Photooxidation of ABS of long-wavelengths

The evolution of the photochemical degradation of ABS has been studied in conditions of long wavelength irradiation (λ's> 300 nm). The main photoproducts involved in the oxidative evolution have been identified by using FTIR spectroscopy and chemical titrations. A particular attention has been devoted to α–β unsaturated ketones that appear as secondary photoproducts. Those ketones present a low photochemical stability when exposed in the range 300–400 nm. Conditions for their formation have been experimentally studied. Formation of oxidation photoproducts has been also studied at the macroscopic level and it has been shown that their repartition in the polymer is heterogeneous. The origins of the heterogenities have been studied.     

Photochemical Study of Chalconoid‐like Compounds: Synthesis of 4‐Flavanoids

The photo‐irradiation of chalconoid‐like compounds; (2E,4E)‐1‐(2‐hydroxyaryl)‐4‐methyl‐5‐phenylpenta‐2,4‐dien‐1‐ones to 4‐flavanoids in solution phase has been described. The molecular scaffold of the probed substrates is responsive to intramolecular cyclization as well as cis‐trans isomerizations and photoproducts corresponding to both of reactions have been realized also. The formation of photoproducts has been explained on the basis of excited state intramolecular proton transfer that further depends upon the different substituents present on the phenolic moiety.     

Studies on photophysics and photochemistry of N-salicylidene-p-(N,N-dimethylamino)aniline

The fluorescence spectra of N-salicylidene-p-(N,N-dimethylamino)aniline have been investigated in various solvents and three kinds of fluorescence were found; they were that of excited intermediate, exciplex and excited dimer. According to the transient absorption spectra and decay kinetic data of photoproducts of the title compound, it has been found that the photoproducts in cyclohexane are a zwitterion and a mixed dimer formed by a zwitterion and an enol; in acetonitrile the photoproducts are a zwitterion, a mixed dimer formed by a zwitterion and an enol and a dimer formed of zwitterion. Photochromic and luminescence mechanisms of the title compound are discussed as well.     

Crystal structures of two macrocyclic diterpenoid photoproducts

The crystal structures of two macrocyclic diterpenoid photoproducts, the chemistry of which has been described in the preceding paper have been determined at 295 K.Compounds I (C₂₀H₂₆O₃) and II (C₂₂H₃₀O₅) are both orthorhombic, P2₁2₁2₁. In I. a = 24.18(1), b = 11.841 c=5.984(2) Å. Z=4. the structure being refined by least squares to a residual of 0.063 for 1234 “observed” reflections. For II. a = 24.30(1), b = 9.413(5). c = 9.149(5) Å. Z = 4 (R = 0.070, 982 “observed” reflections).     

Photoinduced Damage to Cellular DNA: Direct and Photosensitized Reactions(†)

The survey focuses on recent aspects of photochemical reactions to cellular DNA that are implicated through the predominant formation of mostly bipyrimidine photoproducts in deleterious effects of human exposure to sunlight. Recent developments in analytical methods have allowed accurate and quantitative measurements of the main DNA photoproducts in cells and human skin. Highly mutagenic CC and CT bipyrimidine photoproducts, including cyclobutane pyrimidine dimers and pyrimidine (6-4) pyrimidone photoproducts (6-4PPs) are generated in low yields with respect to TT and TC photoproducts. Another striking finding deals with the formation of Dewar valence isomers, the third class of bipyrimidine photoproducts that is accounted for by UVA-mediated isomerization of initially UVB generated 6-4PPs. Cyclobutadithymine (T<>T) has been unambiguously shown to be involved in the genotoxicity of UVA radiation. Thus, T<>T is formed in UVA-irradiated cellular DNA according to a direct excitation mechanism with a higher efficiency than oxidatively generated DNA damage that arises mostly through the Type II photosensitization mechanism. C<>C and C<>T are repaired at rates intermediate between those of T<>T and 6-4TT. Evidence has been also provided for the occurrence of photosensitized reactions mediated by exogenous agents that act either in an independent way or through photodynamic effects.     

