PHOTOCHEMICAL REACTIONS OF AROMATIC KETONES WITH NUCLEIC ACIDS AND THEIR COMPONENTS I—. PURINE AND PYRIMIDINE BASES AND NUCLEOSIDES*.

Abstract— The photochemical reactions of benzophenone and acetophenone with purine and pyrimidine derivatives in aqueous solutions have been investigated by flash photolysis and steady-state experiments. Upon excitation of these two ketones in aqueous solutions, two transient species are observed: molecules in their triplet state and ketyl radicals. The triplet state lifetimes are 65 μsec for benzophenone and 125 μsec for acetophenone. The ketyl radicals disappear by a second order reaction, controlled by diffusion. In the presence of pyrimidine derivatives, the triplet state is quenched and the ketyl radical concentration is decreased without any change in its kinetics of disappearance. Ketone molecules in their triplet state react with purine derivatives leading to an increase in the yield of ketyl radicals due to H-atom abstraction from the purines. Steady-state experiments show that benzophenone and acetophenone irradiated in aqueous solution at wavelengths longer than 290 nm undergo photochemical reactions. The rate of these photochemical reactions is increased in the presence of pyrimidine derivatives and even more in the presence of purine derivatives. Following energy transfer from the triplet state of benzophenone to diketopyrimidines, cyclobutane dimers are formed. The energy transfer rate decreases in the order orotic acid > thymine > uracil. Benzophenone molecules in their triplet state can also react chemically with pyrimidine derivatives to give addition photoproducts. All these results are discussed with respect to photosensitized reactions in nucleic acids involving ketones as sensitizers.     

Studies on the changes in the composition of solvent during photochemical generation of uranous ions in uranium loaded 30 % TBP-n-dodecane-HNO3 system

Present investigation describes our study on photochemical generation of uranous ions and consequent degradation of solvent in the uranium loaded 30 % Tributyl phosphate-n-dodecane-nitric acid system. Samples of 30 % TBP-n-dodecane loaded with uranium were subjected to UV photolysis at 254, 300 and 350 nm respectively. Wavelength dependent formation of U(IV) has been determined spectrophotometrically by measuring absorbance at 656 nm. Additionally, photochemical yield of U(IV) has also been estimated semi quantitatively as a function of time of photolysis. The changes in the solvent composition under different photochemical conditions have been studied by examination of comparative gas chromatographic profiles of the solvent before and after photolysis. Among the wavelength of photo irradiation studied, the yield of U(IV) was found to be optimum at 300 nm with least degradation of PUREX solvent.     

Photolysis of α,β‐Epoxyketones: A Green Synthesis of β‐Hydroxyenones through Tandem H‐Abstraction,Ring Cleavage and Isomerisation

The photochemical behavior of various substituted epoxycarbonyl compounds consisting of more than one possible photo‐labile site (i.e. δ‐hydrogen, β‐hydrogen and epoxide ring) has been investigated. These compounds on photo‐irradiation produced the β‐hydroxyenones in an eco‐friendly green approach. Mechanistically, these photo‐transformations have been envisaged to occur via an intramolecular β‐hydrogen abstraction by the carbonyl group of benzoyl moiety to generate the 1,3‐biradical followed by epoxide ring opening that isomerizes into the photoproducts. The photolysis of the probed epoxy ketones didn’t furnish any photoproduct through δ‐hydrogen abstraction, whatsoever. This exclusive preference for β‐H abstraction over δ‐H abstraction by carbonyl group has been vindicated by the MM2 energy mini‐ mized program for the investigated photochemical substrates. The structures of these photoproducts were established from the analysis of their spectral parameters (IR, ¹H/¹³C NMR and Mass) and single crystal X‐ray crystallography data.     

NMR structural elucidation of photochromic quinolone photoproducts

UV irradiation of 3-benzoyl-2-benzyl-6,7-difluoro-1-propyl-1H-quinolin-4-one leads to the formation of a mixture of photoproducts, with different evolutions. The structure of each of them has been obtained by one- and two-dimensional multinuclear NMR experiments. Two photoenols, which are thermally reversible, have been identified. All of the other photoproducts have been assigned to degradation. Based on the structural identification and the photochemical and thermal evolution of samples, a plausible mechanism is proposed.     

