Main Photoproducts Research Articles

Photochemical reactions of 5-fluoropyrimidine bases with selected alkylamines

Novel photochemical reactions between 5-fluoropyrimidine bases and primary alkylamines are described. Photoreaction of 5-fluorouracil with alkylamines in aqueous solution (pH>8) leads to 5-alkylaminouracils as the main photoproducts. Similarly, photoreaction of 5-fluorocytosine with alkylamines yields 5-alkylaminocytosines.

Photoisomerization reactions of 4-methoxy- and 4-hydroxy-6-methyl-α-pyrones: An experimental matrix isolation and theoretical density functional theory study

Photoisomerization reactions were studied for 4-methoxy- and 4-hydroxy-6-methyl-α-pyrones isolated in low-temperature Ar matrices. Two types of photoproducts: Dewar valence isomer and conjugated aldehyde–ketene were photogenerated upon UV irradiation of the matrix-isolated compounds. After prolonged irradiation practically all the initial material was converted to the corresponding Dewar isomers, whereas the population of aldehyde–ketene photoproducts was marginal. The structure of the main photoproducts was positively identified by comparison of their experimental IR spectra with the spectra theoretically predicted for the Dewar isomers. Good agreement between the experimental observations and the theoretical calculations allowed for reliable assignment of the infrared bands in the spectra of the initial forms of the studied compounds as well as in the spectra of the Dewar isomers. For the 4-hydroxy-6-methyl-α-pyrone the presence of its minor tautomeric form in a low-temperature matrix was detected.

Absolute asymmetric photoreactions of aliphatic amino acids by circularly polarized synchrotron radiation: critically pH-dependent photobehavior.

The absolute asymmetric photoreaction (AAP) of racemic aliphatic amino acids, such as alanine (Ala) and leucine (Leu), by left- and right-handed circularly polarized light (l- and r-CPL) irradiation was investigated in aqueous solutions at various pHs, by using the Onuki-type polarizing undulator installed in an electron storage ring. The magnitude of the optical purity (op) generated and the enantiomer-enriching mechanism operative in the AAP were found to be entirely dependent on the ionic state (and thus pH) of the amino/carboxylic acid moieties. At pH 1, the op of Ala and Leu determined by circular dichroism (CD) spectral measurement gradually developed with CPL irradiation, according to Kagan's equation. In contrast, irradiation at pH 7 gave op's much smaller than the theoretical values predicted by Kagan's equation. However, it turned out that the photodecomposition at pH 7 produces the corresponding alpha-hydroxycarboxylic acids stereoselectively, the CD sign of which is just opposite to that of the remaining amino acid, thus affording the apparently small op. It is concluded that, irrespective of solution pH, the AAP of amino acid proceeds upon CPL irradiation. At pH 1, the photodecomposition of valine, Leu, and isoleucine occurs via a Norrish type II mechanism, which is also applicable to other amino acids possessing a gamma-hydrogen. In the case of amino acids lacking a gamma-hydrogen, such as glycine and Ala, the photodecomposition mechanism is a photodeamination/hydroxylation and a Norrish type I reaction. At pH 7, the main photoproducts were ammonia and alpha-hydroxycarboxylic acids that were produced via photodeamination.

Medium-dependent type selectivity in photoreactions of a crown ether-annelated dibenzobarrelene derivative.

[reaction: see text] A concept for modifying the photochemical properties of a dibenzobarrelene derivative by the combination of supramolecular assembly and solid-state photochemistry is presented. The irradiation of dibenzobarrelene 1b and alkali metal complexes thereof (1:1) in acetonitrile or benzene solution leads preferentially to the dibenzocyclooctatetraene 4b as the photoproduct. In contrast, the dibenzosemibullvalene 5b is formed as the main photoproduct in the solid state because of a strong cation effect.

Photochemical behaviour of dichlorprop [(+/-)-2-(2,4-dichlorophenoxy)propanoic acid] in aqueous solution.