Kinetics of Repair of UV-induced DNA Damage in Repair-proficient and -deficient Cells as Determined by Quantitative Polymerase Chain Reaction

Abstract— Advances in methodologies to monitor gene-specific repair in human cells have facilitated a detailed understanding of the complexity of the nucleotide excision repair system. One of these procedures, quantitative polymerase chain reaction (QPCR), holds significant promise for dissecting the fine structure of the repair of UV-induced DNA damage. This assay was used to study the repair of UV photoproducts in both actively transcribed and nontranscribed genes from human cells that were capable of (1) repair of both cyclobutane pyrimidine dimers and 6-4 photoproducts; (2) removal of neither dinners nor 6-4 photoproducts; (3) strong preferential repair of 6-4 photoproducts relative to dimers; and (4) severely depressed rates of 6-4 photoproducts and dimers. Detailed kinetic analyses revealed that repair of both active and inactive genes can be studied with a very fine degree of precision and that the repair status of the cells can easily be detected by use of the procedures described.     

QUANTUM YIELDS AND SECONDARY PHOTOREACTIONS OF THE PHOTOPRODUCTS OF dTpdT, dTpdC AND dTpdU

Abstract— The photoproducts of the dinucleoside monophosphates, dTpdT, dTpdC and dTpdU, have been purified by high performance liquid chromatography and characterized by UV absorption spectroscopy, fast atom bombardment mass spectrometry and by secondary thermal and photoreactions. Four types of photoproducts were analyzed: (1) cyclobutane dimers including cis-syn isomers and two diastereomers of the trans-syn isomers; (2) 6-4 photoadducts and the corresponding Dewar valence isomers; (3) photohydrates comprising two diastereomers and (4) a new photoproduct resembling nucleobase amine adducts, which occurs only for dTpdC. The quantum yields of formation of these photoproducts and for some secondary photoreactions were measured by kinetic analysis of the photoproduct yield as a function of photon fluence. These results indicate that cis-syn cyclobutane dimers are the photoproducts formed with highest efficiency with dT[p]dC dimers being formed with 50–75% the efficiency of dT[p]dT dimers. The 6-4 photoadducts are formed with 5–10% the efficiency of cis-syn cyclobutane dimers and the 6-4 photoadduct of dTpdC is formed two to three times more efficiently than that of dTpdT. Photohydrates are also formed efficiently due to an equilibrium between stacked and unstacked complexes of the dinucleoside monophosphates. It is shown that three of these photoproducts, namely the cyclobutane dimers of dTpdC, the 6-4 photoadducts and the possible nucleobase amine adduct, undergo photolysis in the UV-B region resulting in either photoreversion or secondary photoreaction.     

Unexpected formation of new photochromic compounds derived from 3,3-diphenyl-3H-naphtho[2,1-b]pyran-1-one

Two new unexpected photochromic compounds were obtained from naphtho[2,1-b]pyran-1-one 1. The reaction of this ketone with the silyl enol ether methyl trimethylsilyl dimethylketene acetal, catalyzed by TiCl₄, afforded the photochromic dihydronaphtho[2,1-b]pyranone 2. The Reformatsky reaction of ketone 1 with ethyl bromoacetate led to the formation of the expected alcohol that under acid treatment gave, unexpectedly, the novel photochromic benzocoumarin 6. UV irradiation compounds 2 and 6 in solution provided thermally stable photoproducts that returned to the initial uncoloured forms under visible irradiation. The photochromic behaviour of these compounds and the structures of the photoproducts formed in these reactions were characterized by 1D and 2D NMR.     

A new on-line solid phase extraction high performance liquid chromatography tandem mass spectrometry method to study the sun light photodegradation of mono-chloroanilines in river water

The study investigates the natural photodegradation pathway of mono-chloroanilines in river waters, with the aim to identify the predominant photoproducts formed. At this purpose a new sensitive on-line SPE HPLC–MS/MS method has been developed with LOQ values equal or lower than the legal threshold concentration levels allowed for mono-chloroanilines in waters. The degradation processes of o-, m- and p-chloroaniline have been investigated subjecting their solutions, prepared both in ultrapure and in river water, to sun light irradiation simulated by a solar box system. The SPE HPLC–MS/MS methodology allowed to evaluate the degradation kinetics, to identify the predominant photodegradation products and to propose the chemical structures. Two photoproducts (aniline and 3-aminophenol), for which standards are available, have also been quantified.     

