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.     

1,2,4-Triazoles—XXXI: Photodimerization of s-triazoleo[4,3-a]pyridines: formation of cyclobutane dimers

Irradiation of several methyl substituted s-triazolo[4,3-a]-pyridines and 3-hydroxy-s-triazolo[4,3-a]pyridines with UV light gave thermally labile cyclobutane photodimers by dimerization of the 5,6-double bond in one molecule with the 7,8-double bond in another.     

MONOFUNCTIONAL 4',5'-PHOTOCYCLOADDUCT BETWEEN 4,5'-DIMETHYLANGELICIN AND THYMINE

Abstract A new fluorescent compound has been isolated from the products of hydrolysis of DNA irradiated in the presence of 4,5'-dimethylangelicin, a monofunctional photosensitizing furocoumarin. The marked similarity of the UV absorption and fluorescence spectra of this photoproduct to those of 4',5'-dihydro-4,5'-dimethylangelicin, as well as its behaviour on photodissociation (254 nm) yielding thymine and 4,5'-dimethylangelicin in equimolecular amounts, are consistent with a cycloadduct between one molecule of 4,5'-dimethylangelicin and one of thymine, and the C₄-cycloaddition occurs through 4',5'–double bond of the furocoumarin and the 5,6-double bond of the pyrimidine.     

Synthesis of (10Z)- and (10E)-19-fluoro-1alpha,25-dihydroxyvitamin D3: compounds to probe vitamin D conformation in receptor complex by 19F-NMR

To study the interaction of vitamin D with its receptor by 19F-NMR, (5Z,10Z)- and (5Z,10E)-19-fluoro-1alpha,25-dihydroxyvitamin D3 were synthesized starting from vitamin D2 via electrophilic fluorination of vitamin D-SO2 adducts as the key step. Regio- and stereoselective electrophilic fluorination at C(19) of vitamin D-SO2 adducts was achieved under the conditions using (PhSO2)2NF and bulky bases. The stereochemistry of the addition and elimination of SO2 of various vitamin D derivatives was studied in detail. SO2 causes Z-E isomerization of the 5,6-double bond of vitamin D and adds to the resulting (5E)-isomer from the sterically less hindered side opposite to the substituent at C(1). Elimination of SO2 from 19-substituted vitamin D-SO2 adducts proceeded exclusively in a suprafacial manner with respect to the diene part under either thermal or reductive conditions. Dye-sensitized photochemical isomerization of 19-fluorovitamin D derivatives was studied in detail. The rapid isomerization at the 5,6-double bond was followed by the slow isomerization at the 10,19-double bond to yield the (5E,10Z)-isomer (by nomenclature of the 1-OH derivatives) as the major product. (10Z)- and (10E)-19-Fluorovitamin Ds were also interconverted thermally probably via the corresponding previtamin D by 1,7-sigmatropic isomerization.     

Biphotonic Ionization of DNA: From Model Studies to Cell

Oxidation reactions triggered by low‐intensity UV photons represent a minor contribution with respect to the overwhelming pyrimidine base dimerization in both isolated and cellular DNA. The situation is totally different when DNA is exposed to high‐intensity UVC radiation under conditions where biphotonic ionization of the four main purine and pyrimidine bases becomes predominant at the expense of singlet excitation processes. The present review article provides a critical survey of the main chemical reactions of the base radical cations thus generated by one‐electron oxidation of nucleic acids in model systems and cells. These include oxidation of the bases with the predominant formation of 8‐oxo‐7,8‐dihydroguanine as the result of preferential hole transfer to guanine bases that act as sinks in isolated and cellular DNA. In addition to hydration, other nucleophilic addition reactions involving the guanine radical cation give rise to intra‐ and interstrand cross‐links together with DNA–protein cross‐links. Information is provided on the utilization of high‐intensity UV laser pulses as molecular biology tools for studying DNA conformational features, nucleic acid–protein interactions and nucleic acid reactivity through DNA–protein cross‐links and DNA footprinting experiments.     

