Electron Adduct Research Articles

Photoinduced Electron-Transfer-Mediated Differential Recognition of Proteins on Plasmonic Surfaces.

Photoinduced enhanced Raman spectroscopy (PIERS) has emerged as an efficient technique for enhancing the vibrational modes of analyte molecules adsorbed on a plasmonic nanoparticle-semiconductor hybrid material through chemical enhancement governed by electron transfer from the semiconductor to the plasmonic nanoparticles under an additional ultraviolet (UV) preirradiation step. The increase in chemical enhancement is imperative in analyzing and detecting pharmaceutically important moieties, such as amino acids and proteins, with a low Raman scattering cross section, even in complex biological environments. Herein, we demonstrate that UV preirradiation induced the creation of additional oxygen vacancies by introducing a low concentration (≈1%) of Ni as a dopant in the 2D platelike morphology of the BiOCl semiconductor; i.e., defect states in the semiconductor can induce charge transfer from the semiconductor to the plasmonic nanoparticles. This phenomenon facilitates electron transfer to the adsorbed analyte on the plasmonic surface. Additionally, we have shown the usefulness of this method in protein immobilization on the substrate surface, followed by the identification of a specific protein in the mixture of proteins. Proteins containing cysteine residues capture these electrons to form a surface-bound thiol group via a transient disulfide electron adduct radical. This allows differential binding of the protein molecules to the semiconductor plasmonic hybrid depending on the concentration of surface cysteine residues in proteins. Through PIERS and principal component analysis, we demonstrate the possibility of probing and distinguishing biomolecules based on their surface composition and secondary structure components even in their mixtures, thus paving the way for efficient analysis of complex biological systems.

Intramolecular Proton Transfer in the Radical Anion of Cytidine Monophosphate Sheds Light on the Sensitivities of Dry vs Wet DNA to Electron Attachment-Induced Damage.

Single-strand breaks (SSBs) induced via electron attachment were previously observed in dry DNA under ultrahigh vacuum (UHV), while hydrated electrons were found not able to induce this DNA damage in an aqueous solution. To explain these findings, crossed electron-molecular beam (CEMB) and anion photoelectron spectroscopy (aPES) experiments coupled to density functional theory (DFT) modeling were used to demonstrate the fundamental importance of proton transfer (PT) in radical anions formed via electron attachment. Three molecular systems were investigated: 5'-monophosphate of 2'-deoxycytidine (dCMPH), where PT in the electron adduct is feasible, and two ethylated derivatives, 5'-diethylphosphate and 3',5'-tetraethyldiphosphate of 2'-deoxycytidine, where PT is blocked due to substitution of labile protons with the ethyl residues. CEMB and aPES experiments confirmed the cleavage of the C3'/C5'-O bond as the main dissociation channel related to electron attachment in the ethylated derivatives. In the case of dCMPH, however, electron attachment (in the aPES experiments) yielded its parent (intact) radical anion, dCMPH-, suggesting that its dissociation was inhibited. The aPES-measured vertical detachment energy of the dCMPH- was found to be 3.27 eV, which agreed with its B3LYP/6-31++G(d,p)-calculated value and implied that electron-induced proton transfer (EIPT) had occurred during electron attachment to the dCMPH model nucleotide. In other words, EIPT, subduing dissociation, appeared to be somewhat protective against SSB. While EIPT is facilitated in solution compared to the dry environment, the above findings are consistent with the stability of DNA against hydrated electron-induced SSB in solution versus free electron-induced SSB formation in dry DNA.

Photoactivated plasmonic nanohybrid fibers with prolonged trapping of excited charge carriers for SERS analysis of biomolecules.

