microsecond flash photolysis of 5-methoxyindole in aqueous solutions has been studied at γ_exc≥ 290 nm. Transients identified in this time realm in neutral solutions are: e_aq^-, the 5-methoxyindole radical cation (γ_max≅ 440 nm), the neutral transient with γ_max≅ 530 nm) and an unidentified oxygen sensitive transient with γ_max≅ 435 nm. Radical cations and e^-_aq are shown to be produced in equal amounts consistent with a photoionization process as the only source of both transients. H⁺ quenching of fluorescence and radical cation production gives equivalent Stern-Volmer constants indicating that photoionization occurs from the fluorescent state. The unidentified oxygen sensitive transient exhibits a pK _a of2–2.5 and is quenched at lower pH values indicating that it also has a fluorescent state precursor.     

SPECTROSCOPIC STUDIES OF CUTANEOUS PHOTOSENSITIZING AGENTS–III. SPIN TRAPPING OF PHOTOLYSIS PRODUCTS FROM SULFANILAMIDE ANALOGS

The photodecomposition of sulfanilamide (4-aminobenzenesulfonamide), sulfacetamide. sulfathiazole. sulfadiazine, carbutamide and tolbutamide has been studied using the spin traps 2-methyl-2-nitrosopropanc and 5,5-dimethyl-l-pyrroline-l-oxide. The following radicals were trapped during the photolysis of sulfanilamide in aqueous solution: H' and HNC₆H₄SO₂NH, (α-fission). SO₂NH₂ and C₆H₄NH₂ (δ fission). H₂NC₆H₄SO₂ and NH₂ (δ-fission). Although the C.,H₄SO₂NH₂ and the SO; radicals were also detected these were not formed directly by homolytic bond fission. Homolytic bond fission was also observed during the irradiation of sulfacetamide (α.δ), sulfadiazine (α). carbutamide (α,δ) and tolbutamide (δ). All of the analogs, with the exception of tolbutamide, generated the SO; radical. Sulfacetamide, sulfadiazine and carbutamide generated the C₆H₄SO_2;NHR radical by some process that did not involve homolytic bond fission. The free radicals generated by these agents may play an important role in their phototoxic and photoallergic effects.     

ULTRAVIOLET PHOTOLYSIS OF RIBOFLAVIN IN PYRIDINE: A PRELIMINARY STUDY

Abstract— Ultraviolet photolysis of riboflavin in pyridine leads to the formation of a red photoproduct. This photoreaction is apparently attributed to the photochemical interaction of pyridine with riboflavin. It is found that the rate of the photoreaction follows 1/2 order kinetics with respect to pyridine. ESR spectrum of the irradiated reaction mixture appears to be identical with that of riboflavin semiquinone obtained from visible light irradiation. Whether or not such a flavin radical is the responsible intermediate in the photoreaction is not established. Possible modes of the reaction are discussed. Absorption, excitation, and emission spectra of the red photoproduct strongly suggest that the structure of the photoproduct is considerably different from that of riboflavin or pyridine, indicating an extensive photolytic process involving covalent bond alterations.     

THE BIPHOTONIC PHOTOIONIZATION OF CHLORPROMAZINE DURING CONVENTIONAL FLASH PHOTOLYSIS: SPIN TRAPPING RESULTS WITH 5,5-DIMETHYL-1-PYRROLINE-N-OXIDE

A novel combination of conventional flash photolysis and electron spin resonance (ESR) spin-trapping has been used to demonstrate that photoionization of chlorpromazine (CPZ), and the concomitant production of hydrated electron, occurs through a stepwise biphotonic mechanism during conventional flash photolysis at wavelengths above 290 nm. The production of hydrated electron in the flash photolysis experiment has been monitored and quantified through the use of the spin trapping agent, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). The effects of nitrous oxide, varying concentrations of CPZ and DMPO, and a range of flash intensities on the ESR spectra of the observed spin adducts of DMPO are discussed. The use of ESR spin trapping to monitor hydrated electron yields in flash photolysis experiments has the potential to permit the use of a much wider range of flash intensities than is typically possible with conventional optical experiments. Thus, there is a greater possibility of distinguishing between monophotonic and biphotonic processes.     

MECHANISM OF THE PHOTOHYDRATION OF PYRIMIDINES: A FLASH PHOTOLYSIS STUDY*

Abstract— Based on comparisons between the microsecond flash photolysis of uracil and substituted uracils and the radiation chemistry of the corresponding 5,6-dihydro-2,4-dioxopyrimidines, it is proposed that the mechanism of photohydration of pyrimidine bases involves the formation of a pyrimidine carbocation. The effects of substitution in the pyrimidine ring and of pH are consistent with a proton transfer from water to the excited singlet state at theC–5 position of the pyrimidine ring. The resultant carbocation is thought to undergo solvolysis to form the photohydrate or eliminate a proton at N-l producing an intermediate isomeric form of the pyrimidine (isopyrimidine) which re-arranges to the parent pyrimidine by a first-order process.