Aqueous solutions of dichlorprop were irradiated under different conditions of pH, wavelength and oxygenation. The photochemical behaviour was found to be complex and many photoproducts were formed. However, at low concentrations the main photoproducts were 4-chloropyrocatechol, 2-chlorophenol, 4-chlorophenol and 2,4-dichlorophenol. Some other photoproducts were identified, namely 2-(4-chloro-2-hydroxyphenoxy)propanoic acid, 2-(3,5-dichloro-2-hydroxyphenyl)propanoic acid and 2,4-dichlorophenyl acetate. From comparison with results previously obtained with mecoprop [2-(4-chloro-2-methylphenoxy)propanoic acid] it appears that the presence of a chlorine atom in position 2 on the ring strongly modifies the photochemical behaviour.

A comparative study of the photophysics and photochemistry of 4-chlorophenol adsorbed on silicalite and β-cyclodextrin

The photochemistry and photophysics of 4-chlorophenol (4-CP) were studied onto two model solid supports, silicalite and β-cyclodextrin (β-CD), using time resolved diffuse reflectance techniques and product degradation analysis. The results have shown that the photochemistry and photophysics of 4-CP are different from solution and depend on the solid. Ground state diffuse reflectance and time resolved luminescence demonstrated the inclusion of the probe in both substrates. 4-CP exhibits room temperature luminescence in both hosts, being structured and much more intense in β-CD. The emission was assigned to phosphorescence of the inclusion complex. Transient absorption demonstrated the formation of the unsubstituted phenoxyl radical and of 4-chlorophenoxyl radical in β-CD. In silicalite only the later was detected. The studies of the photodegradation products indicate that phenol is the main photoproduct in β-CD. In silicalite the chromatographic analysis indicates the presence of products that involve the ring cleavage.

Fenarimol solar degradation pathways and photoproducts in aqueous solution

The degradation photoproducts of the fungicide fenarimol obtained from irradiation of aqueous solutions with sunlight were characterised. The photoproducts resulting from samples with different exposure times were extracted and separated using chromatographic techniques. Seven main photoproducts were detected using high performance liquid chromatography with a photodiode array detector, gas chromatography with mass spectrometry detector and Fourier transform infrared spectroscopy. Structures are suggested for possible photoproducts based on the characterisation results, minimum energy geometry of the parent compound, and the mass spectral behaviour of fenarimol. These correspond to the compounds with m/ z 328 (three structural isomers (a), (b) and (c)), m/ z 294 (two structural isomers (a) and (b)), m/ z 292, 278 and 190. Of the various major products detected, the isomer 328(a) is seen to be particularly unstable under the action of sunlight. The most stable photoproducts are found to be those with m/ z 294(a), 278 and 190. However, upon prolonged solar irradiation all of these break down to produce polar, low molecular weight compounds. Comparison with our own and other results on fenarimol photolysis indicate significant solvent effects on the process. The combination of these structural characterisation results and previous data from spectroscopic and photodegradation kinetics studies allows us to suggest some possible mechanisms for the photodegradation of fenarimol under sunlight.

Photochemical transformations of 5-halogeno-4-thiouridines

A series of 2′,3′,5′-tri-O-acetyl-5-halogeno-4-thiouridines, where halogen = Br, Cl, F, are synthesized and their photochemical transformations upon irradiation with near-UV light (λ > 300 nm) in aqueous acetonitrile solutions investigated. The main photochemical pathways in each case involve intermolecular additions leading to three main photoproducts with different relative distributions. The photoproducts are isolated and their structures determined based on MS, 1H and 13C NMR and UV spectral data.