The quest to enhance Raman spectroscopic signals through the rational design of plasmonic substrates has enabled the detection and characterization of pharmaceutically important molecules with low scattering cross-sections, such as amino acids and proteins, and is helping in making forays into the diverse field of biomedical sciences. This work presents a simple strategy for synthesizing silver nanoparticles-incorporated alumina nanofibers (Ag-AlNFs) utilizing controlled microwave synthesis for enhancing the surface-enhanced Raman chemical enhancement factor through photo-induced charge accumulation at the plasmonic-dielectric interface. The plasmonic-dielectric fibers serve as excellent charge carrier trappers, as evident from the ultrafast transient absorption spectroscopy studies. Apart from chemical enhancement, the increase in electronic surface charge also enables the protein disulfide bonds to capture these electrons and form a transient disulfide electron adduct radical, which converts to free thiol radical on dissociation. This allows protein molecules to bind to the nanoparticle's surface with the favorable silver thiol bond leading to greater surface affinity and larger SERS enhancement. The proposed Ag-AlNFs represent a cost-effective material that can be potentially used to probe biological systems in a label-free manner by photoactivating the SERS substrate for obtaining higher enhancement factors.

Dynamic relaying properties of a β-turn peptide in long-range electron transfer.

The relay stations play a significant role in long-range charge hopping transfer in proteins. Although studies have clarified that many more protein structural motifs can function as relays in charge hopping transfers by acting as intermediate charge carriers, the relaying properties are still poorly understood. In this work, taking a β-turn oligopeptide as an example, we report a dynamic character of a relay with tunable relaying properties using the density functional theory calculations. Our main finding is that a β-turn peptide can serve as an effective electron relay in facilitating long-range electron migration and its relay properties is vibration-tunable. The vibration-induced structural transient distortions remarkably affect the lowest occupied molecular orbital (LUMO) energy, vertical electron affinity and electron-binding mode of the β-turn oligopeptide and the singly occupied molecular orbital (SOMO) energy of the corresponding electron adduct and thus the relaying properties. Different vibration modes lead to different structural distortions and thus have different effects on the relaying properties and ability of the β-turn peptide. For the relaying properties, there approximately is a linear negative correlation of electron affinity with the LUMO energy of the β-turn or the SOMO energy of its electron adduct. Besides, such relaying properties also vary in the vibration evolution process, and the electron-binding modes may be tunable. As an important addition to the known static charge relaying properties occurring in various protein structural motifs, this work reports the dynamic electron-relaying characteristics of a β-turn oligopeptide with variable relaying properties governed by molecular vibrations which can be applied to different proteins in mediating long-range charge transfers. Clearly, this work reveals molecular vibration effects on the electron relaying properties of protein structural motifs and provides new insights into the dynamics of long-range charge transfers in proteins. © 2018 Wiley Periodicals, Inc.

Presolvated electron reactions with methyl acetoacetate: electron localization, proton-deuteron exchange, and H-atom abstraction.

Radiation-produced electrons initiate various reaction processes that are important to radiation damage to biomolecules. In this work, the site of attachment of the prehydrated electrons with methyl acetoacetate (MAA, CH3-CO-CH2-COOCH3) at 77 K and subsequent reactions of the anion radical (CH3-CO•−-CH2-COOCH3) in the 77 to ca. 170 K temperature range have been investigated in homogeneous H2O and D2O aqueous glasses by electron spin resonance (ESR) spectroscopy. At 77 K, the prehydrated electron attaches to MAA forming the anion radical in which the electron is delocalized over the two carbonyl groups. This species readily protonates to produce the protonated electron adduct radical CH3-C(•)OH-CH2-COOCH3. The ESR spectrum of CH3-C(•)OH-CH2-COOCH3 in H2O shows line components due to proton hyperfine couplings of the methyl and methylene groups. Whereas, the ESR spectrum of CH3-C(•)OH-CH2-COOCH3 in D2O glass shows only the line components due to proton hyperfine couplings of CH3 group. This is expected since the methylene protons in MAA are readily exchangeable in D2O. On stepwise annealing to higher temperatures (ca. 150 to 170 K), CH3-C(•)OH-CH2-COOCH3 undergoes bimolecular H-atom abstraction from MAA to form the more stable radical, CH3-CO-CH•-COOCH3. Theoretical calculations using density functional theory (DFT) support the radical assignments.