Vector and scalar correlations in the photodissociation of NCCN

The CN photofragments from the photodissociation of NCCN at 193 nm have been measured by high-resolution transient absorption spectroscopy. Doppler-broadened profiles of isolated rotational lines in the 2-0 and 3–1 vibrational bands of the CN A---X transition were observed under collisionless conditions with a tunable, single-frequency Ti:sapphire ring laser. Analysis of the Dopple profiles reveals a vector correlation between the translation and rotation of CN photoproducts, with the angular momentum of the high rotational states increasingly perpendicular to the recoil velocity. After correction for vector correlations, the laboratory-frame scalar speed distribution of state-selected photoproducts can be determined. The mean squared laboratory velocity is directly related to the average internal energy of coincident CN fragments. The wings of the Doppler profiles indicate that the available energy for a pair of ground state CN photoproducts following 193 nm dissociation of NCCN at 295 K is 5300±150 cm⁻¹, which includes the average vibrational energy of the parent molecules selected by the photolysis laser. Phase space theory with an optimized available energy of 5300 cm⁻¹ produces laboratory speed distributions that are in qualitatively reasonable agreement with the kinetic energy measurements, but overestimate the total internal energy of the photofragments. The measurements are good enough to warrant comparison with more sophisticated models of unimolecular decomposition.     

PHOTOPRODUCTS OF CHLORPROMAZINE WHICH CAUSE RED BLOOD CELL LYSIS

A mechanism for chlorpromazine (CPZ) phototoxicity has been proposed that attributes the response to formation of stable, toxic photoproducts which cause cell membrane disruption. We have characterized these toxic photoproducts as dimers and higher multimers of CPZ. Chlorpromazine solutions (3 or 10 mA/) were irradiated with a medium pressure Hg lamp filtered to exclude λ < 280 nm. Five low mol wt photoproducts were separated by high performance liquid chromatography. Two were identified as CPZ-sulfoxide and promazine. Higher mol wt photoproducts were separated by Sephadex G-50 chromatography into 3 broad bands which were characterized by their absorption and fluorescence spectra. Band A (mol wt > 800) had λ_max^abs= 263 nm, λ_max^fl= 490 nm and Band B (mol wt = 350-800) had λ_max^abs= 255 nm, λ_max^fl= 450 nm. Based on the mol wt of CPZ, Band A contained trimers and higher mol wt compounds and Band B was composed of dimeric structures. BandC(λ_max^abs= 255,310 nm; λ_max^fl= 445 nm) was composed of CPZ (mol wt = 315) and the low mol wt photoproducts. Red blood cell lysis was used as an assay for the ability of photoproducts to cause membrane disruption. Bands A and B, but not Band C, caused cell lysis. These data indicate that the CPZ photoproducts which cause cell membrane disruption are dimers (Band B) and higher multimers (Band A).     

PHOTOCHEMICAL REACTIONS OF THYMINE, URACIL, URIDINE, CYTOSINE AND BROMOURACIL IN FROZEN SOLUTION AND IN DRIED FILMS*

Abstract— The photochemistry of uracil, uridine, cytosine, thymine and broinouracil has been investigated in frozen aqueous solution and in dried films. Essentially the same photoproducts were obtained in the two conditions; however, the yield of photoproducts was considerably greater in frozen solution. Uracil forms a dimer which can exist in two forms. Some kinetic data are presented for the interconversion of these two forms, The mixed dimer of thymine and uracil can also exist in two forms. Uridine forms only one acid stable photoproduct and does not appear to form mixed photoproducts under the conditions used. Two new photoproducts of thymine other than the dimer are described. Cytosine was at first considered to be completely inert hut using more sensitive detecting equipment it has recently been found to form uracil dinier as a result of dinierization and deamination. The most remarkable response was shown by bromouracil. When irradiated by itself it formed no photoproducts but when irradiated in the presence of uracil, uridine, cytosine or NaOH it formed many photoproducts. Most of these products were devoid of bromide, but two still contained bromine. One of these has been identified as the mixed dimer of uracil and bromouracil while the other has been tentatively identified as the dimer of bromouracil. Dimers of thymine or bromouracil were not formed by X-rays.     

