PHOTOCYCLOADDITION OF ACETONE TO URACIL AND CYTOSINE

Abstract— Ultraviolet irradiation (Λ > 280 nm) of uracil in aqueous acetone (1:1) produces cyclobutane uracil dimer and uracil-acetone addition product. The addition product is identified as an oxetane. The same product is also obtained from cytosine irradiated under the same conditions. The cytosine-acetone oxetane apparently undergoes deamination readily. Irradiation (Λ= 265 nm) of the oxetane in aqueous solution produces acetone and uracil with a quantum efficiency of 0–16.     

PHOTOCHEMISTRY OF URACIL. INTERSYSTEM CROSSING AND DIMERIZATION IN AQUEOUS SOLUTION

Abstract— Ultraviolet irradiation of ¹⁴C-uracil in aqueous solution results in the formation of hydrate and dimer photoproducts. The rate of dimerization increases with increasing uracil concentration, and decreases with increasing concentration of oxygen in solution. The kinetics are in agreement with a model previously proposed to account for the reactions, in which dimerization occurs by a reaction involving the triplet state of uracil, but hydration occurs from an excited singlet state. Oxygen reduces dimer formation by quenching the triplet. The quantum yield for intersystem crossing (ISC) to the triplet depends on the irradiation wavelength, increasing from 0.0014 at 280 nm to 0.016 at 230 nm. The ratio of rate constants for reaction of the triplet with oxygen and for dimerization is 1.1; the ratio of rate constants for triplet decay and for dimerization is 5.9 × 10^-5 M. The increase in ISC with photon energy suggests that ISC is favoured from excited vibrational levels. The quantum yield for hydration is about 0.002 at pH 4.5 for all wavelengths, but increases as the pH is decreased.     

紫外线光照产物——胸腺嘧啶二聚体物理化学性质的研究

本文分离和鉴定了紫外线光照胸腺嘧啶冰冻水溶液时迅速形成的低吸收的胸腺嘧啶二聚体,研究了和光化学与光生物学有关的胸腺嘧啶二聚体的一些性质。除了它的酸和热稳定性之外,实验表明,胸腺嘧啶二聚体的化学稳定性受加入的金属离子Ag⁺、Fe⁺⁺、Cu⁺⁺和EB染料的影响;而且胸腺嘧啶二聚体和胸腺嘧啶之间的相互转换依赖于金属离子的种类和浓度。     

ACETONE-SENSITIZED DIMERIZATION OF CYTOSINE DERIVATIVES*

Abstract— Irradiation of cytosine derivatives in aqueous solution in the presence of acetone with light of wavelength > 290 nm results in the formation of cyclobutane-type dimers. A mixture of cis-syn, cis-anti and trans-anti isomers are formed, the trans-anti dimer being the major product. The trans-anti isomers have been isolated as crystalline products and are found to be fairly stable in the solid state. These dimeric products undergo deamination in neutral aqueous solution. In acidic solution monomerization is an important reaction whereas in alkaline solution ring opening is the major mode of degradation. The results of analysis of the photo-products are compared with those of sensitized irradiation of uracil and undine.     

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.     

PHOTOREACTIONS AT 3655Å BETWEEN PYRIMIDINE BASES AND SKIN-PHOTOSENSITIZING FUROCOUMARINS

Abstract After irradiation at 3655 Å of an aqueous frozen solution containing thymine and psoralen, a new photocompound was isolated by column chromatography. It contains a furocoumarin and a pyrimidine-moiety linked together by the formation of a cyclobutane ring (see formulas II and III). By irradiation at 2537 Å in acetic acid solution, the photocompound breaks up again yielding psoralen and thymine. From an aqueous frozen solution containing cytosine and psoralen irradiated at 3655 Å, an analogous photocompound was obtained, which, however, consists of the addition to psoralen of a uracil molecule, instead of a cytosine one (IV, V). It has been stated that the hydrolytic deamination of the cytosine moiety to the uracil one takes place during the working up of the photocompound in aqueous solution after irradiation. Substances with properties similar to those above were obtained from bergapten (5-methoxy-psoralen) and thymine, from psoralen and thymidine or thymidylic acid, irradiated at 3655 Å.
The new substances may be considered as model compounds in explaining the photoreactions which take place between the skin-photosensitizing furocoumarins and DNA upon irradiation at 3655 Å.     

THYMINE HYDRATE FORMED BY ULTRAVIOLET AND GAMMA IRRADIATION OF AQUEOUS SOLUTIONS

Abstract— Evidence is presented for the formation of a thymine hydrate upon ultraviolet (UV) or gamma irradiation of aqueous solutions. The UV quantum efficiency exhibits a dependence on pH similar to that shown for uracil hydration, but the yield is three orders of magnitude smaller than for uracil. Hydration is not affected by wavelength, oxygen, or concentration of thymine. The reversal rate of the photohydrate to thymine is similar to the reversal rates of both isomers of the thymine hydrate formed by γ radiolysis, and depends on pH in the same way as the rate for the uracil photohydrate. The photohydrate of thymine is chromatographically identical to the cis isomer of 6-hydroxy-5, 6-dihydrothymine.     