Radiation Stability of Cations in Ionic Liquids. 1. Alkyl and Benzyl Derivatives of 5-Membered Ring Heterocycles

In order to use hydrophobic room temperature ionic liquids (ILs) as diluents in nuclear separations for advanced fuel cycles, it is desirable to reduce the breakdown of the constituent ions caused by ionizing radiation. In this series, we survey radiation stability for different classes of organic cations used to formulate ILs. While radiolysis of 1-alkyl-3-methylimidazolium cations has been extensively studied, there have not been complementary studies of 1-benzyl derivatives of these cations nor organic cations that are derived from 5-membered ring heterocycles other than imidazole, such as 1,2,4-triazole and thiazole. In part 1, we establish the fragmentation pathways for such cations and quantify product yields for 2.5 MeV electron beam radiolysis of these aromatic cations. Radiolytic reduction of 1-benzyl cations derived from imidazole and 1,2,4-triazole is shown to cause the elimination of benzyl radicals from their electron adducts, whereas this elimination does not occur in the thiazole derivatives due to stabilization of the excess electron as a dimer radical cation. No such elimination occurs in the corresponding 1-alkyl derivatives, but there is significant C-N and C-C bond fragmentation in the aliphatic arms. As such bond dissociation reactions are irreversible, there is significant loss of 1-alkyl cations during the radiolysis. For 1-benzyl derivatives, this electronic excitation causes fragmentation of the C-N bonds in the benzyl arms with the release of the corresponding base and the benzyl carbocation that can subsequently attack this base or add to another cation. Such systems exhibit more predictable fragmentation patterns and yield well-defined products; some of the systems also exhibit increased radiation resistance. The C-N bond fragmentation in the reduced cations can be further suppressed through the use of appropriate electron scavengers, including acids and aromatic imide anions. The observed trends are rationalized using density functional theory calculations, and the implications of these results for the design of IL diluents are examined.

Presolvated Low Energy Electron Attachment to Peptide Methyl Esters in Aqueous Solution: C–O Bond Cleavage at 77 K

In this study, the reactions of presolvated electrons with glycine methyl ester and N-acetylalanylalanine methyl ester (N-aAAMe) are investigated by electron spin resonance (ESR) spectroscopy and DFT calculations. Electrons were produced by γ-irradiation in neutral 7.5 M LiCl-D2O aqueous glasses at low temperatures. For glycine methyl ester, electron addition at 77 K results in both N-terminal deamination to form a glycyl radical and C-O ester bond cleavage to form methyl radicals. For samples of N-acetylalanylalanine methyl ester, electrons are found to add to the peptide bonds at 77 K and cleave the carboxyl ester groups to produce methyl radicals. On annealing to 160 K, electron adducts at the peptide links undergo chain scission to produce alanyl radicals and on further annealing to 170 K α-carbon peptide backbone radicals are produced by hydrogen abstraction. DFT calculations for electron addition to the methyl ester portion of N-aAAMe show the cleavage reaction is highly favorable (free energy equals to -30.7 kcal/mol) with the kinetic barrier of only 9.9 kcal/mol. A substantial electron affinity of the ester link (38.0 kcal/mol) provides more than sufficient energy to overcome this small barrier. Protonated peptide bond electron adducts also show favorable N-C chain cleavage reactions of -12.7 to -15.5 kcal/mol with a barrier from 7.4 to 10.0 kcal/mol. The substantial adiabatic electron affinity (AEA) of the peptide bond and ester groups provides sufficient energy for the bond dissociation.

Urea’s Effect on the Ribonuclease A Catalytic Efficiency: A Kinetic, 1H NMR and Molecular Orbital Study

Understanding of protein-urea interactions is one of the greatest challenges to modern structural protein chemistry. Based in enzyme kinetics experiments and (1)H NMR spectroscopic analysis we proposed that urea, at low concentrations, directly interacts with the protonated histidines of the active center of RNase A, following a simple model of competitive inhibition. These results were supported by theoretical analysis based on the frontier molecular orbital theory and suggest that urea might establish a favorable interaction with the cationic amino acids. Our experimental evidence and theoretical analysis indicate that the initials steps of the molecular mechanism of Urea-RNase A interaction passes through the establishment of a three center four electron adduct. Also, our results would explain the observed disruption of the (1)H NMR signals corresponding to H12 and H119 (involved in catalysis) of the RNase A studied in the presence of urea. Our interaction model of urea-amino acids (cationic) can be extended to explain the inactivation of other enzymes with cationic amino acids at the active site.