Photo- and thermal-oxidation of polyethylene: Comparison of mechanisms and influence of unsaturation content

The behaviour of polyethylene with different contents in vinyl and t-vinylene groups have been studied by photooxidation with λ ≥ 300 nm light or by thermooxidation at a temperature of 100 °C. The oxidation was studied by infrared spectroscopy and it was shown that the same oxidation products were obtained, but with different relative concentrations depending on the conditions of ageing, i.e. photochemical or thermal conditions. The mechanisms by which the oxidation products are formed were recalled. The differences between photo- and thermo-oxidation were evidenced on the basis of the stability of ketones that do not accumulate in photochemical conditions, as a result of Norrish reactions. The influence of the initial amount of unsaturated groups on the rates of oxidation was characterized. It was shown that the concentration of unsaturations had no effect on the rate of photooxidation but dramatically influenced the stability in thermooxidative conditions.     

Identification of flurbiprofen and its photoproducts in methanol by gas chromatography-mass spectrometry

A sample of 10 mM flurbiprofen in methanol (or ethanol) was photoirradiated with sixteen 8 W low-pressure quartz mercury lamps irradiated at 306 nm in a Panchum PR-2000 photochemical reactor. In total, four major photoproducts derived from each sample were observed from the HPLC chromatogram. The photoproducts were separated and their structures elucidated by various spectroscopic methods. Alternatively, using GC-MS, 11 major photoproducts were observed. A reaction scheme of flurbiprofen in methanol is proposed: the photochemical reaction routes occur mainly via esterification and decarboxylation, followed by oxidation with singlet oxygen to produce a ketone, alcohols and other derivatives.     

Analysis of barbiturates by micro-high-performance liquid chromatography with post-column photochemical derivatization

This study attempts to combine the advantages of microcolumn high-performance liquid chromatography (μ-HPLC) with those of post-column photochemical derivatization in barbiturate analysis. Some barbiturates are photochemically unstable, leading to photoproducts that show maximum absorption at 270 nm and not the typical one at ≈220 nm. For this purpose, a laboratory-built photoreactor has been developed to work with μ-HPLC instruments. Its performance is satisfactory in the forensic analysis of barbiturates.     

FLUORESCENCE AND PHOTOCHEMISTRY OF OLIGOCYTIDYLIC AND POLYCYTIDYLIC ACIDS IN AQUEOUS SOLUTION

Abstract —In addition to the monomer-like fluorescence, a long-wavelength emission (Λ^max_em= 410 nm) has been detected in the dinucleoside 5'-5' pyrophosphate (CppC) at room temperature. This emission looks very similar to that previously reported for the acidic forms of Poly C (Poly C. Poly C⁺ and Poly I. Poly C. Poly C⁺). Only the monomer-like emission (Λ^max_em= 330 nm) can be detected in neutral Poly C, acidic CppC, and the neutral or protonated forms of the dinucleoside phosphate CpC.A correlation between the room temperature fluorescence of oligo and polycytidylic acids and their photochemical behaviour is found. Irradiation of all the polymeric samples at both neutral and acid pH results in the formation of minor photoproducts. They have been characterized by their absorbance (in the range 300–400 nm) and their fluorescence spectra. The same product is obtained in all cases where the monomer-like fluorescence only is detected. Distinct products are formed in neutral CppC and in the acidic Poly C forms.
The results are discussed with respect to the conformation of the oligo and polycytidylic acids and possible relationships between the 410–420 nm emission and adduct formation. An excimer is proposed as a common, intermediate excited state in both radiative deactivation and adduct formation in neutral CppC and the acidic Poly C forms.     

Photoinduced fluorimetric determination of folic acid and 5-methyltetrahydrofolic acid in serum using the kinetic evolution of the emission spectra accomplished with multivariate second-order calibration methods

Second-order multivariate calibration methods in combination with a continuous flow system, which allows for the continuous on-line irradiation of the analytes, have been employed for the determination of folic acid and its main metabolite 5-methyltetrahydrofolic acid in serum samples. An experimental central composite design, together with response surface methodology, has been used to find the optimum instrumental variables to perform the photochemical reaction. The time evolution of the emission spectra of the generated photoproducts, in the range 330-540 nm, after irradiation at 275 nm for 20 min, provided the three-way data set employed. On the basis of the differences on the kinetic rates of the photoreaction of both analytes, direct determination of the compounds in human plasma has been accomplished. The second-order methods assayed were parallel factor analysis (PARAFAC), self-weighted alternating trilinear decomposition (SWATLD), and unfolded partial least-squares (U-PLS), multidimensional partial least-squares (N-PLS), and bilinear least-squares (BLLS), all three in combination with the residual bilinearization procedure (RBL).     

