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.     

PHOTOCHEMISTRY OF PHEOMELANIN: ACTION SPECTRUM FOR SUPEROXIDE PRODUCTION

Abstract— The action spectrum for superoxide production from aerated aqueous solutions of pheomela-nin was determined by utilizing the nitroblue tetrazolium-superoxide dismutase assay for superoxide. Superoxide production was greatest in the UVC regions, but continued well into the visible wavelengths. The marked increase in superoxide production noted in the UVC-UVB regions of the spectrum suggests that superoxide production may be involved in a number of actinic disorders in fair-skinned humans.     

配合物光化学回顾与展望

本文简述了配合物光化学的近期进展和将来趋势.     

PHOTOLYSIS OF PHEOMELANIN PRECURSORS: AN ESR-SPIN TRAPPING STUDY

The photolysis of 5-S-cysteinyldopa, 2,5-S,S-dicysteinyldopa, 4-hydroxybenzothiazole and cysteinyldopa melanins has been studied by ESR-spin trapping methods using 5.5-dimethyl-1-pyrroline-1-oxide and 2-methyl-2-nitrosopropane as spin traps for hydrated electrons, hydrogen atoms, and carbon-centered alanyl radicals. The photochemistry of these materials is shown to resemble dopa in that both photoionization and photohomolysis occur. However, unlike dopa, the cysteinyldopa also forms the carbon-centered alanyl radicals via photodecomposition of the cysteinyl group. Action spectra and quantum yields are reported. Mechanism(s) of radical formation are discussed.     

PHOTOCHEMISTRY OF FLAVINS—I. CONVENTIONAL AND LASER FLASH PHOTOLYSIS STUDY OF ALLOXAZINE

Abstract— The photobleaching of alloxazine in buffered aqueous solution has been studied by means of flash photolysis using conventional and laser excitation sources. Several transient species have been characterized. The alloxazine triplet state (Λ_max 420 nm and 550 nm, times; = 9 μs) was identified with the aid of low-temperature comparison experiments in ethanol. Transient absorption with Λ_max 440 nm, which appears after decay of the triplet state, and whose second-order decay is pH-dependent, is postulated to be due to the semiquinone radical (AH₂*) and a radical derived from alloxazine by addition of water and loss of a hydrogen atom (HAOH*), which are in equilibrium with their conjugate cation radicals. The results of experiments in the presence of oxygen indicate that these species are not primarily formed from the triplet state. The enhanced second-order decay of the flavin radicals in oxygen-containing solutions is interpreted in terms of their reaction with the peroxy radicals. The proposed mechanisms account for the production of hydroxylated alloxazines.     

PHOTOCHEMISTRY OF BIOLOGICAL MOLECULES – II. PHOTOLYSIS OF DIPEPTIDES IN THE SOLID STATE

Abstract— Photolysis with light of 2537 Å of a series of aliphatic dipeptides in the solid state has been shown to lead to bond rupture and free radical formation. The structures of the radicals have been determined by ESR techniques and in general, the free electron has been shown to reside on the carbon atom attached to the nitrogen atom of the peptide bond. Dipeptides containing phenylalanine residues show ESR spectra typical of the free amino-acid in the terminal position.     

PHOTOCHEMISTRY OF BIOLOGICAL MOLECULES-V. A MECHANISTIC STUDY OF THE PHOTOLYSIS OF CYCLOALANYLALANINE IN THE SOLID STATE

Abstract— Earlier studies of the photodamage induced by 254 nm irradiation of linear alanine peptides in the solid state have been supplemented by an investigation into the gaseous photoproducts from the cyclic dipeptide, 3,6-dimethyl-2,5-diketopiperazine. The trans and cis isomers have been prepared and the photoproducts compared with those from the DL-mixture. The conformation of the molecule does influence the yield of gaseous products. CO was produced by peptide bond rupture with concomitant release of hydrogen. CO₂ was also produced. The use of N- and C-deuterated analogues together with relevant crystallographic and EPR data has enabled a detailed study of the mechanism of photode-gradation to be made, from which it is concluded that the methyl protons are not inert but rather are the major source of the hydrogen observed on photolysis.     