Pulse radiolysis study on gatifloxacin — A fluoroquinolone antibiotic

The reactions between gatifloxacin (GFX) and various one-electron oxidants, such as ·OH, N3·, Br2·−, and SO4·−, have been studied by pulse radiolysis techniques. The GFX radical anion formed in the reaction of GFX with eaq− could either be protonated or deprotonated, and the absorption of GFX radical anion was located at 390 nm. The transient species produced by the reaction of GFX with ·OH radical shows a broad band in the 380–600 nm region with a shoulder, while the oxidation by N3·, SO4·−, and Br2·− results in an absorption band with λmax = 370 nm. At neutral condition (pH 7), the rate constants of GFX reacting with ·OH, N3·, Br2·−, SO4·− and eaq− are estimated to be 1.0 × 1010, 3.1 × 109, 2.8 × 109, 3.0 × 109, and 1.8 × 1010 dm3 mol−1 s−1, respectively. From the pH dependence on the formation of electron adducts and on the rate constant of GFX with eaq−, the pKa of GFX radical anion is estimated to be 5.5 and 9.3.

Synthesis and Spectroscopic Characterization of Manganese(II), Iron(III) and Cobalt(III) Complexes of Macrocyclic Ligand. Potential of Cobalt(III) Complex in Biological Activity

A new series of manganese(II), iron(III) and cobalt(III) complexes of 14-membered macrocyclic ligand, (3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane-1,8-diamine) have been prepared and characterized by elemental analyses, IR, UV-VIS, <TEX>$^1H$</TEX>- and <TEX>$^{13}C$</TEX>- NMR spectra, magnetic susceptibilities, conductivities, and ESR measurements. Molar conductance measurements in DMF solution indicate that the complexes are electrolytes. The ESR spectrum for cobalt(III) complex in <TEX>$CD_3OD+10%D_2O$</TEX> after exposure to <TEX>$^{60}Co-{\gamma}$</TEX>-rays at 77 K using a 0.2217 M rad <TEX>$h^{-1}$</TEX> vicrad source showed <TEX>$g_{\perp}$</TEX> > <TEX>$g_{\parallel}$</TEX> > <TEX>$g_e$</TEX>, indicating that, the unpaired electron site is mainly present in the <TEX>$d_z2$</TEX> orbital with covalent bond character. In this case, the ligand hyperfine tensors are nearly collinear with <TEX>${\gamma}$</TEX>-tensors, so there is no major tendency to bend. Therefore, little extra delocalization via the ring lobe of the <TEX>$dz^2$</TEX> orbital occurs. However, the ESR spectrum in solid state after exposure to <TEX>$^{60}Co-{\gamma}$</TEX>-rays at 77 K showed <TEX>$g_{\parallel}$</TEX> > <TEX>$g_{\perp}$</TEX> > <TEX>$g_e$</TEX>, indicating that, the unpaired electron site is mainly present in the <TEX>$d_x2_{-y}2$</TEX> ground state as the resulting spectrum contains a large number of randomly oriented molecules provided that, the principle directions of g and A tensors. Manganese (II) complex 2, <TEX>$[H_{12}LMn]Cl_4.2H_2O$</TEX>, showed six isotropic lines characteristic to an unpaired electron interacting with a nucleus of spin 5/2, however, iron(III) complex 3, <TEX>$[H_{12}LFe]Cl_5.H_2O$</TEX>, showed spectrum of a high spin <TEX>$^{57}Fe$</TEX> (I=1/2), <TEX>$d^5$</TEX> configuration. The geometry of these complexes was supported by elemental analyses, IR, electronic and ESR spectral studies. Complex 1 showed exploitation in reducing the amount of electron adducts formed in DNA during irradiation with low radiation products.

