Reaction of nucleic acids bases and their derivatives with peroxodisulfate ion

Reaction with peroxodisulfate ion was investigated, that is, reaction of 1,3-dimethyluracil, 1,3-dimethylthymine, and caffeine with carbon radicals formed from decarboxylation of carboxylic acids, oxidation of the methyl group at 5-position of thymines, and halogenation of nucleic acids bases and their derivatives with alkali halides.     

Oxidation of nucleosides and nucleotides by peroxosulfate ions

Treatment of 5-methylpyrimidine nucleosides and nucleotides with sodium peroxodisulfate in sodium phosphate buffer solution at pH 7.0 at 75 degrees C resulted in the selective oxidation of the methyl group. On the other hand, oxidation of thymidine by potassium peroxomonosulfate gave thymidine glycols.     

The reactions of the hydroxymethyl radical with 1,3-dimethyluracil and 1,3-dimethylthymine

Hydroxymethyl radicals .CH2OH, generated by the radiolysis of methanol (0.5 mol dm-3) in N2O-saturated aqueous solutions, were reacted with 1,3-dimethyluracil or 1,3-dimethylthymine (10(-3) mol dm-3). The products were identified and their G values determined. It has been concluded that in 1,3-dimethyluracil .CH2OH attack occurs only at C(6) while in 1,3-dimethylthymine there is partitioning between addition (two-thirds) and H-abstraction from the C(5)-methyl group (one-third). A rate constant for CH2OH addition to 1,3-dimethyluracil of about 10(4) dm3 mol-1 s-1 is estimated. Complexities that may arise in the radiolysis of pyrimidines such as 1,3-dimethylthymine, apparently as a consequence of the formation of 5-alkylidenepyrimidines, are discussed. A value of 0.15 has been estimated for the disproportionation/combination ratio for the hydroxymethyl radical self-termination reaction.     

Pulse radiolytic studies on uracil and uracil derivatives. Protonation of their electron adducts at oxygen and carbon

The electron adducts of uracil, 1,3-dimethyluracil and 1,3-dimethylthymine, known to protonate rapidly in aqueous solution at oxygen, are now shown to undergo a slower protonation at C(6) producing a radical centred at C(5), a reaction which can be catalysed by buffer.     

The SO4(.-)-induced chain reaction of 1,3-dimethyluracil with peroxodisulphate

The sulphate radical SO4(.-) reacts with 1,3-dimethyluracil (1,3-DMU) (k = 5 X 10(9) dm3 mol-1 s-1) thereby forming with greater than or equal to 90 per cent yield the 1,3-DMU C(5)-OH adduct radical 4 as evidenced by its absorption spectrum and its reactivity toward tetranitromethane. Pulse-conductometric experiments have shown that a 1,3-DMU-SO4(.-) aduct 3 as well as the 1,3-DMU radical cation 1, if formed, must be very short-lived (t1/2 less than or equal to 1 microsecond). The 1,3-DMU C(5)-OH adduct 4 reacts slowly with peroxodisulphate (k = 2.1 X 10(5) dm3 mol-1 s-1). It is suggested that the observed new species is the 1,3-DMU-5-OH-6-SO4(.-) radical 7. At low dose rates a chain reaction is observed. The product of this chain reaction is the cis-5,6-dihydro-5,6-dihydroxy-1,3-dimethyluracil 2. At a dose rate of 2.8 X 10(-3) Gys-1 a G value of approximately 200 was observed ([1,3-DMU] = 5 X 10(-3) mol dm-3; [S2O8(2-)] = 10(-2) mol dm-3; [t-butanol] = 10(-2) mol dm-3). The peculiarities of this chain reaction (strong effect of [1,3-DMU], smaller effect of [S2O(2-)8]) is explained by 7 being an important chain carrier. It is proposed that 7 reacts with 1,3-DMU by electron transfer, albeit more slowly (k approximately 1.2 X 10(4) dm3 mol-1 s-1) than does SO4(.-). The resulting sulphate 6 is considered to hydrolyse into 2 and sulphuric acid which is formed in amounts equivalent to those of 2. Computer simulations provide support for the proposed mechanism. The results of some SCF calculations on the electron distribution in the radical cations derived from uracil and 1-methyluracil are also presented.     