INVESTIGATION OF PROTRIPTYLINE PHOTOPRODUCTS WHICH CAUSE CELL MEMBRANE DISRUPTION

Abstract— Protriptyline (PTL; N-methyl-5H-dibenzo[a,d]cycloheptene-5-propylamine) hydrochloride is a skin photosensitizing agent in humans. The fluorescence and photochemical behavior of PTL varies with the solvent. In water, 40% ethanol and ethanol in the hydrochloride salt of PTL has a fluorescence quantum yield of 0.81. The fluorescence quantum yield of PTL free base in selected organic solvents is between 0.41 and 0.17; in ethanol it remains at 0.81. Photolysis of PTL in acidic aqueous solution yields at least five photoproducts which were separated by high-performance liquid chromatography. Three of these photoproducts lysed red blood cells. One of the photoproducts has been identified as a cyclobutyl photodimer of PTL based on its mass spectrum and UV absorption and its ability to undergo photore-versal with 254 nm irradiation. The others were not cyclobutyl dimers. The yield of lytic products decreased as the ethanol content was increased and were not formed from PTL free base in any solvent.     

Photochemical Hydrogen Abstraction and Electron Transfer Reactions of Tetrachlorobenzoquinone with Pyrimidine Nucleobases

Pentachlorophenol, a widespread environmental pollutant that is possibly carcinogenic to humans, is metabolically oxidized to tetrachloroquinone (TCBQ) which can result in DNA damage. We have investigated the photochemical reaction dynamics of TCBQ with two pyrimidine type nucleobases (thymine and uracil) upon UVA (355 nm) excitation using the technique of nanosecond time-resolved laser flash photolysis. It has been found that 355 nm excitation populates TCBQ molecules to their triplet state ³TCBQ^*, which are highly reactive towards thymine or uracil and undergo two parallel reactions, the hydrogen abstraction and electron transfer, leading to the observed photoproducts of TCBQH· and TCBQ·^- in transient absorption spectra. The concomitantly produced nucleobase radicals and radical cations are expected to induce a series of oxidative or strand cleavage damage to DNA afterwards. By characterizing the photochemical hydrogen abstraction and electron transfer reactions, our results provide potentially important molecular reaction mechanisms for understanding the carcinogenic effects of pentachlorophenol and its metabolites TCBQ.     

Photochemical Rearrangements of 6-Methoxymethylbicyclo[4.4.0]Dec-1-En-3-One and 6-Methoxymethylbicyclo[4.3.0]Non-1-En-3-One

The irradiation of the title compounds 6 and 12 in isooctane and benzene was investigated. The photochemical reactions took place sluggishly upon n→π^* excitation (λ > 300 nm) in deoxygenated solutions but more rapidly in the presence of air; deconjugation to yield β,γ-unsaturated ketones and lumiketone rearrangement were the main primary photochemical reactions. In contrast, intramolecular hydrogen abstraction by the C_α-carbon was the main reaction upon π→^* excitation (λ = 254 nm).     

PHOTOINITIATED DNA DAMAGE BY MELANOGENIC INTERMEDIATES IN VITRO

Cysteinyldopas, metabolic by-products of activated melanocytes, are photochemically unstable in the presence of biologically relevant ultraviolet radiation (i.e. wavelengths > 300 nm). Initial photochemical processes involve free radical production; continued photolysis yields polymeric photoproducts. Radicals produced during 5SCD photolysis were trapped by 5,5-dimethyl-l-pyrrolidine-l-oxide (DMPO) and identified by their ESR spectra. Further characterization by use of nitroso spin trap (2-methvl-2-nitrosopropane-MNP) demonstrated that homolytic cleavage of the -S-CH₂ bond of the 5SCD cysteinyl side chain is a significant photochemical pathway. The potential photobiological significance of these reactive intermediates was investigated in vitro using isolated nucleic acids. Radiolabeled 5-[³⁵S]-cysteinyldopa was found to photobind to calf thymus DNA with 300 nm light activation. Under similar conditions, 5-S-cysteinyldopa also induced single strand breakage of ³H-radiolabeled superhelical, circular pBR322 plasmid DNA. The implications of the 5SCD photoinitiated DNA damage and the production of highly reactive free radicals in this process are discussed with respect to the etiology of various skin cancers, particularly malignant melanoma.     