Photochemical behaviour of 4-chloro-2-methylphenoxyacetic acid

The kinetics of phototransformation of MCPA is studied under various irradiation conditions. The photocatalytic transformation on TiO2 in aqueous suspension is almost specific and leads to 4-chloro-2-methylphenol (P 7) as the main photoproduct. The same product is obtained when MCPA is irradiated on silica in the absence of water. The direct phototransformation is more complex: the reaction is not influenced by oxygen but it depends on the pH of the solution and on the irradiation wavelength. With the anionic form irradiated between 254 and 350 nm, photohydrolysis of C–Cl bond is almost quantitative (yield > 86%). It leads to the hydroxylated photoproduct P2. With the molecular form the main product P5 results from a photochemical rearrangement of the molecule. With both forms, some other photoproducts are also identified and quantified, particularly methylhydroquinone (P 1) and P7. 2-Methylphenol (P6) is only obtained with the anionic form as a minor product. However, irradiation of solutions in sunlight or with lamps emitting mainly at 365 nm (about 2 and 6% of the light is emitted at 334 and 313 nm, respectively) yields P7 as the main photoproduct. Its formation is self-accelerated. This wavelength effect is attributed to reactions induced by quinonic compounds formed as intermediates since the disappearance of MCPA is more efficient in presence of quinonic products. P 7 is also the major photoproduct when phototransformation is induced by nitrite ions or Fe(III) perchlorate. Besides, it appears from Microtox ® test that photoproducts formed at wavelength shorter than 350 nm are more toxic to the marine bacterium Vibrio fischeri than the initial compound. © 2001 Elsevier Science B.V. All rights reserved.

Phototransformation of propiconazole in aqueous media.

The photolysis of propiconazole in pure water, in water containing humic substances, and in natural water was investigated. The reaction rates were determined, and the main photoproducts were identified with the help of HPLC-mass spectrometry and by NMR. The quantum yield for direct photolysis was 0.11 +/- 0.01 at the maximum of absorption (269 nm). Photocyclization after HCl elimination and photohydrolysis of the cyclized intermediate were the main reaction pathways at 254 nm. By contrast, oxidation prevailed over dechlorination in simulated or natural solar light. Humic substances (10 mg x L(-)(1)) and naturally occurring chromophores contained in natural water enhanced the rate of propiconazole photodegradation in solar light. Half-life in June in Clermont-Ferrand (latitude 46 degrees N) was found to be 85 +/- 10 h in pure water and 60 +/- 10 h in natural water; showing that photodegradation of propiconazole in natural waters involves both direct photolysis and photoinduced reactions.

Dissipation of triadimefon on the solid/gas interface

The dissipation of triadimefon, as pure solid and in the Bayleton 5 commercial formulation, was studied under controlled and natural conditions. Volatilization and photodegradation were shown to be the main dissipation processes. The volatilization results can be described by an empirical model assuming exponential decay of the volatilization rate. The filler of the commercial formulation is determinant for the volatilization but has little effect on the photodegradation rates. The main photoproducts were identified and a reaction mechanism proposed.

Fluorescence spectroscopy of normal mouse skin exposed to 5-aminolaevulinic acid and red light

Photobleaching and phototransformation of protoporphyrin IX (PpIX) was investigated in normal mouse skin. The PpIX was induced by topical application of 5-aminolaevulinic acid (ALA). Exposure to laser light (635 nm) caused photobleaching of PpIX fluorescence and formation of fluorescent products. Analysis of the fluorescence spectra revealed appearance of new fluorescent photoproducts during light exposure. The main photoproduct, supposedly chlorin-type photoprotoporphyrin (PPp), exhibited fluorescence with an emission maximum at 675 nm. The other products exhibited main fluorescence peaks at around 588 and 623 nm that can presumably be attributed to an endogenous metallo-porphyrin and water-soluble porphyrin(s), respectively. Our results indicate that light exposure causes alterations in the enzymatic pathway of PpIX synthesis from ALA and leads to accumulation of intermediate water-soluble porphyrins. ALA-induced porphyrins are transported away from the treated area and partly deposited in remote skin sites.

Photosensitivity of Aqueous Sodium Nitroprusside Solutions: Nitric Oxide Release versus Cyanide Toxicity

Photolysis of the pentacyanonitrosylferrate (II) ion in sodium nitroprusside (SNP) solutions involves a competition between photosubstitution and photo-oxidation reactions, where the nitrosyl and cyanide ligands can be released as free nitric oxide (NO) or NO+ and free CN- or CN radical. We have irradiated aqueous SNP solutions at several narrow wavelength ranges in the UV/Vis region (314–576 nm), with the aim of investigating the photolability of the CN and NO ligands. Kinetics of photolysis were used to characterize the photosensitivity of SNP solutions in the range 314–576 nm. Spectral changes in the UV/Vis and IR regions upon irradiation and assays for the detection of free CN- and NO, provided additional evidences for the absence of photoprocesses leading to the release of CN- with λirr > 480 nm. In this condition, the main photoproducts were found to be the [FeIII(CN)5(H2O)]2- ion and NO, formed in a intramolecular photo-oxidative process. Irradiation with unfiltered UV/Vis light and with λirr < 480 nm, lead to the release of both CN- and NO and to a mixture of ferrocyanide and ferricyanide products. These results confirm that the exposure of SNP solutions to UV/Vis light below λirr = 480 nm can lead to cyanide poisoning and further support that above this wavelength limit, a selective NO release from SNP can be achieved, with possible implications for its biochemical action.