Über Pterinchemie. 73. Mitteilung [1]. Zum Verlauf der katalytischen Reduktion von 7-Methylpterin

On the Pathway of the Catalytic Reduction of 7-Methylpterin The catalytic hydrogenation of 7-methylpterin (VII) in a neutral solution occurs first by the reduction of the 7,8-double bond (thermodynamically-controlled reaction) followed by the reduction of the 5,6-double bond. On the contrary, in an acidic medium like CF₃COOH, the 5,6-double bond is reduced first (kinetically-controlled reaction). The dihydro-intermediate then undergoes a [1,2]-H-rearrangement leading to the formation of the thermodynamically more stable 7-methyl-7,8-dihydropterin (XV) which on further reduction gives 7-methyl-5,6,7,8-tetrahydropterin (VIII). The catalytic reduction of 7-methyl-7,8-dihydropterin (XV) with deuterium gives stereoselectively a sole product with D at C(6) in the equatorial position.     

PHOTOCHEMICAL REACTIONS OF CYTOSINE N-METHYL ANALOGS IN ALCOHOLIC SOLUTION

Abstract— –The photochemical reactions of 1-methylcytosine (la), 1, N^4-dimethylcytosine (Ib), and 1, N⁴. N^4-trimethylcytosine (Ic) in isopropanol have been investigated. In all three reactions addition of isopropanol occurs across the 5,6-double bond of the pyrimidine ring. For example, in the case of Ir photo-reaction with isopropanol leads to formation of 5,6-dihydro-6–(2-hydroxy-2-propyl)-l, N⁴, N^4-trimethylcytosine (IIc). In all three reactions 5,6-dihydro-l-methyluracil (III) was also isolated as a product; in the case of Ic, 5,6-dihydro-1, N⁴, N⁴-trimethylcytosine (IV) was isolated as well. Evidence is presented that indicates cytosine, cytidine, and 2',3'-isopropylidene cytidine also form adducts with isopropanol. Photoinduced reaction of Ic with ethanol leads to formation of 5.6-dihydro-6–(l-hydroxy-l-ethyl)-l, N⁴. N^4-trimethylcytosine (V).
Both acetone sensitization and di-t-butyl peroxide initiated free radical reactions of Ib with isopropanol leads to formation of an adduct identical to that formed in the unsensitized photochemical reaction of Ib with isopropanol.     

MECHANISM OF THE PHOTOHYDRATION OF PYRIMIDINES: A FLASH PHOTOLYSIS STUDY*

Abstract— Based on comparisons between the microsecond flash photolysis of uracil and substituted uracils and the radiation chemistry of the corresponding 5,6-dihydro-2,4-dioxopyrimidines, it is proposed that the mechanism of photohydration of pyrimidine bases involves the formation of a pyrimidine carbocation. The effects of substitution in the pyrimidine ring and of pH are consistent with a proton transfer from water to the excited singlet state at theC–5 position of the pyrimidine ring. The resultant carbocation is thought to undergo solvolysis to form the photohydrate or eliminate a proton at N-l producing an intermediate isomeric form of the pyrimidine (isopyrimidine) which re-arranges to the parent pyrimidine by a first-order process.     

Unconventional nucleotide analogues: Reaction of carbenes with uracil and uridine derivatives

Carbenes 2a-d (:CCl₂, :CBr₂, :CFC1, :CHCOOC₂H₅) add to the 5,6-double bond of uracil derivatives la-e to give adducts in modest to good yields The unsymmetrically substituted carbenes :CFC1 2c and CHCOOC₂H₅, 2d lead, as expected, to the formation of mixtures of endo and exo-isomers 3c-n which were separated and identified via their ¹H NMR spectra. Reaction of 2d with pyrimidine derivatives 5a,b resulted in products which can be rationalized in terms of addition at either the 5,6 or the 3,4 double bond. Addition of carbenes 2a-c to uridine derivatives gave diastereomeric adducts 12-16 which were isolated and identified. The two diastereomeric series, referred to as A and B, have been correlated and assigned the configurations 5S, 6S and 5R, 6R, respectively, on the basis of X-ray structure analysis of 12B The adducts of 2a,c and the uridine derivatives 11a,b have been deprotected to give 7,7-dihalocyclothymidines.     