Radiation induced degradation of ketoprofen in dilute aqueous solution

The intermediates and final products of ketoprofen degradation were investigated in 0.4mmoldm−3 solution by pulse radiolysis and gamma radiolysis. For observation of final products UV−vis spectrophotometry and HPLC separation with diode array detection were used, and for identification MS was used. The reactions of •OH lead to hydroxycyclohexadienyl type radical intermediates, in their further reactions hydroxylated derivatives of ketoprofen form as final products. The hydrated electron is scavenged by the carbonyl oxygen and the electron adduct protonates to ketyl radical •OH is more effective in decomposing ketoprofen than hydrated electron. Chemical oxygen demand and total organic carbon content measurements on irradiated aerated solutions showed that using irradiation technology ketoprofen can be mineralised. The initial toxicity of the solution monitored by the Daphnia magna test steadily decreases with irradiation. Using 5kGy dose no toxicity of the solution was detected with this test.

Radicals Formed in N-Acetylproline by Electron Attachment: Electron Spin Resonance Spectroscopy and Computational Studies

In this study, the reactions of electrons with N-acetylproline are investigated by electron spin resonance (ESR) spectroscopy and density functional theory. Electrons are produced by γ irradiation or by photoionization of K(4)Fe(CN)(6) in neutral 7.5 M LiCl-D(2)O aqueous glasses at low temperatures with identical results. Electrons are found to add to both the peptide bond and the carboxyl group of the acetyl-proline moiety at 77 K. On annealing, both the electron adducts undergo fragmentation of the peptide bond between the nitrogen and the α carbon of the peptide structure. However, the peptide bond electron adduct radical reacts much more rapidly than the carboxyl group electron adduct radical. The DFT calculations predict that the carboxyl adduct is substantially more stable than the peptide bond adduct, with the activation barrier to N-Cα cleavage 3.7 kcal/mol for the amide electron adducts and 23 kcal/mol for the carboxyl electron adducts in inagreement with the relative reactivity found by experiment.

ChemInform Abstract: One-Electron-Reduction Potentials of Pyrimidine Bases, Nucleosides, and Nucleotides in Aqueous Solution. Consequences for DNA Redox Chemistry.

The reduction potentials in aqueous solution of the pyrimidine bases, nucleosides, and nucleotides of uracil (U) and thymine (T) were determined using the technique of pulse radiolysis with time-resolved spectrophotometric detection. The electron adducts of U and T were found to undergo reversible electron exchange with a series of ring-substituted N-methylpyridinium cations with known reduction potential. From the concentrations of the pyrimidine electron adducts and the reduced N-methylpyridinium compounds at electron-transfer equilibrium, the thermodynamical equilibrium constants wereobtained and from these the reduction potentials

Comparison of Isoelectronic 8-HO-G and 8-NH2-G Derivatives in Redox Processes

8-Oxo-7,8-dihydroguanine (8-oxo-G) is the major lesion of oxidatively generated DNA damage. Despite two decades of intense study, several fundamental properties remain to be defined. Its isoelectronic 8-aminoguanine (8-NH(2)-G) has also received considerable attention from a biological point of view, although its chemistry involving redox processes remains to be discovered. We investigated the one-electron oxidation and one-electron reduction reactions of 8-oxo-G and 8-NH(2)-G derivatives. The reactions of hydrated electrons (e(aq)(-)) and azide radicals (N(3)(*)) with both derivatives were studied by pulse radiolysis techniques, and the transient absorption spectra were assigned to specific tautomers computationally by means of time-dependent DFT (TD-B3LYP/6-311G**//B1B95/6-31+G**) calculations. The protonated electron adducts of 8-NH(2)-G and 8-oxo-G showed a substantial difference in their absorption spectra, the unpaired electron being mainly delocalized in the imidazolyl ring and in the six-membered ring, respectively. On the other hand, the deprotonated forms of one-electron oxidation of 8-NH(2)-G and 8-oxo-G showed quite similar spectral characteristics. In a parallel study, the one-electron reduction of 8-azidoguanine (8-N(3)-G) afforded the same transient of one-electron oxidation of 8-NH(2)-G, which represents another example of generation of one-electron oxidized guanine derivatives under reducing conditions. Moreover, the fate of transient species was investigated by radiolytic methods coupled with product studies and allowed self- and cross-termination rate constants associated with these reactions to be estimated.