Photocatalysed degradation of cyromazine in aqueous titanium dioxide suspensions: comparison with photolysis

The fate of cyromazine ( N-cyclopropyl-1,3,5-triazine-2,4,6-triamine), an insect growth regulator, has been studied upon irradiation as well as its photocatalytic treatment (advanced oxidation technology). Most of the organic compounds occurring during the photodegradation have been identified by means of liquid chromatography and mass spectrometry coupled techniques (LC–MS). The main photoproducts of the first degradation steps are amide-triazines and melamine. Simultaneously, carboxylic acids, such as acrylic, pyruvic and glycolic acids have been identified by LC–UV. These acids are resulting from the cyclopropyl degradation. The melamine (1,3,5-triazine-2,4,6-triamine) is transformed into cyanuric acid (2,4,6-trihydroxy-1,3,5-triazine) by means of three successive deaminations. This compound remains very stable in aqueous solution.

Photostability and phototoxicity studies on diltiazem

The photostability of diltiazem was studied in aqueous solutions at different pH values. Firstly, the hydrolysis of the drug to desacetyldiltiazem in alkaline medium was evaluated and then the drug photodegradation under exposure to UVA-UVB radiation (solar simulator) was monitored by HPLC methods. The main photoproduct was isolated and characterized as diltiazem-S-oxide on the basis of the NMR and mass spectra. The HPLC method was also applied to the selective analysis of diltiazem in commercial formulations. Tests on mutagenicity and photomutagenicity of the drug were also carried out using Salmonella typhimurium TA 102 strain. In this testing the drug neither was mutagenic nor toxic.

Photochemical reactions of β-2,4,6-trimethyl-1,3,5-trithiane in solution

The mechanism for the photoreaction of β-2,4,6-trimethyl-1,3-5-trithiane (β-TMT) in acetonitrile was investigated using steady-state and laser flash photolysis. The main photoproducts were determined to be the thioester CH 3C(S)SCH(CH 3)SCH 2CH 3 and the α-TMT isomer. These products were identified by analytical and spectroscopic methods. The quantum yield for β-TMT disappearance was measured as Φ β-TMT=0.43, and quantum yields for the formation of products were Φ thioester=0.32 and Φ α-TMT=0.10. A short-lived ( τ=13 μs) intermediate was observed in the flash photolysis experiments that was seen to decay into the thioester (the stable photoproduct). A detailed mechanism of the primary and secondary processes was proposed and discussed that accounts for the experimental results.

Photodegradation of the pharmaceutical drug diclofenac in a lake: Pathway, field measurements, and mathematical modeling

Vertical concentration profiles of diclofenac were measured in Lake Greifensee (Switzerland) under mixed (February/December) and stratified (July) lake conditions. The concentrations ranged from 1 to 12 ng/L and were lower in summer than in winter, especially near the lake surface, pointing to an efficient elimination of diclofenac by photodegradation in the lake. Laboratory experiments confirmed the rapid photodegradation of diclofenac in water when exposed to sunlight. First-order reaction rates varied seasonally according to actual solar radiation (half-lives, tau = 0.2-1.7 h). The initial photoproduct was 8-chlorocarbazole-1-acetic acid, which photodegraded even faster than the parent compound. Carbazole-1-acetic acid, previously reported as the main photoproduct, was only formed in the presence of a H-source, such as methanol. In the absence of a H-source and of air, hydroxycarbazole-1-acetic acid was formed. However, hydroxycarbazole-1-acetic acid was not observed in the presence of air and, thus, under conditions similar to those in a lake, likely because of its rapid further photooxidation. Computer simulations using a one-dimensional lake model taking actual solar radiation and flushing data of the lake into account confirmed that photolysis is the predominant elimination pathway for diclofenac in Lake Greifensee. These calculations further showed that the expected concentrations of the photoproduct 8-chlorocarbazole-1-acetic acid are less than the current detection limits of approximately 3 ng/L.