Acid-Catalyzed Photosubstitution of 5-Fluoro-1,3-Dimethyluracil with Substituted Benzenes

Abstract

The substantial photosubstitution of 5-fluoro-1,3-dimethyluracil with substituted benzenes is first achieved by the addition of trifluoroacetic acid to the reaction mixture. The presence of protons is essential for the formation of 5-aryl-1,3-dimethyluracils.     

Reaction of substituted pyrimidines with photochemically generated t-BuO* radicals

Humans are exposed to various organic peroxides through chemical, pharmaceutical and cosmetic products. On photolysis, these peroxides produce alkoxyl radicals and hydroxyl radicals. The reaction of *OH radicals with DNA and its constituents have been extensively studied, but very little is known about the reactions of alkoxyl radicals with DNA and its constituents. In view of this, the oxidation of pyrimidine bases viz., thymine, uracil, cytosine, 5-bromouracil, 6-methyluracil and 1,3-dimethyluracil by t-BuO* radicals in aqueous solution at pH 7.5 has been carried out. The reaction between pyrimidine and t-BuO* is followed by measuring the absorbance of pyrimidine at the respective lambdamax. The rates of oxidation of pyrimidines are calculated from the plot of absorbance vs time. The rates of oxidation of pyrimidines have been found to increase with increase in [t-BuOOH], [pyrimidine] and light intensity. The quantum yields are calculated from the initial rates of oxidation of pyrimidine and the measured light intensity at 254 nm the wavelength at which t-BuOOH is activated to give radicals. The quantum yields are found to depend on [pyrimidine] as well as on [t-BuOOH] while they are independent of light intensity. The product analysis was carried out on HPLC with UV-visible detector. The corresponding 5,6-dihydroxypyrimidine and isobarbituric acid have been identified by comparing the retention times of the authentic samples. On the basis of experimental results and product analysis, it is suggested that t-BuOOH on photolysis gives t-BuO* radical, which initiates the reaction by adding to C (5) or C (6) position of pyrimidine base, leading to the formation of pyrimidine base radical via hydrolysis. The pyrimidine radical further reacts with t-BuO* radical to give the final product. This study predicts the probable transient pyrimidine radicals.     

Synthesis of monosaccharides by oxidation of furfural derivatives

dl-Ribose, dl-arabinose, and dl-xylose have been synthesized from 2-furaldehyde. Condensation of 2-furaldehyde with pinacol followed by oxidation of the product with bromine water or lead tetra-acetate and reduction with sodium borohydride gave dl-2-(1,4-dihydroxy-cis-but-2-enyl)-4,4,5,5-tetramethyl-1,3-dioxolane (4a). Hydroxylation of the dibenzoate of 4a followed by debenzoylation and hydrolysis gave dl-ribose and dl-arabinose. Epoxidation of 4a with monoperoxysuccinic acid followed by hydrolysis gave dl-xylose.     

New Aspects in the Free-Radical Chemistry of Pyrimidine Nucleobases

The contribution will cover three aspects:

i) It has been known for some time that OH radicals and H atoms react with the pyrimidines by adding to the C(5)-C(6) double bond, but only the u.v.-spectra of the sum of these radicals have been reported so far. It will be shown how to arrive at the individual spectra of the C(5) and the C(6) adduct radicals.

ii) α-Hydroxyalkyl radicals are known to inactivate biologically active DNA. In contrast to the electrophilic radicals H and OH they are nucleophilic and the hydroxymethyl radicals add exclusively at the C(6) position of 1,3-dimethyluracil (k ～ 10⁴dm³ mol^?1 s^?1). In the corresponding thymine derivative this reaction also occurs, but one third of the hydroxymethyl radicals abstract an H-atom from the C(5)-methyl group thereby forming an allylic radical. In the course of these reactions pyrimidines with an exocyclic double bond are formed. These products react much more rapidly with hydroxymethyl radicals than the starting material leading to highly hydroxymethylated material at very low doses.

iii) The direct effect of ionizing radiation which would produce a pyrimidine base radical cation can be mimicked by reacting the pyrimidine with SO₄^?, a very good electron acceptor. In water, the radical cation of 1,3-dimethyluracil is rapidly (t_1/2 2μs) converted into the C(5) OH adduct radical. In the presence of peroxodisulphate a chain reaction sets in which leads to the cis-glycol.

The relevance of these findings to radiobiological aspects of nucleic acid research will be discussed.     

