Hydrogen Atom Donor Research Articles

Mechanistic Studies of the 5-Iodouracil Chromophore Relevant to Its Use in Nucleoprotein Photo-Cross-Linking

The photoreactivity of the 5-iodouracil chromophore was investigated toward understanding photo-cross-linking of nucleic acids bearing the chromophore to functionality in associated proteins. Irradiation of 5-iodouridine (IU) in the presence of a 10-fold excess of N-acetyltyrosine N-ethylamide (1) at 308 nm with a XeCl excimer laser or at 325 nm with a HeCd laser yields uridine (U) and N-acetyl-m-(5-uridinyl)tyrosine N-ethylamide (2) in a 1:2 mole ratio. In the presence of N-acetylphenylalanine N-ethylamide, uridine and analogous ortho, meta, and para regioisomeric adducts (3o, 3m, and 3p) were formed in a similar U to adduct mole ratio. The primary photochemical process leading to products was established as carbon−iodine bond homolysis in the first excited singlet state from a deuterium labeling experiment, photoacoustic calorimetry, and quantum yield measurements. Photoreduction of IU in 2-propanol-d solvent gave U with no deuterium incorporation. Photoacoustic calorimetric measurements established that triplet benzophenone transferred energy to IU with a rate constant of 2 × 109 M-1 s-1. Further, the reaction of IU with 1 to form 2 was sensitized by benzophenone; however, comparison of quantum yields upon direct and sensitized excitation indicated that, at most, only a small portion of the reactions occurred via the triplet state. With direct excitation of IU, quantum yields as a function of the concentration of 1 showed that U and adduct 2 resulted from a common intermediate proposed to be the 5-uridinyl radical. Uridine formation was enhanced by the presence of hydrogen atom donors at the expense of formation of 2. Quantum yields were independent of excitation wavelength in the region 310−330 nm but not the reaction medium. The quantum yield of uridine formation but not adduct formation was approximately an order of magnitude higher in 90% acetonitrile−10% water than in pH 7 water. The results are discussed in terms of high-yield cross-linking of nucleic acids bearing the 5-iodouracil chromophore to associated proteins in light of cocrystal X-ray structural data.

Fluorine Substitution Enhances the Reactivity of Substituted Phenyl Radicals toward Organic Hydrogen Atom Donors

Phenyl radicals with different positively charged, chemically inert substituents in the para position were generated by collision-activated dissociation of para-substituted iodobenzene ions in the gas phase inside a Fourier-transform ion cyclotron resonance mass spectrometer. The reactivity of the radicals was examined toward organic hydrogen atom donors. The findings parallel those made by others for neutral radicals in solution. For example, the efficiency of hydrogen atom abstraction increases in the expected order phenol < thiophenol < benzeneselenol. Inspite of great exothermicity, hydrogen atom abstraction by the charged phenyl radicals occurs at a small fraction of the gas-phase collision rate, suggesting the presence of a significant barrier on the reaction coordinate. Fluorine substitution on the phenyl ring was found to drastically enhance the reaction rate for all the substrates studied. For example, hydrogen abstraction from thiophenol by the (2,3,5,6-tetrafluoro-4-dehydrophenyl)trimethylphosp...

Reduction of a water-soluble iron(III) porphyrin and an iron(II) nitrosyl porphyrin by benzophenone ketyl radical

Reactions of benzophenone ketyl radicals with the water-soluble iron(III)porphyrin, [FeIII(tpps)]3–[tpps =meso-tetrakis(p-sulfonatophenyl)porphyrinate], and a nitric oxide adduct of the iron(II) form of the porphyrin, [FeII(tpps)(NO)]4–, have been studied by laser photolysis in aqueous solutions. Reactions of ferricytochrome c and metmyoglobin with the ketyl radicals were also studied for comparison with the iron(III) porphyrin. Benzophenone ketyl radicals were produced from the reaction of the triplet state of benzophenone with the glucose used as a suitable hydrogen atom donor. The transient absorption spectrum observed after 266 nm laser pulse excitation for the [FeIII(tpps)]3– solution containing benzophenone and glucose shows that the iron(III) porphyrin is reduced by the ketyl radical to the iron(II) form. Ferricytochrome c and metmyoglobin were found also to be reduced to the iron(II) form by the ketyl radical. The rate constants for the reductions of [FeIII(tpps)]3–, ferricytochrome c and metmyoglobin were determined to be 3.0 × 109, 4.5 × 108 and 2.2 × 108 dm3 mol–1 s–1 respectively. Nitric oxide irreversibly reacts with [FeII(tpps)]4– to yield [FeII(tpps)(NO)]4– in an aqueous solution. The transient absorption spectrum observed 500 µs after laser pulsing for the [FeII(tpps)(NO)]4– solution containing benzophenone and glucose indicates that the ketyl radical reacts with [FeII(tpps)(NO)]4– to form a new transient species which displays the spectrum with a peak at 418 nm in the Soret band region. The transient species has been identified as a one-electron adduct of [FeII(tpps)(NO)]4– where the added electron possibly exists on the coordinated NO moiety.