Studies on the photostability and in vitro phototoxicity of Labetalol

The purpose of this study was to obtain information on the photochemical and phototoxic properties of Labetalol, a beta-blocker drug. Preliminary information on the drug photoreactivity was achieved using a flow system with a photochemical reactor on-line with a diode array detection system. Photophysical and photochemical investigations on the drug were performed in aqueous solutions at different pH values using spectrophotometric and fluorimetric methods; the photodegradation quantum yield was found to be 2.7×10 −3 at pH 5.8 and 1.5×10 −2 at pH 11.5. Forced photodegradation of labetalol solutions under exposure to UVA–UVB radiations (xenon arc lamp) was monitored by reversed-phase liquid chromatography. The main photodegradation products were isolated and characterized by NMR and mass spectrometry; labetalol was found to give 3-amino-1-phenylbutane and salicylamide-4-carboxaldehyde as the main photoproducts. Preliminary phototoxic testings on human keratinocyte cultures were performed evaluating the viability of the cells by the neutral-red uptake assay; mutagenic and photomutagenicity tests were also carried out based on Salmonella typhimurium strains. As a result, labetalol was found to be photolabile,mainly in alkaline medium, but evidences of significant phototoxic and photomutagenic effects by the drug were not observed.

Methanol–pyridine complexes trapped in argon and nitrogen matrices: Infrared induced isomerization and theoretical calculations

Selective vibrational excitations of OH stretching modes of methanol–pyridine hydrogen-bonded complexes trapped in solid argon or nitrogen at 7 K were carried out in the range 3400–3280 cm−1. This proved an efficient way for inducing isomerization processes within heterodimers and larger aggregates. The main photoproducts are non-H-bonded species, with OH and CO frequencies, respectively, close to 3665 and 1020 cm−1. These unbonded species are unstable, with lifetimes strongly temperature dependent in the range 7–20 K. The possible structures of the heterodimer have been calculated theoretically according to a two-step method. The potential energy surface is firstly explored using a semiempirical method, then the properties of its minima are calculated in the framework of the density functional method. The deepest minimum corresponds to the quasilinear N⋯HO hydrogen-bonded structure, with vibrational properties in good agreement with the matrix data obtained after deposition at low concentration in both dopants. Several other minima were examined in which H bonding is either weak (OH… π electrons) or nonexistent. The vibrational perturbations are weak in all cases, but the identification of the photoproduct of the stable dimer to the form involving the OH… π interaction can be discarded. The stable forms of the mixed trimers were also calculated. (Methanol)2–pyridine has a cyclic structure, the methanol dimer being tied to pyridine through a strong OH⋯N hydrogen bond and a weak CH⋯O interaction. Methanol–(pyridine)2 is also cyclic, with a OH⋯N bond slightly stronger than in the heterodimer. Comparison with experimental data allows identification of this heterotrimer in nitrogen matrices, in the presence of an excess of pyridine with respect to methanol. Among its photolysis products, one class in which the OH group is perturbed on the oxygen atom has been identified.

Photochemical behaviour of 2-halogenoanilines in aqueous solution

The main photoproducts formed in aqueous solutions of 2-chloro-, 2-bromo- and 2-fluoroaniline (2-ClA, 2-BrA and 2-FA) are 2-aminophenol, aniline and 1,3-cyclopentadiene-1-carbonitrile. They result from a heterolytic mechanism more efficient with 2-FA than for the two other congeners. Quantum yields of disappearance are in the range 0.02–0.04 for 2-ClA and 2-BrA and 0.10–0.13 for 2-FA, according to the conditions. The formation of aniline occurs only in the absence of oxygen. In the presence of oxygen 2-aminophenol initially formed is phototransformed into 2-aminophenoxazin-3-one.Three isomers of diaminobromobiphenyls were identified in the photolysis of 2-BrA. Similar products are formed with 2-ClA but with a lower yield. This formation of biphenyl derivatives is attributed to the reactivity of the excited triplet state.