Production and characterization of new families of polyglucuronic acids from TEMPO–NaOCl oxidation of curdlan

Curdlan from Agrobacterium sp. was oxidized using 2,2,6,6,-tetramethylpiperidine-1-oxyl radical (TEMPO)–NaBr–NaClO systems at pH 11. The effects of oxidation conditions on degrees of oxidation and polymerization of the products obtained were studied using SEC–MALLS, NMR and IR analyses. Different families of water-soluble β-(1,3)-polyglucuronic and β-(1,3)-polyglucoglucuronic acid sodium salts were quantitatively generated with a yield of 80% and without significant loss of their molecular weights.Given that β-(1,3)-polyglucuronic acids prepared from the regioselective oxidation of curdlan by the TEMPO–NaBr–NaClO systems regularly consist of the glucuronic acid repeating unit; they may open new biotechnological fields for the utilizations of water soluble forms of curdlan.     

IR study of base stacking interactions.

For D2O solutions of 1,3-dimethyluracil, cytidine, caffeine, inosine and 2'-deoxyadenosine the concentration dependence of IR spectra (1800-1400 cm-1) have been found which reflects stacking association of these compounds. A method is proposed to use this data to obtain thermodynamic parameters of association and the molecular spectra in the monomer and associated forms. The homoassociation constant for 1,3-dimethyluracil was estimated as k=0.65. Stacking is shown to change radically the spectra, inducing a high-frequency shift of carbonyl vibrations and a decrease in the intensity of skeletal stretching vibration bands. This allows one to distinguish between the stacking interactions and hydrogen bonding. A conclusion is made about the considerable contribution of stacking interactions into the change of IR spectra of DNA and other polynucleotides following conformational transitions.     

The chemistry of castanospermine. Direct oxidation of the tetraacetate to the corresponding gamma-lactam

From the products of oxidation of tetra-O-acetylcastanospermine (2) with N-bromosuccinimide, only the gamma-lactam 11 was isolated (19%). Treatment of this lactam with DBU in dichloromethane afforded the elimination product 12, while deacetylation with sodium methoxide in methanol gave tetraol 13 together with diene 14. The X-ray crystal structure of compound 13 is reported.     

Triazole-fused sugars from nitroalkene-containing C-glycosides by a tandem 1,3-dipolar cycloaddition and intramolecular Michael addition

A synthetic method to triazole-fused sugars by treatment of nitroalkene-containing C-glycosides with sodium azide is described. Initial experiments conducted at room temperature gave only the 1,3-dipolar cycloaddition products. However, at elevated reaction temperature the tandem β-elimination/cycloaddition/Michael addition yielded 1,5-disubstituted triazole-fused sugars.     

Synthesis of 2-bromomethyl-3-hydroxy-2-hydroxymethyl-propyl pyrimidine and theophylline nucleosides under microwave irradiation. Evaluation of their activity against hepatitis B virus

Alkylation of 2-methylthiopyrimidin-4(1H)-one (1a) and its 5(6)-alkyl derivatives 1b-d as well as theophylline (7) with 2,2-bis(bromomethyl)-1,3-diacetoxypropane (2) under microwave irradiation gave the corresponding acyclonucleosides 1-[(3-acetoxy-2-acetoxymethyl-2-bromomethyl)prop-1-yl]-2-methyl-thio pyrmidin-4(1H)-ones 3a-d and 7-[(3-acetoxy-2-acetoxymethyl-2-bromomethyl)prop-1-yl]theophylline (8), which upon further irradiation gave the double-headed acyclonucleosides 1,1 '-[(2,2-diacetoxymethyl)-1,3-propylidene]-bis[(2-(methylthio)-pyrimidin-4(1H)-ones] 4a-c, and 7,7 '-[(2,2-diacetoxymethyl)-1,3-propylidene]-bis(theophylline) (9). The deacetylated derivatives were obtained by the action of sodium methoxide. The activity of deacetylated nucleosides against Hepatitis B virus was evaluated. Compound 5b showed moderate inhibition activity against HBV with mild cytotoxicity.     

A Raman-active competitive inhibitor of OMP decarboxylase

6-Cyanouridine 5'-phosphate was shown to act as a competitive inhibitor of yeast OMP decarboxylase, with a K(i) value of 1.1 x 10(-5)M. Upon binding by the active site of yeast OMP decarboxylase (EC 4.1.1.23), the Raman stretching frequency of the nitrile group of 6-cyanouridine 5'-phosphate decreases from 2240 to 2225 cm(-1). Based on the behavior of a model compound, 6-cyano-1,3-dimethyluracil, and on vibrational calculations, the observed change in stretching frequency is attributed to desolvation of the ligand, and distortion of the ligand in which the nitrile group moves out of the plane of the pyrimidine ring. Similar distortions may play a role in substrate activation by OMP decarboxylase, contributing to the catalytic process.     