Kinetics of Dialkylaminium Cation Radical Reactions: Radical Clocks, Solvent Effects, Acidity Constants, and Rate Constants for Reactions with Hydrogen Atom Donors

Kinetics of Dialkylaminium Cation Radical Reactions: Radical Clocks, Solvent Effects, Acidity Constants, and Rate Constants for Reactions with Hydrogen Atom Donors

Antioxidative activity and scavenging effects on active oxygen of xylose‐lysine maillard reaction products

AbstractThe antioxidative activity and scavenging effects on active oxygen of Maillard reaction products (MRP) prepared by heating xylose and lysine (XL) at a molar ratio 1:2 and pH 9.0 for 1.5 h (XL‐1 to XL‐5) were investigated. The antioxidative activity and browning intensity of XL MRP increased with increasing duration of reaction, but XL‐1 and XL‐2 MRP showed greater reducing power than other samples (P &lt; 0.05). All XL MRP showed scavenging activity on 1,1‐diphenyl–2‐picrylhydrazyl (DPPH) radical. XL‐1, XL‐2 and XL‐3 MRP exhibited about 50% reduction in absorbance of superoxide. Scavenging effects of XL MRP on DPPH radical and superoxide were markedly decreased after decolorization with Sep‐Pak Cartridge C18, indicating that the browning pigment might contain components that can scavenge superoxide or donate hydrogen atoms. The electron paramagnetic resonance spectra indicated that XL MRP had scavenging activity on the hydroxyl radical; this scavenging effect depended on dose (r = 0.99) and increased with increasing duration of reaction. Based on these data, the antioxidative activity of XL MRP may be attributed to the combined effects of reducing power, donation of hydrogen atoms and scavenging of active oxygen.

Indirect Electroreductive Cyclization of N-Allyl and N-Propargylamides Using a Nickel(II) Complex as an Electron-Transfer Catalyst: Selective Formation of Halogenated and Non-halogenated Pyrrolidinones.

Nickel(II) complex-catalyzed indirect electroreduction of N-allyl and N-propargyl-α-bromoamides conducted in dimethylformamide with 2 eq of a hydrogen atom donor, diphenylphosphine, afforded the corresponding pyrrolidinones as the sole cyclized product in good yields, while that of N-allyl-α-iodoamides in acetonitrile gave the iodinated pyrrolidinones. Under both conditions the trans-C-3: C-4 pyrrolidinones were formed predominantly

DNA Cleavage by Neocarzinostatin Chromophore.Establishing the Intermediacy of Chromophore-Derived Cumulene and Biradical Species and Their Role in Sequence-Specific Cleavage