PHOTOCHEMISTRY OF BIOLOGICAL MOLECULES-IV. GASEOUS PRODUCTS FROM THE PHOTOLYSIS OF ALANINE PEPTIDES IN THE SOLID STATE

Abstract— Earlier studies based on ESR measurements of the photodamage induced by 254 nm irradiation of solid state alanine peptides have been supplemented by a new mechanistic probe involving mass spectrometric and gas-solid-chromatographic analyses of the gaseous products viz., carbon monoxide, carbon dioxide, hydrogen, methane and ethane. Decarboxylation, a major reaction, is temperature sensitive and independent of carbon monoxide formation. Carbon monoxide does not arise from the carboxyl group but rather by peptide bond rupture, which may lead to the generation of at least some of the hydrogen gas detected. The use of N-deuterated-DL-alanyl-DL-alanine shows that the methyl radicals generated can abstract hydrogen atoms from the solid state peptide, and the formation of H₂ and HD implicate an imine intermediate in the photodegradation.     

PREBIOTIC FORMATION OF HIGHER MOLECULAR WEIGHT COMPOUNDS FROM THE PHOTOLYSIS OF AQUEOUS ACETIC ACID

In the study of chemical evolution, experiments simulating primitive earth conditions have shown that many compounds of biological significance can be produced from simple precursors. In an attempt to search for the origin of polycarboxylic acids, the UV irradiation of aqueous acetic acid was performed. Among the products detected were succinic, citric and malonic acids which are key compounds in metabolic pathways, suggesting that these compounds might have been formed on the primitive Earth prior to life itself.     

PHOTOSENSITIZED OXIDATION OF BIOMATERIALS and RELATED MODEL COMPOUNDS

Aluminium trisulfonatophthalocyanine (A1PCS), a dye being widely advocated for use in photodynamic therapy, produces singlet oxygen with a quantum yield of 0.34 in oxygenated water at pH 7. Triplet A1PCS abstracts an electron from a variety of amines and phenols, the rate of electron transfer depending upon the thermodynamic driving force, forming the A1PCS radical anion. This latter species reduces molecular oxygen to superoxide ions with high efficiency. The triplet state also abstracts an electron from biological components, including NADH, vitamin C, cysteine, methionine, tyrosine, tryptophan, uracil, and guanine, but not from DNA. These results suggest that photoinduced electron abstraction from appropriate biomaterials could compete with singlet oxygen production under in vivo conditions.     

ON THE MECHANISMS OF PHOTOLYSIS OF COMPOUNDS I AND II OF HORSERADISH PEROXIDASE AT 77 K*

Abstract—When Compounds I and II of horseradish peroxidase in glycerol/water glasses at 77 K were irradiated with the light of a mercury lamp, some changes in optical spectra as well as the development of strong EPR signals of a free-radical type were detected. In case of photolysis of Compound I the light of wavelengths around 400 nm was the most effective, while only short wavelength (≥ 280 nm) light affected Compound II. The results of experiments with thawing and freezing the product of photolysis of Compound I (called Intermediate Y) provide further evidence for the suggestion that the ferric state of the heme iron is generated in the course of the photolysis. The quantum yields of the photolysis of Compound I at different wavelengths of irradiating light were obtained and mechanisms of the photolysis of Compounds I and II are proposed.     

PHOTOCHEMICAL PROBES OF BIOMEMBRANE STRUCTURE AND FUNCTION. CYCLOHEXANONE PHOTOCHEMISTRY IN A MODEL SYSTEM

Abstract— The photochemical reactions of cyclohexanone have been studied in the system sodium octanoate/1-decanol/water which is known to form bilayers under the conditions used. Cyclohexanone photochemistry was also studied in aqueous media, 1-decanol, and decane. The photoproducts from the bilayer system result predominantly from reaction of the carbonyl group of cyclohexanone at the hydroxy methylene group of 1-decanol. These products include cyclohexanol, decanal, and n -decyl hexanoate. It is concluded that cyclohexanone in the bilayer system is largely concentrated near the hydroxy methylene group of 1-decanol.     