ULTRAVIOLET PHOTOCHEMISTRY OF THYMINE IN AQUEOUS SOLUTION

Abstract— We have investigated the ultraviolet photochemistry of thymine in aqueous solution. Four isomeric dimers are produced, and the yield of each has been measured as a function of thymine concentration, oxygen concentration, and temperature. At low thymine concentration, dimerization proceeds via the triplet state, while at high concentration it arises mainly from aggregates, probably from a singlet precursor. We have determined the ratios of rate constants for the triplet state mechanism and the quantum yield for dimerization from aggregates. The quantum yield for dimerization from the triplet state in thymine is smaller by a factor of about 10 than that in uracil, which in turn is smaller by another factor of 10 than that in orotic acid. It increases with the energy of the exciting radiation in a manner similar to the behaviour of uracil and orotic acid. On the other hand, dimer formation from aggregates is independent of photon energy. Dimerization from aggregates decreases with increasing temperature, while the total production of dimers from the triplet state is independent of temperature.     

PHOTOSENSITIZED REDUCTION OF PYRIMIDINE BASES BY TRYPTOPHAN*

Abstract— Thymine and uracil were chemically altered when irradiated with UV light in aqueous solution containing tryptophan as a photosensitizer. The reaction is inhibited by oxygen and is therefore not an example of photodynamic action. Unlike the pyrimidine bases, purine bases were not altered under similar reaction conditions. Two major photoproducts were identified. One of the products was identified as the reduced base, dihydrouracil or dihydrothymine. The quantum yield for formation of dihydrouracil was 0.24 times 10^--⁴ to 13.6 times 10^--⁴, depending upon the concentrations of uracil and the reaction temperature. Radical scavengers such as cysteine and nitrous oxide decreased the rate of dihydrouracil formation. Other indole compounds also sensitized the photoreduction of uracil, their quantum yields ranging from <1 × 10^--⁵ for tryptamine to 1.3 times 10^--³ for indole-3-acetic acid. A reaction mechanism is presented whereby the pyrimidines are reduced by electron transfer from a metastable charge transfer complex originating from the first excited singlet state of tryptophan.     

PHOTOREACTIONS OF 5-BROMOURACIL IN THE PRESENCE OF CYSTEINE AND GLUTATHIONE

Abstract —Sulfhydryl compounds such as cysteine and glutathione enhance the photochemical reactivity of 5-bromouracil irradiated in aqueous solution with ultraviolet light (Λ > 240 nm). Production of uracil and addition of the sulfhydryl compound to the 5-bromouracil residue are the major photoreactions.     

γ- and ultraviolet-radiation-induced solid-state polymerization of maleimide in a few binary systems

The solid-state polymerization of maleimide by γ- and ultraviolet irradiation was carried out in binary systems with succinimide, maleic anhydride, and acenaphthylene. Polymaleimide obtained from the solid-state polymerization of maleimide by γ-rays was amorphous, while that obtained from the solid-state polymerization by ultraviolet rays was highly crystalline. In the maleimide–succinimide system the rate of polymerization reached a maximum nearly at the eutectic composition when the polymerization was carried out by γ-irradiation. With ultraviolet irradiation the rate of polymerization became higher with increasing content of succinimide in the feed. In the maleimide–maleic anhydride system a copolymer of both constituents was formed by γ-irradiation, but almost no homopolymer was produced. On the other hand, two kinds of polymers, a crystalline copolymer and an amorphous one, were produced by ultraviolet irradiation. The results were compared with those obtained from the copolymerization in solution. In the maleimide-acenaphthylene system the main products with ultraviolet irradiation was the dimer of acenaphthylene.     

KINETICS OF DIMER FORMATION AND PHOTOHYDRATION IN ULTRAVIOLET-IRRADIATED POLYURIDYLIC ACID

Abstract— The decrease in absorbance at 2600 Å of poly-U following u.v. irradiation is analyzed quantitatively in terms of uracil-dimer formation and photohydration of uracil residues. At all wave lengths tested between 2300 and 2800 Å both dirner formation and hydration occur as well as dimer breakage. The cross section for uracil dirner breakage is large at 2300 and decreases to a relatively small value at 2800 Å. Consequently, at the shorter wave lengths the steady state for dimer formation and breakage is reached at lower doses than for the longer wave lengths. Absorbance decreases caused by longer wave lengths can be partially restored by irradiation with 2380 Å. At high doses, for all wave lengths, the main photoproducts are hydrated uracil residues. The maximum number of dimers that may be formed increases with wavelength. The absorbance loss caused by photohydration may be reversed by acidification and heat. For 2650 Å irradiation, 95 per cent of the absorbance is restored by a combination of the 2380 Å and thermal treatment. The values for the photochemical reactions of poly-U are fairly close to those for poly-T and for uridine.     