Experiments establishing the intermediacy of the cumulene 3 and the biradical 4 in the cleavage of double-stranded DNA by neocarzinostatin chromophore (1) and methyl thioglycolate (2) are described. It is shown that, in the presence of millimolar concentrations of 2, ≥95% of DNA cleavage arises via the cumulene 3; pathways of DNA cleavage not involving 3 are, at best, minor. The following detailed mechanism emerges for the DNA cleavage reaction. The rate-determining step in the damage of DNA by neocarzinostatin chromophore and the thiols glutathione (GSH), cysteine (CySH), or methyl thioglycolate at physiologically relevant concentrations and pH values is shown to be thiol addition to the chromophore. Evidence is presented to support the notion that the addition of thiols to 1, whether 1 is free in solution or bound to DNA, is an inherently efficient process. The addition of GSH or CySH is shown to proceed via a ternary complex of DNA, thiol, and chromophore. In neither case does DNA accelerate (catalyze) the thiol addition reaction; rather, it is found to induce a modest decrease in the rate of thiol addition versus control solutions lacking DNA. The greater concentration of DNA-bound chromophore versus chromophore free in solution offsets the attenuated rate of thiol addition to the former. The site of activation appears not to be critical, however, because it is likely that the cumulene intermediate produced is sufficiently long-lived to equilibrate among DNA binding sites. In support of this idea, it is shown that the sequence specificity of DNA cleavage by externally generated cumulene is identical to that of the presumptive cumulene formed in situ from 1 and 2 in the presence of DNA. It is proposed that the species that determines the sequence specificity of DNA cleavage is thecumulene intermediate. The experimental evidence suggests that the cumulene intermediate undergoes cycloaromatization while bound to DNA and that the biradical formed in this cycloaromatization reaction is a highly reactive and poorly selective intermediate. The yield-determining step in the production of thiol adducts from 1 is found to be the quenching of the biradical intermediate by hydrogen atom transfer. It is shown that double-stranded calf thymus DNA and the water-soluble 1,4-cyclohexadiene derivative 14 are approximately equally effective in trapping of the biradical intermediate at concentrations of 5 mM (base pairs) and 1 M, respectively, supporting the idea that the biradical is generated as a DNA-bound species. Although the data do not rule out the possibility that DNA may catalyze the cycloaromatization reaction, this proposal is considered to be unlikely. It is shown that thiol activation of 1 in the presence of single-stranded calf thymus DNA or a heterogeneous mixture of cellular RNA, but not bovine serum albumin, likely occurs as a ternary complex with the biopolymer. Furthermore, single-stranded DNA and heterogeneous cellular RNA are shown to serve as effective hydrogen atom donors for quenching of the biradical product of thiol activation, suggesting that biopolymers other than double-stranded DNA are potential targets for neocarzinostatin-induced damage.

Characterization of the excited-state reactivity of a persistent aryl-substituted allyl free radical

A family of stable aryl-substituted allyl radicals 1 (9H-fluoren-9-yl-9-[9H-fluroren]-9-ylidenephenylmethyls) has been characterized by electronic absorption spectroscopy, semiempirical (AMI) molecular orbital calculations, and cyclic voltammetry. Photolysis of 1a in aerated solvents that are poorer hydrogen atom sources than toluene resulted in oxygenation (O OX ∼ 5×10 -4 ) and cleavage products. A thermally reversible photocyclization mechanism is proposed to explain the oxygenation of 1a in inert solvents. Enhanced excited-state reactivity toward hydrogen and halogen atom donors was observed during the steady-state photolysis of 1a

Flash photolysis and time-resolved electron spin resonance studies of triplet benzophenone quenching by hindered amine light stabilizers (HALS). A comparison of HALS amines and aminoethers as electron and hydrogen atom donors

Transient absorption spectroscopy and time-resolved electron spin resonance (TR-ESR) were employed to understand the electron and hydrogen atom transfer quenching of triplet benzophenone ( 3BP) in acetonitrile and benzene by amine and aminoether derivatives of 2,2,6,6-tetramethylpiperidine (hindered amine light stabilizers, HALS). The amines ( I and II) are efficient quenchers of 3BP. The observed quenching rate constants were found to be in the range 6 × 10 8–6 × 10 9 M −1 s −1 and depend on the structure of the amine and the solvent. On the other hand, the aminoethers ( III and IV) do not exhibit any quenching at all. Reactions of 3BP with amines I and II in benzene yield ketyl radicals (detected by transient absorption spectroscopy) and the corresponding aminyl or aminomethyl radicals (detected by TR-ESR). Neither experimental method provided any evidence for products of the reactions of 3BP with aminoethers III and IV. The experimental results are discussed from the viewpoint of the mechanism of polymer stabilization by HALS.

Evidence for Induction of DNA Double-Strand Breaks at Paired Radical Sites

Isolated plasmid pBR322 DNA was irradiated in the gas explosion apparatus in the presence of 10 mmol dm-3 GSH. By varying the time of the oxygen shot relative to the 5-ns pulse of electrons, the chemical repair kinetics of the oxygen-dependent free-radical precursors of DNA single- and double-strand breaks (SSBs and DSBs) can be determined. The first-order repair rate of the SSB precursors was 1370 s-1 in comparison to 2900 s-1 for DSB precursors. Under these conditions the oxygen enhancement ratio for SSBs was 3.0 in comparison to 7.5 for DSBs. This twofold difference in chemical repair rate may be interpreted on the basis of the free-radical precursor of a DSB consisting of two radicals, one on either strand of the DNA. With the chemical repair of one or other of these radicals by hydrogen atom donation from GSH, a DSB is not produced. This process will occur at twice the rate of the chemical repair of an SSB precursor consisting of a single radical. These data are consistent with the concept that DSBs are formed at the sites of clustered energy depositions with the production of a paired radical.