Synthesis of new mannosyl, galactosyl and glucosyl theophylline nucleosides with potential activity as antagonists of adenosine receptors. DEMA-induced cyclization of glycosylideneiminouracils

The synthesis of D-mannosyl, D-galactosyl and D-glucosyl theophylline nucleosides by diethoxymethyl acetate (DEMA)-induced cyclization of 4-amino-5-glycosylideneimino-1,3-dimethyluracil is reported. 8-Methyltheophylline derivatives of the same sugars were also prepared by Ac(2)O/H(+)-induced cyclization of their imine precursors. This approach has allowed beta-D-mannopyranosyl-, alpha-D-galactofuranosyl- and beta-D-glucofuranosyltheophylline nucleosides to be synthesized for the first time. The inhibition of specific binding at A(1), A(2A), A(2B) and A(3) adenosine receptors in the mannose derivatives is also reported.     

Oxidation of diiron and triiron sulfurdithiolato complexes: mimics for the active site of [FeFe]-hydrogenase

The oxidation of the hexacarbonyl(1,3-dithiolato-S,S')diiron complexes 4a-4c with varying amounts of dimethyldioxirane (DMD) was systematically studied. The chemoselectivity of the oxidation products depended upon the substituent R (R=H, Me, 1/2 (CH2)(5)). For R=H, four oxidation products, 6a-6d, have been obtained. In the case of R=Me, three products, 7a-7c, were formed, and for R=1/2 (CH2)(5), only complex 8 was observed. These observations are due to steric and electronic effects caused by the substituent R. Additionally, oxidation of the triiron complex 5 with DMD was performed to yield the products 9a and 9b. X-Ray diffraction analyses were performed for 6a-6d, 7a, and 7c, as well as for 9a and 9b. The electronic properties were determined by density-functional theory (DFT) calculations.     

Strong metal-oxygen interaction in uracils. X-ray crystal structure of bis-(1,3-dimethyluracil)dichlorocopper(II).

An X-ray structure determination showed that in the compound bis-(1,3-dimethyluracil)-dichlorocopper(II) the uracil moiety is strongly bonded to the metal via the O(4) atom. Changes in the i.r. spectra on metal binding are discussed. Details of the structural parameters have been deposited as Supplementary Publications SUP 50091 (3 pages) at the British Library Lending Division, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1978) 169, 5.     

Aromatic ring cleavage of a β-biphenyl ether dimer catalyzed by lignin peroxidase of phanerochaete chrysosporium

Under aerobic conditions homogeneous lignin peroxidase catalyzed the oxidation of 1-(4'-methoxyphenyl)-2-(2″,5′-dimethoxy-4″-phenylphenoxy)-1,3-dihydroxypropane (I) to yield four products: 1-(4'-methoxy-phenyl)-1,2,3-trihydroxypropane (X), 4-[-hydroxy--(4'-methoxyphenyl)-methyl]-1,3-dioxolane-2-one (V), 4-(4'-methoxyphenyl)-5-hydroxymethyl-1,3-dioxolane-2-one (VI) and 5-hydroxy-5-carbomethoxy-4-phenyl-oxol-3-en-2-one (VIII). V, VI and VIII are all products of ring opening reactions. When the reaction was conducted under anaerobic conditions, the substrate was oxidized but no ring-cleaved products were detected. During the oxidation of I, 4 atoms of ¹⁸O from ¹⁸O₂ were incorporated into the lactol product VIII.     

Synthesis and biological activity of 2-(4,5-dihydroisoxazol-5-yl)-1,3,4-oxadiazoles

Acid hydrazides were coupled with acrylic acid derivatives and cyclodehydration gave 1,3,4-oxadiazoles. Lastly, in-situ nitrile oxide formation from aryl oximes treated with sodium hypochlorite, and subsequent 1,3-dipolar cycloaddition to the exomethylene moiety delivered 2-(4,5-dihydroisoxazol-5-yl)-1,3,4-oxadiazoles. This library was evaluated in a high-throughput screen at Dow AgroSciences. Several compounds were active against fungal pathogens and pest insects.