Blocking-Deblocking of the NH-Functionality of Uracil and its Derivatives

We have been interested in various 5,6-dihydrouracils and 5,6-dihydroorotic acid derivatives as possible inhibitors of dihydrouracil dehydrogenase, dihydroorotase, and dihydroorotate dehydrogenase.² The known methods for the synthesis of 5,6-dihydrouracils and 5,6-dihydroorotic acid derivatives are either a low yield cyclization process³ or a catalytic hydrogenation⁴ procedure which frequently led to the elimination of desired functionality⁵. In order to obviate these difficulties, we have recently developed⁶ a mild non-catalytic method for the reduction of 5,6-double bond of uracil and orotic acid derivatives. By using lithium tri-sec-butyl borohydride^7–8 we have been able to reduce N₁, N₃-dialkyl uracil and orotic acid derivatives to the corresponding 5,6-dihydrouracil and orotic acid derivatives as shown in scheme (1).     

Chlorimine formation and deamination of the model nucleobase 1-methylcytosine

Treatment of the model nucleobase 1-methylcytosine with chlorine in aqueous solution gives, depending on the reaction conditions, 5-chloro-1-methylcytosine ( 3 ) or its hemiprotonated form 2 , 5,5-dichloro-6-hyd-roxy-5,6-dihydro-4-chlorimino-1-methylpyrimidin-2-one ( 5 ), and 5,5-dichloro-6-hydroxy-5,6-dihydro-1-methyl-uracil ( 4 ). While substitution of H5 of 1-methylcytosine by chlorine giving 2 and 3 , and subsequent addition of hypochlorous acid to the 5,6-double bond is not unexpected, chlorination of the exocyclic amino group 5 and deamination to give 4 is novel. The X-ray structures of 4 and 5 are reported.     

Independent generation of the 5-hydroxy-5,6-dihydrothymidin-6-yl radical and its reactivity in dinucleoside monophosphates

Hydroxyl radical is a major reactive oxygen species produced by gamma-radiolysis of water or Fenton reaction. It attacks pyrimidine bases and gives the 5-hydroxy-5,6-dihydropyrimidin-6-yl radical as the major product. Here we report the synthesis of all four stereoisomers of 5-hydroxy-6-phenylthio-5,6-dihydrothymidine (T*), which, upon 254 nm UV irradiation, give rise to the 5-hydroxy-5,6-dihydrothymidin-6-yl radical (I). We also incorporated the photolabile radical precursors into dinucleoside monophosphates d(GT*) and d(TT*) and characterized major products resulting from the 254-nm irradiation of these dinucleoside monophosphates. Our results showed that, under anaerobic conditions, the most abundant product emanating from the 254-nm irradiation of d(GT*) and d(TT*) is an abasic site lesion. Products with the thymine portion being modified to thymine glycol and 5-hydroxy-5,6-dihydrothymine were also observed. In addition, we demonstrated that radical I can attack the C8 carbon atom of its 5' neighboring guanine and give rise to a novel cross-link lesion. Moreover, LC-MS/MS results showed that gamma-radiation of d(GT) under anaerobic condition yielded the same type of cross-link lesions.     

PHOTO-OXIDATION OF THYMINE SENSITIZED BY 2-METHYL-1,4-NAPHTHOQUINONE: ANALYSIS OF PRODUCTS INCLUDING THREE NOVEL PHOTO-DIMERS

Abstract —The near-UV photosensitization of thymine by 2-methyl-1,4-naphthoquinone in oxygenated aqueous solution leads to the formation of nine major thymine-derived products. Six of the photoproducts, representing 70% of the initial thymine loss, are also known to be formed by γ-ray or X-ray irradiation of oxygenated water solutions of thymine. They include the cis and trans isomers of 5-hydroperoxy-6-hydroxy-5,6-dihydrothymine, the cis and trans thymine glycols, 5-hydroxy-5-methylhydantoin, and N-formyl-N'-pyruvylurea. The three remaining major products, accounting for a further 25% of the initial thymine loss, are thymine di-adducts of a type not previously reported. Preliminary characterization of these novel photoproducts, based on their UV absorption, mass spectra, and proton and carbon-13 nuclear magnetic resonance spectra, suggests that in each of the three, the thymine moieties are linked by an N(1)-C(5) or N(1)-C(6) bond. A thymine cation radical is implicated as the common precursor of all 9 products. Accordingly. pyrimidine photosensitization by 2-methyl-1,4-naphthoquinone serves as a model system for studying the chemical consequences of radiation-induced ionization of the DNA bases.     