ROLE OF COMPLEX FORMATION IN THE PHOTOSENSITIZED DEGRADATION OF DNA INDUCED BY N'-FORMYLKYNURENINE

Abstract— N'-Formylkynurenine derivatives efficiently bind to DNA or polynucleotides. Homopolynucleotides and DNA display marked differences in the binding process. Association constants are derived which indicate that the oxidized indole ring is more strongly bound to DNA than the unoxidized one. Irradiation of such complexes with wavelengths greater than 320 nm induces pyrimidine dimer formation as well as DNA chain breaks. Complex formation is shown to play an important role in these photosensitized reactions.
The photodynamic action of N'-formylkynurenine on DNA constituents is negligible at neutral pH but guanine and xanthine derivatives are sensitizable at higher pH. Thymine dimer splitting can occur in aggregated frozen aqueous solutions of N'-formylkynurenine and thymine dimer but this photosensitized splitting is negligible in liquid solutions at room temperature.     

紫外光照下尿嘧啶在磷酸盐水溶液中的新型光化学反应研究

在中压汞灯光照下,无机磷酸盐能促进尿嘧啶水溶液(pH=8)的光解作用(磷酸盐效应),发生嘧啶碱基的光解取代反应,主要光解产物为6-磷酸基尿嘧啶(C₄H₅N₂O₆P).通过元素分析,UV,IR,EIMS,¹HNMR,¹³CNMR,³¹PNMR等测试手段和方法,确定了光解产物的组成和结构.实验表明,在中压汞灯的发射光谱(连续光谱)中,对磷酸盐效应起作用的波长为190～220nm.     

Photochemistry of tobacco mosaic virus RNA. Effect of HCN

Abstract— In attempting to sort out possible mechanisms of photoreactivation of tobacco mosaic virus RNA (TMV-RNA) inactivated by ultraviolet radiation (u.v.) in buffer of ionic strength 0.25, we have investigated the effect of HCN on the quantum yield for u.v. inactivation of TMV-RNA and on the percent photoreactivation of inactivated TMV-RNA. Some photo-products produced by irradiation of model substances, polyuridylic acid (poly U) and polycytidylic acid (poly C), in the presence of HCN have also been studied. The ratio of the quantum yield for inactivation of TMV-RNA in the presence of HCN to that in the absence of HCN is 1.5, under non-photoreactivating conditions. By comparison, the ratio of the initial rates of loss of uracil residues in poly U under comparable conditions is 1.6; by contrast, the rate of loss of cytosine residues in poly C is unaffected by HCN. This similarity of ratios between poly U and TMV-RNA suggests that two of the mechanisms of u.v. inactivation of TMV-RNA at high ionic strength are akin to known reactions of uracil residues in poly U, i.e. hydrate and dimer formation. The photohydration reaction in poly U, as measured by the heat reversal of hydrated residues to uracil residues, is almost abolished by HCN, and the rate of dimerization, as measured by the appearance of dimer containing oligonucleotides following enzymatic hydrolysis of irradiated poly U, is reduced to half by HCN. HCN does not affect the rate of hydration of cytosine residues in poly C. Since photoreactivation of RNA inactivated in presence of HCN is only 60 per cent of that in absence of HCN it is suggested that uracil dimers are somehow involved in photoreactivation of TMV-RNA inactivated at high ionic strength.     

STUDIES ON THYMINE AND URACIL TRIPLET EXCITED STATE IN ACETONITRILE AND WATER

Abstract— Thymine and uracil triplet-triplet absorption spectra and triplet excited state lifetimes have been observed in acetonitrile and water by nanosecond laser flash spectroscopy. A study of triplet energy transfer from these pyrimidines to retino! has also allowed an estimation of the triplet extinction coefficient ε_TT of thymine and uracil. These ε_TT were then used to determine the triplet quantum yields Ø_T of both pyrimidines in acetonitrile and water.     