Flash Photolysis of Cutinase: Identification and Decay Kinetics of Transient Intermediates Formed upon UV Excitation of Aromatic Residues

Aromatic amino acids play an important role in ultraviolet (UV)-induced photochemical reactions in proteins. In this work, we aim at gaining insight into the photochemical reactions induced by near-UV light excitation of aromatic residues that lead to breakage of disulfide bridges in our model enzyme, Fusarium solani pisi cutinase, a lipolytic enzyme. With this purpose, we acquired transient absorption data of cutinase, with supplemental experimental data on tryptophan (Trp) and lysozyme as reference molecules. We here report formation kinetics and lifetimes of transient chemical species created upon UV excitation of aromatic residues in proteins. Two proteins, lysozyme and cutinase, as well as the free amino acid Trp, were studied under acidic, neutral, and alkaline conditions. The shortest-lived species is assigned to solvated electrons (lifetimes of a few microseconds to nanoseconds), whereas the longer-lived species are assigned to aromatic neutral and ionic radicals, Trp triplet states, and radical ionic disulphide bridges. The pH-dependent lifetimes of each species are reported. Solvated electrons ejected from the side chain of free Trp residues and aromatic residues in proteins were observed 12 ns after excitation, reaching a maximum yield after ∼40 ns. It is interesting to note that the formation kinetics of solvated electrons is not pH-dependent and is similar in the different samples. On the other hand, a clear increase of the solvated electron lifetime is observed with increasing pH. This observation is correlated with H3O+ being an electron scavenger. Prolonged UV illumination of cutinase leads to a larger concentration of solvated electrons and to greater absorption at 410 nm (assigned to disulphide electron adduct RSSR •−), with concomitant faster decay kinetics and near disappearance of the Trp• radical peak at 330 nm, indicating possible additional formation of TyrO• formed upon reaction of Trp• with Tyr residues. Prolonged UV illumination of cutinase also leads to a larger concentration of free thiol groups, known to originate from the dissociation of RSSR •−. Additional mechanisms that may lead to the near disappearance of Trp• are discussed. Our study provides insight into one key UV-light-induced reaction in cutinase, i.e., light-induced disruption of disulphide bridges mediated by the excitation of aromatic residues. Knowledge about the nature of the formed species and their lifetimes is important for the understanding of UV-induced reactions in humans that lead to light-induced diseases, e.g., skin cancer and cataract formation.

Reaction of Hydrated Electrons with Guanine Derivatives: Tautomerism of Intermediate Species

Here, we show that two tautomers are produced by the protonation of the guanine-electron adduct. The fate of electron adducts of a variety of substituted guanosines was investigated by radiolytic methods and addressed computationally by means of time-dependent DFT (TD-B3LYP/6-311G**//B1B95/6-31+G**) calculations. The reaction of e(aq-) with guanosine and 1-methylguanosine produces two transient species, whereas the reaction with N2-ethylguanosine and N2,N2-diethylguanosine produces only one. The two short-lived intermediates, which show a substantial difference in their UV-visible spectra, are recognized to be two purine tautomers (i.e., iminic 18 and aminic 19 forms). The tautomerization 18 --> 19 occurs with a rate constant of ca. 1.5 x 106 s(-1) , and theory suggests that it is a water-assisted process.

The kinetics of radical reactions in the radiolysis of aqueous solutions of organic nitro compounds

Pulsed radiolysis and computer simulation of gamma radiolytic decomposition of organic nitrates in aqueous solutions were performed to determine the rate constants for reactions with the participation of intermediates determining the mechanism of the process. 2,4,6-Trinitrotoluene, 2,4-dinitrotoluene, and cyclic nitramine, cyclotrimethylene-trinitramine, were used as substrates. The bimolecular rate constants for the reactions of hydrated electrons (e aq − ) and hydroxyl radicals (•OH) with the substrates and constants for the recombination of electron adducts and carbon-centered radicals (the products of the detachment of the H atom from the nitro compound molecule by the OH radical) were determined by direct measurements with the use of high-speed spectrophotometry. Computer simulation of the reaction scheme was used to estimate the rate constant for significant reactions, monomolecular forward and back reactions of electron adducts and electron transfer to molecular oxygen, and refine the rate constant for the reaction of e aq − with tert-butanol.