Acylation of the 4,5- and 5,6-double bond isomers of 3-steroidal thiazolidines: The chemical stability of spiro-3-steroidal 4,5- and 5,6-double bond isomers

It has been shown that the 5,6-double bond steroidal thiazolidines can be N-acylated if there is no substituent in the 4'-position on the thiazolidine ring; substituents in the 4'-position of the thiazolidine sterically hinder acylation. On the other hand, the 4,5-double bond steroidal thiazolidine isomers decomposed on attempted acylation. The lability of these 4,5-double bond isomers was attributed to the contribution of a hyperconjugative resonance form to the structure of the 4,5-double bond isomers.     

Electronic aspects of the photodimerization of pyrimidine bases and their derivatives

Results of calculations for the lower excited triplet and singlet states of pyrimidine bases and a number of their derivatives, calculated by the self-consistent field (SCF) method for open shells and by the configuration interaction (CI) method for self-consistent orbitals, are presented. It is shown that excitation to the T₁ and S₁ states primarily affects the C₅—C₆ bond while triplet excitation is almost completely localized on this bond. This leads to exceptionally pronounced weakening of the C₅—C₆ bond in the T₁ state. All other conditions being equal, the conditions in the T₁ state are more favorable for the dimerization of the bases than in the S₁ state. A correlation between the degree of localization of the triplet excitation at the C₅—C₆ bond and the ease of photodimerization has been established. The experimental facts associated with the formation of thymine free radicals in DNA are explained.The authors thank G. G. Dyadyusha and O. V. Shramko for discussing our results. The program for calculation of the bond orders by the CI method was kindly given to us by G. M. Kagan and G. I. Kagan.     

DNA Damage Induced by Late Spring Sunlight in Antarctica

Sunlight ultraviolet (UV) radiation constitutes an important environmental genotoxic agent that organisms are exposed to, as it can damage DNA directly, generating pyrimidine dimers, and indirectly, generating oxidized bases and single-strand breaks (SSBs). These lesions can lead to mutations, triggering skin and eye disorders, including carcinogenesis and photoaging. Stratospheric ozone layer depletion, particularly in the Antarctic continent, predicts an uncertain scenario of UV incidence on the Earth in the next decades. This research evaluates the DNA damage caused by environmental exposure to late spring sunlight in the Antarctic Peninsula, where the ozone layer hole is more pronounced. These experiments were performed at the Brazilian Comandante Ferraz Antarctic Station, at King’s George Island, South Shetlands Islands. For comparison, tropical regions were also analyzed. Samples of plasmid DNA were exposed to sunlight. Cyclobutane pyrimidine dimers (CPDs), oxidized base damage and SSBs were detected using specific enzymes. In addition, an immunological approach was used to detect CPDs. The results reveal high levels of DNA damage induced by exposure under the Antarctic sunlight, inversely correlated with ozone layer thickness, confirming the high impact of ozone layer depletion on the DNA damaging action of sunlight in Antarctica.     

Studies on the preparation and exchange reactions of 5-deuterated uracils

The mechanism of base catalyzed proton exchange at the 5-position of uracil and its N-methylated derivatives has been studied. These reactions proceed by addition — elimination across the 5,6-double bond when the 1-nitrogen is substituted with a methyl group, or with anchimeric assistance of the N-1 anion if the 1-position is unsubstituted. The base catalyzed hydrolyses of 1,3-dimethyluracil and 3-methyluracil also appear to proceed through hydrated intermediates. A facile method for an acid catalyzed preparation of 5-deuterated uracils is described as well as a simple and accurate method for analysis of deuterium content.     