The photochemistry of uracil: a reinvestigation

Results from a re-examination of the photochemical reactions undergone by uracil (Ura) are presented. Irradiation of Ura in frozen aqueous solution at -78.5°C produces two diastereomeric (6-4) products, namely the cis and trans isomers of 5-hydroxy-6-4'-(pyrimidin-2'-one)-5,6-dihydrouracil. Upon heating in 0.1m HCl, each of these compounds decomposes to form 6-4'-(pyrimidin-2'-one)uracil. In addition, evidence for production of a hydrate of a trimer of Ura is presented, Irradiation of this compound at 254nm forms Ura and a (6-4) adduct as products. The compounds 5-4'-(pyrimidin-2'-one)uracil and 6-hydroxy-5,6-dihydrouracil were also present after Ura was irradiated in frozen aqueous solution. Cyclobutane dimers (CBDs) are formed when Ura is irradiated in the frozen aqueous state, in fluid aqueous and acetonitrile solution and in the presence of photosensitizers (e.g. acetone). Published information, concerning the identity and relative quantitative importance of the four CBD isomers (cis-syn, cis-anti, trans-syn and trans-anti) formed in these photochemical systems, is incomplete and often in substantial disagreement. Using chemical methods in conjunction with HPLC, the identity and relative amounts of the four dimers have been determined in each of these systems. Consequently, a number of inconsistencies found in the literature concerning dimer product identity and quantitative distribution have been resolved.     

KINETICS OF THE ULTRAVIOLET-INDUCED DIMERIZATION OF THYMINE IN FROZEN SOLUTIONS*

Abstract— It is known that thymine forms dimers when aqueous solutions are irradiated with ultraviolet light while in the frozen state, but does not form dimers when solutions are irradiated in the liquid state. The eutectic point of aqueous thymine solutions was found to be. —0.02°C. Since the irradiation of frozen solutions is always carried out at lower temperatures, the dimerization must be occurring in the solid state. Activation energies and quantum yields for dimer formation were determined by irradiating 1–mm layers of thymine solution at —5°C to — 707deg;C for various lengths of time. As expected, the activation energy was zero. After measuring the amount of radiation scattered by samples of ice, the extreme values for the quantum yield were found to be 0.73 and 4.08. The lower limit assumed that all the scattered light was absorbed by thymine; the upper limit assumed that none was absorbed. Since the theoretical maximum quantum yield is 2, the best estimate of the quantum yield is considered to be between 1 and 2.     

Functional monomers and polymers 159 synthesis and photochemical reactions of polymers containing thymine photodimer units in the main chain

Polyamides containing thymine photodimer units in the main chain were synthesized, and their photolysis by ultraviolet irradiation below 260 nm were studied in film state. Photodimers of thymine derivatives were obtained by photochemical reaction of the carboxylic acid derivatives of thymine in aqueous solution irradiated above 270 nm. An attempt was made to resolve the isomers of the photodimers, and the two kinds of cis isomers [cis–syn(head to head), and cis–anti(head to tail)] were isolated successfully. The polyamides were prepared by condensation of the photodimers with diamine using an activated ester method. The photodissociation of the thymine photodimer in the polymer main chain caused the breakage of the polymer chains, leading to the production of oligomers and dimer compounds containing thymine bases at the ends of the molecule. The dissociation rate of the polymer did not depend on the kind of the thymine photodimer which was in the main chain of the polymer.     

H-NMR studies of duplex DNA decamer containing a uracil cyclobutane dimer: implications regarding the high UV mutagenecity of CC photolesions

To determine the origin of the UV-specific CC to TT tandem mutation at the CC site, we made a duplex DNA decamer containing a uracil cis-syn cyclobutane dimer (CBD) as the deaminated model of a cytosine dimer. Two-dimensional 1H-NMR spectroscopy studies were performed on this sequence where two adenines (Ade) were opposite to the uracil dimer. Two imino protons of the uracil dimer were found to retain Watson-Crick hydrogen bonding with the opposite Ade, although the 5'-U(NH) of the dimer site showed unusual upfield shift like that of the 5'-T(NH) of the TT dimer, which seemed to be associated with deshielding by the flanking base rather than with reduced hydrogen bonding. (McAteer et al. 1998, J. Mol. Biol. 282:1013-1032). Hydrogen bondings at the dimer site were also supported by detecting typical strong nuclear Overhauser effects (NOE) between two imino protons and the opposite Ade H2 or NH2. But sequential NOE interactions of base protons with sugar protons were absent at the two flanking nucleotides of the 5' side of the uracil dimer and at the intradimer site, contrasting with its thymine analog where sequential NOE was absent only at the A4-T5 step. In addition, NOE cross peak for U5(NH) <--> A4(H2) was detected, although the NOE interactions of U6(NH) with A7(H2) and A17(H2) were not observed in contrast to the thymine dimer duplex. This different local structural alteration may be affected by the induced right-hand twisted puckering mode of cis-syn cyclobutane ring of the uracil dimer in the B-DNA duplex, even though the isolated uracil dimer had left-hand twisted puckering rigidly. In parallel, these observations may be correlated with observed differences in mutagenic properties between cis-syn UU dimer and cis-syn TT dimer.