Formation of Spectral Intermediate G−C and A−T Anion Complex in Duplex DNA Studied by Pulse Radiolysis

The dynamics of electron adducts of 2'-deoxynucleotides and oligonucelotides (ODNs) were measured spectroscopically by nanosecond pulse radiolysis. The radical anions of the nucleotides were produced within 10 ns by the reaction of hydrated electrons (e(aq)(-)) and were protonated to form the corresponding neutral radicals. At pH 7.0, the radical anion of deoxythymidine (dT(*-)) was protonated to form the neutral radical dT(H)(*) in the time range of microseconds. The rate constant for the protonation was determined as 1.8 x 10(10) M(-1) s(-1). In contrast, the neutral radical of dC(H)(*) was formed immediately after the pulse, suggesting that the protonation occurs within 10 ns. The transient spectra of excess electrons of the double-stranded ODNs 5'-TAATTTAATAT-3' (AT) and 5'-CGGCCCGGCGC-3' (GC) differed from those of pyrimidine radicals (C and T) and their composite. In contrast, the spectra of the electron adducts of the single-stranded ODNs GC and AT exhibited characteristics of C and T, respectively. These results suggest that, in duplex ODNs, the spectral intermediates of G-C and A-T anions complex were formed. On the microsecond time scale, the subsequent changes in absorbance of the ODN AT had a first-order rate constant of 4 x 10(4) s(-1), reflecting the protonation of T.

Pulse radiolysis of tetrazolium violet in aqueous and aqueous-alcoholic solutions under oxidative and reductive conditions

The radiolytic reduction of colourless tetrazolium salts to coloured formazans in liquid and solid state is suggested for dosimetry purposes. In order to clarify the reaction mechanism, a pulse radiolysis study was conducted in aqueous and aqueous-alcoholic solutions under oxidative and reductive conditions. Under reducing conditions, fast formation of the electron adduct tetrazolinyl radical was observed: coloured formazan final product formed during the decay of electron adduct. Both the decay of the tetrazolinyl radical and the formation of the formazan were found to be second order. The spectra of the formazan were similar in neutral and alkaline solutions, but with higher absorbance in the latter solutions due to the higher molar absorption coefficient. Under oxidative conditions formazan did not form; hydroxylated products through OH-adducts were observed in the pH range studied.

One-Electron Reduction of 8-Bromo-2-aminoadenosine in the Aqueous Phase: Radiation Chemical and DFT Studies of the Mechanism

Two tautomeric forms of one-electron oxidized 2-aminoadenosine (2AA) have been produced by reactions of hydrated electrons (e aq-) with 8-bromo-2-aminoadenosine (8-Br-2AA) at natural pH, whereas only one tautomer is formed by oxidation of 2AA. Tailored experiments by pulse radiolysis and time-dependent DFT (TD-B3LYP/6-311G**//B1B95/6-31+G**) calculations allowed the definition of the reaction mechanism in some detail. The electron adducts of 8-Br-2AA protonated at C8 eject Br- and produce the two short-lived tautomers (8 and 9). The first observable species decays by first-order kinetics to produce the second intermediate, which is also obtained by oxidation of 2AA by SO4*-. The rate of tautomerization (k taut = 4.5 x 104 s-1) is strongly accelerated by phosphate and is retarded in D2O (kinetic isotope effect 7). B1B95/6-31+G** calculations showed that the tautomerization is a water-assisted process. In acidic or basic solutions, the "instantaneous" formation of one-electron oxidized 2AA or its deprotonated forms has been produced by reactions of e aq- with 8-Br-2AA. gamma-Radiolysis of 8-Br-2AA in aqueous solutions followed by product studies led to the formation of 2AA as a single product.