On the stability and radical deoxygenation of tertairy xanthates.

Abstract Xanthates of tertiary alcohols were isolated and properly characterized. Radical deoxygenation reactions under various conditions to the corresponding alkanes were successfully carried out. Comparative data of various initiatiors and hydrogen atom donors for xanthate deoxygenation are reported.

Indirect electroreductive cyclisation of N-allylic and N-propargylbromo amides and o-bromoacryloylanilides using nickel(II) complexes as electron-transfer catalysts

Nickel(II) complex-catalysed indirect electroreduction of N-allylic and N-propargyl-α-bromo amides and o-bromoacryloylanilides gave the corresponding 5-membered lactams. The product distribution was affected by the ability of the solvent to donate hydrogen atom. The electroreduction of N-allyl-N(bromoacetyl)toluene-p-sulfonamide 1a in DMF and acetonitrile yielded as main product the 4-methylpyrrolidinone 2a(41%) and the 4-(bromomethyl)pyrrolidinone 3a(33%), respectively. On addition of 2 mol equiv. of a hydrogen-atom donor (Ph2PH) to the reaction of bromo amide 1a in acetonitrile, compound 1a provided compound 2a(58%) as the sole cyclised product.

F430 model chemistry. Mechanistic investigation of the reduction, coupling, and dehydrohalogenation of alkyl halides by the nickel(I) octaethylisobacteriochlorin anion

The reactions of Ni I (OEiBC) - with alkyl halides and p-toluene sulfonates and with hydrogen atom and proton donors were investigated in a wide range of solvents. The ease of reduction of Ni II (OEiBC) by sodium amalgam and the stability of the Ni I (OEiBC) - generated was greater in more polar solvents.

Synthesis of novel 2-(3′-dialkylaminopropoxy)-thioxanthone derivatives. photochemistry and evaluation as photoinitiators of butyl acrylate polymerization

We report the synthesis of novel derivatives of thioxanthone with built-in tertiary amino groups. The aim of tailoring these new structures was to have photoinitiators which could react into the photocurable formulation so that the chromophore becomes attached to the polymer matrix, so reducing the possibility of it being extracted by solvents. The general photochemistry was studied by measurements of u.v. absorption, fluorescence and phosphorescence; a photoreduction study was carried out. Luminescence properties have shown the importance of the built-in amino group in the behaviour of the singlet and triplet excited states. Photoreduction quantum yields of aminopropoxythioxanthones are low, confirming that the amino group acts as a quencher of the excited state as well as a hydrogen atom donor. Polymerization quantum yields, determined by photocalorimetry in the presence of oxygen, are slightly higher with aminopropoxythioxanthones than with simple 2-propoxythioxanthone.

The effect of radical trapping reagents upon formation of poly(α‐tetrahydrothiophenio para‐xylylene) polyelectrolytes by the wessling soluble precursor method

AbstractMolecular weights were studied by gel permeation chromatography of derivatized poly(α‐tetrahydrothiophenio para‐xylylene) chloride produced by aqueous or methanolic base‐induced polymerization of 1,4‐bis(tetrahydrothiopheniomethyl) benzene dichloride, both with and without a variety of added polymerization inhibiting agents. Efficient radical scavenging agents such as 2,2,6,6‐tetramethylpiperidinoxyl and hydrogen atom donor 2,4,6‐tri‐tert‐butylaniline reduced the degree of polymerization of the reactive α‐(tetrahydrothiophenium chloride)‐para‐xylylene intermediate produced in this chemistry, and in some cases completely suppressed formation of high polymer. These two traps did not affect the equilibrium production of the para‐xylylene by UV‐Vis spectral analysis; hence they must affect the subsequent polymerization chain propagation steps in the mechanism. Electron spin resonance studies of polymerization in the presence of 0.00025 equiv of TEMPO showed disappearance of the spin label, a result consistent with a radical scavenging process. The results suggest that production of high molecular weight poly(α‐tetrahydrothiophenio para‐xylylene) chloride proceeds through a radical chain propagation sequence. © 1992 John Wiley &amp; Sons, Inc.

