DNA Damage from Sulfite Autoxidation Catalyzed by a Nickel(II) Peptide

Reduction of Np(VI) with hydrazinopropionitrile via water-mediated proton transfer

Effectively adjusting and controlling the valence state of neptunium (Np) is essential in its separation during spent fuel reprocessing. Hydrazine and its derivatives as free-salts can selectively reduce Np(VI) to Np(V). Reduction mechanisms of Np(VI) with hydrazine and four derivatives have been explored using multiple theoretical methods in our previous works. Herein, we examine the reduction mechanism of Np(VI) with hydrazinopropionitrile (NCCH₂N₂H₃) which exhibits faster kinetics than most other hydrazine derivatives probably due to its σ-π hyperconjugation effect. Free radical ion pathways I, II and III involving the three types of hydrazine H atoms were found that correspond to the experimentally established mechanism of reduction of two Np(VI) via initial oxidation to [NCCH₂N₂H₃]⁺˙, followed by conversion to NCCH₂N₂H (+2H₃O⁺) and ultimately to CH₃CN + N₂. Potential energy profiles suggest that the second redox stage is rate-determining for all three pathways. Pathway I with water-mediated proton transfer is energetically preferred for hydrazinopropionitrile. Analyses using the approaches of localized molecular orbitals (LMOs), quantum theory of atoms in molecules (QTAIM), and intrinsic reaction coordinate (IRC) elucidate the bonding evolution for the structures on the reaction pathways. The results of the spin density reveal that the reduction of the first Np(VI) ion is the outer-sphere electron transfer, while that of the second Np(VI) ion is the hydrogen transfer. This work offers new insights into the nature of reduction of Np(VI) by hydrazinopropionitrile via water-mediated proton transfer, and provides a basis for designing free-salt reductants for Np separations.

Roles of Sulfites in Reverse Osmosis (RO) Plants and Adverse Effects in RO Operation

More than 60 years have passed since UCLA first announced the development of an innovative asymmetric cellulose acetate reverse osmosis (RO) membrane in 1960. This innovation opened a gate to use RO for commercial use. RO is now ubiquitous in water treatment and has been used for various applications, including seawater desalination, municipal water treatment, wastewater reuse, ultra-pure water (UPW) production, and industrial process waters, etc. RO is a highly integrated system consisting of a series of unit processes: (1) intake system, (2) pretreatment, (3) RO system, (4) post-treatment, and (5) effluent treatment and discharge system. In each step, a variety of chemicals are used. Among those, sulfites (sodium bisulfite and sodium metabisulfite) have played significant roles in RO, such as dechlorination, preservatives, shock treatment, and sanitization, etc. Sulfites especially became necessary as dechlorinating agents because polyamide hollow-fiber and aromatic thin-film composite RO membranes developed in the late 1960s and 1970s were less tolerable with residual chlorine. In this review, key applications of sulfites are explained in detail. Furthermore, as it is reported that sulfites have some adverse effects on RO membranes and processes, such phenomena will be clarified. In particular, the following two are significant concerns using sulfites: RO membrane oxidation catalyzed by heavy metals and a trigger of biofouling. This review sheds light on the mechanism of membrane oxidation and triggering biofouling by sulfites. Some countermeasures are also introduced to alleviate such problems.

A ratiometric fluorescent probe for the rapid and specific detection of HSO3 - in water samples

Hydrosulphite (HSO₃ ^- ), as a common and important chemical reagent, is widely used in everyday life, however excessive use and abuse of HSO₃ ^- can cause potential harmful effects on the environment and in biological health. In this paper, we describe the design and preparation of a colorimetric and ratiometric fluorescence probe for the visual detection of HSO₃ ^- (excitation wavelengths were, respectively, 336 nm and 520 nm). This method showed some advantages including simple preparation, high selectivity, fast response, and significant colour and fluorescence ratio (F₄₅₀ /F₅₉₄ ) changes in the presence of HSO₃ ^- . In addition, this probe was used successfully for the detection of HSO₃ ^- in real water samples and showed a good recovery rate range.

Designed Metal-ATCUN Derivatives: Redox- and Non-redox-Based Applications Relevant for Chemistry, Biology, and Medicine

The designed “ATCUN” motif (amino-terminal copper and nickel binding site) is a replica of naturally occurring ATCUN site found in many proteins/peptides, and an attractive platform for multiple applications, which include nucleases, proteases, spectroscopic probes, imaging, and small molecule activation. ATCUN motifs are engineered at periphery by conjugation to recombinant proteins, peptides, fluorophores, or recognition domains through chemically or genetically, fulfilling the needs of various biological relevance and a wide range of practical usages. This chemistry has witnessed significant growth over the last few decades and several interesting ATCUN derivatives have been described. The redox role of the ATCUN moieties is also an important aspect to be considered. The redox potential of designed M-ATCUN derivatives is modulated by judicious choice of amino acid (including stereochemistry, charge, and position) that ultimately leads to the catalytic efficiency. In this context, a wide range of M-ATCUN derivatives have been designed purposefully for various redox- and non-redox-based applications, including spectroscopic probes, target-based catalytic metallodrugs, inhibition of amyloid-β toxicity, and telomere shortening, enzyme inactivation, biomolecules stitching or modification, next-generation antibiotic, and small molecule activation.