THE VACUUM ULTRAVIOLET PHOTOCHEMISTRY OF NUCLEOTIDES

Abstract— Photodestruction of nucleotides by vacuum UV (λ < 200 nm) and near UV radiation in thin films has been studied; the products were analyzed by thin-layer chromatography. Vacuum UV photolysis leads to the rupture of the N-glycosidic bond with liberation of bases, the quantum yield of the process being about 2 × 10^-4 for guanosine-5'-monophosphate. No bases have been found among the products of near UV photolysis.
Vacuum UV irradiation of aqueous solutions of adenosine-5'-monophosphate and guanosine-5'-monophosphate produces, along with bases, some other fluorescent substances. The identity of photoproducts of vacuum UV photolysis with those of γ-radiolysis is established. The mechanism of photochemical conversion is discussed.     

SPECTROSCOPIC PROPERTIES OF PSORALEN DERIVATIVES SUBSTITUTED BY CARBETHOXY GROUPS AT THE 3,4 AND/OR 4,5' REACTION SITE

Abstract— The newly synthesized linear psoralen derivative 3-carbethoxypsoralen has been shown recently to behave as a monofunctional derivative and has attracted some interest in the psoriasis treatment. In a first attempt to understand, by the fluorescence technique, the molecular mechanism by which it interacts with DNA, a spectroscopic study of the molecule was undertaken. The fluorescence of 3-carbethoxypsoralen at room temperature resembles that of 8-methoxypsoralen with a ten times higher quantum yield. 365 nm irradiation of dilute solutions of 3-carbethoxypsoralen rapidly leads to the formation of two types of highly fluorescent photoproducts. Type 1 photoproducts (λ_em^max= 425 nm, λ_exc^max= 360 nm) have been identified as the result of the addition of a solvent molecule to the 4,5' reaction site of the molecule. Their fluorescence intensity is a hundred times higher for 3-carbethoxypsoralen than for 8-methoxypsoralen. They become negligible when the 4',5' reaction site carries also a carbethoxy group. Type 2 photoproducts exhibit a somewhat different emission (λ_em^{max =} 443 nm, λ_exc^max= 413 nm). They are probably the result of an opening of the furocoumarin molecule. The implications of the peculiar photochemical properties of 3-carbethoxypsoralen are discussed in view of its biological activity. In addition, the use of fluorescence in monitoring the photobinding of psoralens to DNA is also discussed     

UV-Induced unimolecular photochemistry of diketene isolated in cryogenic inert matrices

Diketene (C(4)H(4)O(2)) monomers were isolated in cryogenic Ar (15 K) and Xe (30 K) matrices. The infrared (IR) spectra of the freshly deposited matrices show that diketene monomers exclusively adopt the 4-methylene-oxetan-2-one form. In situ photochemical transformations of diketene were induced by tunable UV laser light. Diketene was found to be photostable when exposed to near-UV irradiations (λ> 300 nm). Irradiations in the middle-UV domain showed different types of photochemical reactivity occurring upon irradiations with 280 > λ > 240 nm and λ = 225 nm. The photoproducts were characterized by IR spectroscopy supported by B3LYP/6-311++G(d,p) calculations. Upon irradiation in the 280 > λ > 225 nm range, diketene was found to decompose in two ways: (i) with production of two parent ketene molecules (O═C═CH(2)), and (ii) with production of cyclopropanone (CP) plus carbon monoxide. For irradiations in the 280 > λ > 240 nm range, diketene exhibited two additional reactions: (iii) decomposition to allene (H(2)C═C═CH(2)) and carbon dioxide, and (iv) isomerization into cyclobutane-1,3-dione (CB). Of the above photoproducts, CP and CB were consumed by the same UV irradiations that resulted in their generation. Positive spectroscopic identification of CP and CB turned out to be possible with near-UV irradiations: CP decomposes to ethylene and carbon monoxide upon irradiation with λ = 345 nm; CB decomposes exclusively to two parent ketene molecules, without isomerization back to diketene or decarbonylation, upon irradiation with λ = 330 nm. Natural bond orbital (NBO) analysis showed that the two lowest excited singlet states of diketene are almost degenerate in energy and correspond to π* orbitals of C═C and C═O moieties. The NBO calculations helped to establish that the third excited singlet state, in terms of energy, has σ*(3s) Rydberg character, in accord with the literature.     