RIBOFLAVIN-PHOTOSENSITIZED OXIDATIVE DEGRADATION OF A VARIETY OF LIGNIN MODEL COMPOUNDS

Oxidation of several lignin model compounds with alkylated paraphenolic groups by photosensitizing riboflavin (RF). rose bengal (RB) and methylene blue (MB) was examined. Photosensitizing RF cleaved l–(3'-4'-diethoxyphenyl)-1.3 dihydroxy-2-(4-methoxyphenyl)propane (I). 4-ethoxy-3-methoxyphenylglycerol-(3-guaiacyl ether (II) and l-(4'-ethoxy-3'-methoxyphenyI)-1,3 dihydroxypropane (IV) at their respective C_α-C_β bonds. Riboflavin also oxidized 3.4-diethoxy-benzaldehyde (VI) to the corresponding acid, and hydroxylated the conjugated olefin l-(4'-ethoxy-3'-methoxyphenyl)1.2 propene (III) to yield the initial product IV. In contrast, MB and RB hydroxylated III but had no effect on I, II, IV or VI under identical conditions. This suggested RF effected transformations via a hydrogen radical abstraction (Type I) rather than a ¹O₂ mediated reaction.
To confirm this, the effects of deuterium oxide (D_:0) and oxygen pressure on the photosensitizing dye reactions were examined. The effect of D₂0 on tryptophan (Trp) oxidation and hydroxylation of III by MB and RB was significant, indicating involvement of ¹O₂. D₂O had no significant effect on cleavage of the diarylpropane (I) and the olefin (III) by RF, indicating these reactions did not proceed by a Type II mechanism. While O₂ pressure effect on Trp oxidation by MB was insignificant, it had a large negative effect on cleavage of I by RF. These results, coupled with the relatively slow oxidation rate of Trp by RF. indicate that photosensitizing RF produces ¹O₂ inefficiently and is not effecting oxidation of these lignin model compounds via a Type II mechanism.     

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.     

梳形支化聚苯乙烯模型化合物的合成及表征

本文用阴离子聚合的聚苯乙烯进行氯甲基化,再将此带有活泼氯的甲基化聚苯乙烯作为主链与活性聚苯乙烯阴离子偶合接枝,制备了12个具有等长度支链沿主链无规分布的、结构明确的梳形聚苯乙烯模型化合物,并用GPC—粘度计联用装置对之进行了表征.     

星形支化聚苯乙烯模型化合物的合成及表征

本文用阴离子聚合方法,首先合成了线形‘活’的聚苯乙烯大分子阴离子,然后用二乙烯基苯为偶联成核剂,制备了十三个4-80臂较窄分子量分布的等臂长规则星形支化聚苯乙烯的模型化合物,并用GPC—粘度计联用装置对之进行了表征。     

PHOTOCHEMISTRY OF DI(9-ANTHRYL)- AND BENZYL-SUBSTITUTED SILANES AND RELATED COMPOUNDS1

Abstract

By irradiation, both di(9-anthryl)dimethylsilane and germane afforded a new type of [4 + 2] intramolecular addition product that underwent quantitative cycloreversion by thermolysis. The photoproduct from the former was converted to a novel naphthotriptycene derivative. A phosphorus analogue, di(9-anthryl)phenylphosphine, however did not undergo the reaction. Photochemical reactions of benzyltrimethylsilane and acyloxymetyl(benzyl)dimethylsilanes (RC(O)OCH₂SiMe₂CH₂Ph: R = CH₃ and R = t - Bu) are also reported for the first time. The mechanisms of these new photochemical reactions are discussed.     

PHOTOCHEMISTRY OF RETINOCHROME

Abstract— Retinochrome is a photopigment found in the visual cells of cephalopods. It has been considered to act as a supplier of the 11- cis -retinal required for synthesis of rhodopsin, because its all-trans chromophore is isomerized to 11- cis form in the light. Light and thermal reactions of squid retinochrome were investigated by low-temperature spectrophotometry.
On irradiation with green light at liquid-nitrogen temperature, retinochrome (λ_max 496 nm, – 190°C) is converted mainly to an intermediate lumiretinochrome (λ_max 475 nm, – 190°C), its chromophore being changed to 11- cis -retinal. On irradiation with blue light at - 190°C, retinochrome is changed to a photosteady–state mixture (λ_max 487 nm, – 190°C) composed mainly of retinochrome and lumiretinochrome, since lumiretinochrome is partially regenerated back to retinochrome. Similarly, irradiation of lumiretinochrome with blue light also results in the same photosteady-state mixture, which can be completely reverted to lumiretinochrome on re-irradiation with green light.
Lumiretinochrome is stable at a wide range of temperatures from – 190°C to about – 20°C. Above – 20°C, it is further converted, thermally, into metaretinochrome (λ_max 470 nm), which is the same bleached product as has been observed on irradiation of retinochrome at room temperatures. Thus, the light-bleaching process of retinochrome is rather simple compared with that of rhodopsin.