Relaxation pathways of excited N-(triphenylmethyl)salicylidenimine in solutions

Excited state relaxation of N-(triphenylmethyl)-salicylidenimine (MS1) in protic and aprotic solvents has been investigated by means of absorption pump-probe spectroscopy with femtosecond time resolution and fluorescence spectroscopy with picosecond time resolution. Short-lived excited states and long-lived photoproducts have been identified from the differential absorption spectra. Excited states and photoproducts were different under excitation of enol-closed and cis-keto tautomers. As a result, the commonly accepted excited state relaxation model of aromatic anils, which assumes an ultrafast transformation of excited enol-closed tautomers into cis-keto tautomers, has been modified. Performed quantum chemical calculations suggest that hydrogen-bonded ethanol molecules facilitate formation of cis-keto tautomers and are responsible for their different relaxation pathways in comparison with relaxation of excited enol-closed tauromers. Fluorescence decay on a nanosecond time scale was attributed to aggregated MS1 molecules.     

Further insight in the photochemistry of DNA: structure of a 2-imidazolone (5-4) pyrimidone adduct derived from the mutagenic pyrimidine (6-4) pyrimidone photolesion by UV irradiation

Pyrimidine (6-4) pyrimidone photoproducts represent one of the major mutagenic and carcinogenic class of DNA damage produced by UV exposure. At present, besides their conversion to their Dewar valence isomer, (6-4) photoproducts are generally believed to be photostable, and the observed biological properties of Paterno-Büchi-derived photoproducts are, thus far, exclusively attributed to these two types of compounds. Using a model system (2) relevant to DNA photochemistry, we have observed that the 5'-base moiety of the (6-4) thymine dimer 3, under far-UV radiation, is able to undergo a ring contraction leading to a 2-oxoimidazoline, 1. This unprecedented secondary photochemical reaction constitutes the first report of a major photomodification affecting (6-4) photoproducts and strongly questions the biological stability of the (6-4) adducts under UV light with 2-imidazolone (5-4) pyrimidone adducts being possibly another source of endogenous DNA damage.     

UNIQUE DNA REPAIR PROPERTY OF AN ULTRAVIOLET-SENSITIVE (radC MUTANT OF Dictyostelium discoideum

Abstract— Dictyostelium discoideum is an organism that shows higher UV resistance than other organisms, such as Escherichia coli and human cultured cells. We examined the removal of cyclobutane pyrimidine dimers (CPD) and 6–4 photoproducts from DNA in the radC mutant and the wild-type strain using an enzyme-linked immunosorbent assay with monoclonal antibodies. Wild-type cells excised more than 90% of both CPD and 6–4 photoproducts within 4 h. Dictyostelium discoideum appeared to have a special repair system, because 6–4 photoproducts were repaired faster than CPD in E. coli and human cultured cells. In radC mutant cells, although only 50% of CPD were excised from DNA within 8 h, effective removal of 6–4 photoproducts (80% in 8 h) was observed. Excision repair-deficient mutants generally cannot remove both CPD and 6–4 photoproducts. Though the radC mutant shows deficient excision repair, it can remove 6–4 photoproducts to a moderate degree. These results suggest that D. discoideum has two kinds of repair systems, one mainly for CPD and the other for 6–4 photoproducts, and that the radC mutant has a defect mainly in the repair enzyme for CPD.     