Study of a reaction between 2,3-dichloro-1,4-naphthoquinone and N,N'-diphenyl thiourea involving an EDA adduct as intermediate

The reaction between 2,3-dichloro-1,4-naphthoquinone and N,N'-diphenyl thiourea in acetonitrile medium, which yields the product, 2,3-(N,N'-diphenylthioureylene)-naphtho-1,4-quinone has been found to take place in two ways--thermal and photochemical. The thermal (dark) reaction occurs through an electron donor-acceptor (EDA) adduct as intermediate with evolution of HCl and kinetic data fit into the scheme A + B<==>AB(fast)-->product(slow) Formation constant of the EDA adduct and the rate constant of the slow process have been determined at four different temperatures from which the enthalpy of formation of AB has been determined. The photochemical reaction has been studied with 360 nm ordinary light and also with 365 and 370 nm laser beams. Use of laser causes about 10(3)-fold increase in the rate of the reaction but does not affect the quantum yield. The final product has been isolated and characterised by elemental analysis, 1H and 13C NMR, IR spectroscopy and mass spectrometry.     

The photochemical attachment of the O-glucoside of 3-hydroxykynurenine to alpha-crystallin: a model for lenticular aging

The young human lens contains a small metabolite from tryptophan called the O-glucoside of 3-hydroxykynurenine (3-HKG). Its function is to absorb most radiation between 295 and 400 nm, preventing it from reaching the retina. With age the concentration of this component decreases while the lens crystallins acquire covalently attached chromophores. This study investigates the photochemical attachment of 3-HKG to lens alpha-crystallin. Initial studies showed that alpha-crystallin photolyzed in the presence of 3-HKG developed a fluorescence (emission, 440 nm) and UV-visible spectrum similar to that found in aged human lens proteins. Extensive studies were then performed on the tryptic HPLC maps as monitored by photodiode array and fluorescent detection. Numerous photoproducts with either blue (emission, > 400 nm) or green (emission, > 500 nm) fluorescence were formed in addition to nonfluorescent compounds with absorption maxima above 300 nm. Comparisons were made between these model photoproducts and peptide maps from alpha-crystallin isolated from old human lenses. In terms of retention time and UV-visible spectra at least two of the peptides that appear in the model system are also present in the human samples. It is concluded that one of the aging processes in the human lens is the photochemically induced attachment of 3-HKG to lens proteins.     

Intramolecular NH/pi complexes of 2-allylaniline derivatives in the ground and excited states

The photochemical and photophysical properties of three 8-allyl-1,2,3,4-tetrahydroquinolines (1a-c) have been studied. These compounds exhibit a 2-allylaniline-like photochemical behavior, undergoing photocyclization to lilolidines (3a-c). The absorption, emission, and excitation spectra of 1a-c, employing convenient model compounds for comparison, demonstrate the formation of a NH/pi intramolecular ground-state complex (AB). This species can absorb light at long wavelengths (330-340 nm), giving rise to the corresponding excited complex AB*. Emission from AB* is red-shifted (420 nm) with respect to that observed when the monomer A is excited (lambda(exc) = 300 nm). These experimental results have been rationalized by means of density-functional theory calculations.     

Purine photoproducts

Abstract— DNA purine modifications by ultraviolet irradiation have not been as extensively studied as those of pyrimidines. However, a number of such reactions have been identified. These include photochemical addition of amino acids, photoalkylation by alcohols, amines and other compounds, photochemical activation of procarcinogens to mutagenic electrophiles, and formation of covalent linkages between DNA purines and adjacent bases. The recent characterization of two adenine-adenine di-adducts and the finding of endonucleases from two sources that incise ultraviolet-irradiated DNA at purine photoproducts indicate the possible biological importance of these moieties.