Neighbouring group participation in the reaction of 7-oxo-ent-kaur-16-ene derivatives with diacetoxyiodobenzene. Synthesis of gibberellin analogues

The reaction of different 7-oxo-ent-kaur-16-ene derivatives with diacetoxyiodobenzene has been evaluated for the preparation of gibberellin analogues. Thus, the reaction of 7-oxo-ent-kaur-16-en-18-oic acid methyl ester (3) with this reagent afforded 4-epi-GA₁₂ dimethyl ester (6). This reaction constitutes a good procedure for the preparation of this type of compounds. In some cases, alternative reactions that led to the introduction in the substrate of a conjugated 5,6-double bond or to the formation of a ketal at the 6-position were also produced. The formation of these compounds, or of gibberellin analogues, depends on the neighbouring group participation of the different C-18 and C-19 substituents at C-4.     

PHOTOCHEMICAL REACTION BETWEEN PROTEIN AND NUCLEIC ACIDS: PHOTOADDITION OF TRYPTOPHAN TO PYRIMIDINE BASES*1

Abstract— The photochemical interactions between tryptophan and nucleic acid bases were studied. When aqueous solutions of tryptophan were irradiated (Λ > 260 nm) at neutral pH in the presence of each of the nucleic acid bases, pyrimidine bases but not purine bases were altered. Air was found to decrease the rate of reaction. Two classes of photoproducts were isolated by thin layer and ion-exchange chromatography and partially characterized. One was the dihydro-pyrimidine forms of the base (see Reeve and Hopkins, 1979) and the other consisted of tryptophan-pyrimidine photoadducts. Four tryptophan-uracil and two tryptophan-thymine adducts each with a 1:1 molecular stochiometry were found. Spectroscopic measurements and a positive reaction with Ehrlich's reagent indicate that the indole nucleus remained intact, but that the pyrimidine base was reduced at the 5, 6 double bond. The absence of a positive ninhydrin reaction and the effect of pH on the quantum yield of the photoadduct formation indicated that the ionized a-amino acid group of tryptophan was involved in photoadduct formation. Indole derivatives lacking an a-amino group were also found to form photoadducts with pyrimidine bases. The experimental results are consistent with a reaction mechanism involving tryptophan excitation to the first excited singlet state as the initial event. Radical scavenging experiments indicate that tryptophan free radicals, formed by electron dissociation from the excited state, react with the ground state pyrimidine.     

STUDIES ON THE PHOTO-C4-CYCLO-ADDITION REACTIONS BETWEEN SKIN-PHOTOSENSITIZING FUROCOUMARINS AND NUCLEIC ACIDS*

Abstract— Among the natural or synthetic furocoumarins (psoralens) a group exists which has interesting biological properties, the best known of which is skin-photosensitization. The mechanism of action has remained unclarified for a long time. Furocoumarins lack photooxidative properties; they act by a mechanism that does not require oxygen and are therefore different from photodynamic substances. Photosensitizing furocoumarins when irradiated at 365 nm react with nucleic acids to give a C₄-cyclo-addition to the 5,6-double bond of the pyrimidine bases engaging their 3,4- or 4‵,5‵-double bond. Differences exist in the behaviour of the various furocoumarins; psoralen reacts equally well with native DNA, with denatured DNA and with RNA, whereas bergapten, xanthotoxin and 8-methylpsoralen at room temperature react to a much greater extent with native DNA than with denatured DNA and with RNA. A temperature effect has also been observed. In the case of native DNA an intercalation, occurring in the dark, of furocoumarins between two adjacent base pairs of the double helix is suggested as the first step in the reaction. The photoreaction is not accompanied by breaks in the polynucleotide chain or by conformational modifications of the macromolecule. A parallelism was observed between the order of activity of the substances of this group for photoreaction with native DNA and for skin-photosensitization. Ehrlich ascites tumor cells lose completely their capacity of transmitting the tumor after irradiation in the presence of psoralen, bergapten and xanthotoxin. By hydrolysis of DNA extracted from Ehrlich ascites tumor cells irradiated in the presence of psoralen a photoadduct between psoralen and thymine was isolated.