UV IRRADIATION OF NUCLEIC ACIDS: CHARACTERIZATION OF PHOTOPRODUCTS OF THYMIDYLYL-(3'→5')-2'-DEOXY-5-FLUOROURIDINE

Abstract— The acetone-sensitized irradiation using UV-B (ultraviolet light, 280–320 nm; sunlamps) of thymidyl-yl(3'→5')deoxyfluorouridine monophosphate produces two main photoproducts. The distribution of these photo-products is dependent on the pH of the irradiation solution. At pH 6, the cis-syn cyclobutane-type photodimer is the major product, whereas at high pH (8–10) a photoadduct is the major product. These photoproducts have been identified and structurally characterized by H-1 and C-13 NMR spectroscopy. The photoadduct arises from defluorination of the 5-fluorouracil moiety. The structure of the photoadduct maintains the sugar-phosphate backbone of the starting material (d-TpF), and contains a saturated thymine moiety with an added Thy(C6-hydroxyl) and a Thy(C5)-(C5)Ura covalent bond.     

The influence of the magnetic field on the photosubstitution reaction of coordination complexes

Photolyses of rhodium(III) complexes, Rh(NH₃)₅X²⁺ (X = Cl^? and Br^?), under intense magnetic fields, e.g. λ_excit = 360 nm and H = 24 kG, have been investigated. The magnetic field quenches the photoaquation of the ammonia and enhances the photoaquation of the acido ligand, X = Cl^? or Br^? by 10% for H = 24 kG. The implication of two different precursors in the formation of the photoproducts is discussed.     

Intramolecular interactions in the triplet excited states of benzophenone-thymine dyads

Time-resolved and product studies on the synthesized dyads 1 and 2 have provided evidence that the benzophenone-to-thymine orientation strongly influences intramolecular photophysical and photochemical processes. The prevailing reaction mechanism has been established as a Paterno-Büchi cycloaddition to give oxetanes 3-6; however, the ability of benzophenone to achieve a formal hydrogen abstraction from the methyl group of thymidine has also been evidenced by the formation of photoproducts 7 and 8. These processes have been observed only in the case of the cisoid dyad 1. Adiabatic photochemical cycloreversion of the oxetane ring is achieved upon direct photolysis to give the starting dyad 1 in its excited triplet state. The photobiological implications of the above results are discussed with respect to benzophenone-photosensitized damage of thymidine.     

Intermolecular and intramolecular photocycloaddition reactions of fluoroarenes

The meta photocycloaddition of para-fluorotoluene to cyclopentene yields 1-methyl-10-fluoro-endo- and -exo- tetracyclo[6.3.0.0^2,11.0^3,7]undec-9-ene as major photoproducts, while the intramolecular photoaddition of 5-(para-fluorophenyl)pent-1-ene gives exclusively 7-fluorotetracyclo[3.2.1.0^2,8.3^4,8]undec-6-ene. A reaction pathway via zwitterionic intermediates is proposed.     

PHOTOREACTIVATION IN A phrB MUTANT OF Escherichia coli K-12: EVIDENCE FOR THE ROLE OF A SECOND PROTEIN IN PHOTOREPAIR

Abstract In Escherichia coli , the light-dependent repair of pyrimidine dimers in UV-irradiated DNA is now accepted as being due to enzymatic photoreactivation (PR) by a 50 kDa enzyme, photolyase (EC 4.1.99.3). The gene for this enzyme has been mapped at 16.2 min and designated phr . This gene was earlier described as phr B, another locus phr A having been proposed in association with PR. The relevance of the putative phr A gene has now been placed in doubt. The recent report of the discovery of a photoreactivating enzyme in Drosphila melanogaster . which specifically repairs pyrimidine (6–4) pyrimidone photoproducts ([6–4] photoproducts), and that E. coli does possess a protein with specific affinity for the (6–4) photoproduct, has cast new light on the prospective role of phr A in PR. We have determined the nucleotide sequence of the putative phr A gene, which suggests it codes for a protein of 38 kDa. When the putative phr A gene was cloned into an expression vector and transformed into a phr A phr B mutant of E. coli , a level of photorepair was observed, which could correspond to repair of (6–4) photoproducts.