Reactions of N,N?-diphenyl-p-quinonediimine with hydroquinone, ?-tocopherol, and other antioxidants

Phenols (hydroquinone and α-tocopherol) and heterocyclic aminophenols (2,2,4-trimethyl-6-ethoxy-1,2-dihyroquinoline) reduce N,N′-diphenyl-p-quinonediimine (A) to the diamine. With difunctional hydrogen atom donors, the reaction rate is proportional to the concentrations of the reactants. The effective rate constants have been determined over a range of temperatures. In the reaction of A with α-tocopherol, plots of rate versus initial concentrations are nonlinear, 2,6-Di-tert-butyl-4-methylphenol and 2,2,4-trimethyl-6-ethoxy-1,2-dihyroquinoline do not react with A under the same conditions.

Amino acids and their derivatives as radioprotective agents

Numerous amino acids and their analogs are capable of protecting biological systems from the toxic effects of ionizing radiation. These radioprotective agents can be classified into two broad groups, depending upon the presence or absence of a free or potentially free sulfhydryl group. The sulfhydryl-containing compounds have been studied extensively and are thought to exert their radioprotective effects by several mechanisms, including free radical scavenging and hydrogen atom donation. Several non-sulfhydryl-containing amino acids are also being investigated for their radioprotective effects. These agents are less well known than the familiar sulfhydryl compounds, but possess very interesting protective qualities. In short, the study of amino acids and their derivatives as radioprotective agents continues to contribute to an understanding of processes involved in radiation toxicity and to offer new compounds with potential application to situations of human exposure.

Effects of trypanothione on the biological activity of irradiated transforming DNA.

Held et al. (1984a,b) demonstrated previously that glutathione (GSH), a negatively charged thiol, is significantly less efficient in the hydrogen atom donation repair reaction with radicals induced by radiation in transforming DNA (t-DNA) than are other thiol compounds. Fahey et al. (1991a,b) postulated that the charge on thiols can influence their ability to radioprotect DNA. GSH, which is excluded from the vicinity of DNA due to its negative charge, is less protective than neutral or positively charged thiols. We have investigated this phenomenon further with trypanothione, the conjugate of glutathione and spermidine, N1,N8-bis (L-gamma-glutamyl-L-hemicystinyl-glycyl)-spermidine. Trypanothione exists in aerobic solution largely as the disulphide (T(S)2) but is maintained in the cell in the reduced form (T(SH)2) by means of an NADPH-dependent flavo-enzyme, trypanothione reductase (TR). Experimental data show that T(S)2 in the presence of TR radioprotects t-DNA in the absence of oxygen much better than GSH or spermidine alone or in combination. Little radioprotection by T(S)2 is seen when TR is not present. The results obtained with reduced trypanothione at low concentrations suggest that radioprotection of t-DNA in hypoxia occurs predominantly by H atom donation and slightly by .OH radical scavenging, and the protection is greater than that by GSH or spermidine because the polyamine moiety in trypanothione allows a greater concentration of GSH near the DNA molecule.

ChemInform Abstract: An Improved Radical Chain Procedure for the Deoxygenation of Secondary and Primary Alcohols Using Diphenylsilane as Hydrogen Atom Donor and Triethylborane‐Air as Initiator.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Active site of DNA photolyase: tryptophan-306 is the intrinsic hydrogen atom donor essential for flavin radical photoreduction and DNA repair in vitro

DNA photolyases repair cyclobutadipyrimidines (Pyr()Pyr) in DNA by photoinduced electron transfer. The enzyme isolated from Escherichia coli contains methenyltetrahydrofolate (MTHF), which functions as photoantenna, and FADH2, which is the redox-active cofactor. During purification, FADH2 is oxidized to the blue neutral radical form, FADH., which has greatly diminished activity. Previous nanosecond flash photolysis studies [Heelis, P.F., Okamura, T., & Sancar, A. (1990) Biochemistry 29, 5694-5698] indicated that excitation of FADH. either directly by absorbing a photon or indirectly by electronic energy transfer from MTHF excited singlet state yielded an FADH. quartet which abstracted a hydrogen atom from a nearby tryptophan to generate the catalytically competent FADH2 from of the enzyme. Using site-directed mutagenesis, we replaced all 15 photolyase tryptophan residues by phenylalanine, individually, in order to identify the internal hydrogen atom donor responsible for photoreduction. We found that W306F mutation abolished photoreduction of FADH. without affecting the excited-state properties of FADH. or the substrate binding (KA approximately 10(9) M-1) of the enzyme. The specificity constant (kcat/km) was approximately 0 for the mutant enzyme in the absence of reducing agents in the reaction mixture, indicating that photoreduction of FADH. is an essential step for photorepair by photolyase in vitro. Chemical reduction of FADH. of the mutant enzyme restored the specificity constant to the wild-type level.