Role of Iron Sulfide Phases in the Stability of Noncrystalline Tetravalent Uranium in Sediments

Uranium (U) in situ bioremediation has been investigated as a cost-effective strategy to tackle U contamination in the subsurface. While uraninite was believed to be the only product of bioreduction, numerous studies have revealed that noncrystalline U(IV) species (NCU(IV)) are dominant. This finding brings into question the effectiveness of bioremediation because NCU(IV) species are expected to be labile and susceptible to oxidation. Thus, understanding the stability of NCU(IV) in the environment is of crucial importance. Fe(II) minerals (such as FeS) are often associated with U(IV) in bioremediated or naturally reduced sediments. Their impact on the stability of NCU(IV) is not well understood. Here, we show that, at high dissolved oxygen concentrations, FeS accelerates NCU(IV) reoxidation. We hypothesize that either highly reactive ferric minerals or radical S species produced by the oxidation of FeS drive this rapid reoxidation of NCU(IV). Furthermore, we found evidence for the contribution of reactive oxygen species to NCU(IV) reoxidation. This work refines our understanding of the role of iron sulfide minerals in the stability of tetravalent uranium in the presence of oxygen in a field setting such as contaminated sites or uranium-bearing naturally reduced zones.

A mitochondrion-targeted dual-site fluorescent probe for the discriminative detection of SO32− and HSO3− in living HepG-2 cells

Sulfur dioxide, known as an environmental pollutant, produced during industrial productions is also a common food additive that is permitted worldwide. In living organisms, sulfur dioxide forms hydrates of sulfite (SO₂·H₂O), bisulfite (HSO₃⁻) and sulfite (SO₃²⁻) under physiological pH conditions; these three exist in a dynamic balance and play a role in maintaining redox balance, further participating in a wide range of physiological and pathological processes. On the basis of the differences in nucleophilicity between SO₃²⁻ and HSO₃⁻, for the first time, we built a mitochondrion-targeted dual-site fluorescent probe (Mito-CDTH-CHO) based on benzopyran for the highly specific detection of SO₃²⁻ and HSO₃⁻ with two diverse emission channels. Mito-CDTH-CHO can discriminatively respond to the levels of HSO₃⁻ and SO₃²⁻. Besides, its advantages of low cytotoxicity, superior biocompatibility and excellent mitochondrial enrichment ability contribute to the detection and observation of the distribution of sulfur dioxide derivatives in living organisms as well as allowing further studies on the physiological functions of sulfur dioxide.

Rational design and sensing mechanism of a dual-site fluorescence probe for HSO₃⁻ and SO₃²⁻.

Review on low- and high-frequency sonolytic, sonophotolytic and sonophotochemical processes for inactivating pathogenic microorganisms in aqueous media

Ultraviolet and ultrasound-based advanced oxidation processes (AOPs) are gaining considerable research attention for water treatment and disinfection. Compared to low-frequency ultrasound (LFUS, <100 kHz), high-frequency ultrasound (HFUS, >100 kHz and MHz range) for water disinfection remains much less investigated. The present review aims at surveying and discussing literature data on microbial inactivation in non-food aqueous media using HFUS alone and with AOPs. More specifically, the review covers sonophotolytic (US/UV) processes under sequential and simultaneous modes as well as sonophotochemical processes, where both low and high frequencies were applied. Addressing a state-of-the-art biomedical research, we have attempted to provide more insight into mechanical and sonochemical mechanisms of inactivation under ultrasonic exposure. Sonoporation, intracellular generation of reactive oxygen species (ROS), energy stimulation of aquaporins to deliver ROS, and injection of extracellular ROS into sonoporated cells have all been identified as primary ways of inactivation. Application of ultrasound in the 0.2-2 MHz range and mercury-free light sources to support the Minamata Convention on Mercury is an ongoing challenge for effective elimination of microbial pathogens from water and wastewater through sonophotolytic and sonophotochemical AOPs.

SO2 Donors and Prodrugs, and Their Possible Applications: A Review

SO2 is widely recognized as an air pollutant and is a known cause of acid rain. At a sufficiently high level, it also causes respiratory diseases. A much lesser known side of SO2 is its endogenous nature and possible physiological roles. There is mounting evidence that SO2 is produced during normal cellular metabolism and may possibly function as a signaling molecule in normal physiology. The latter aspect is still at the stage of being carefully examined as to the validity of classifying SO2 as a gasotransmitter with endogenous signaling roles. One difficulty in studying the biological and pharmacological roles of SO2 is the lack of adequate tools for its controllable and precise delivery. Traditional methods of using SO2 gas or mixed sulfite salts do not meet research need for several reasons. Therefore, there has been increasing attention on the need of developing SO2 donors or prodrugs that can be used as tools for the elucidation of SO2's physiological roles, pharmacological effects, and possible mechanism(s) of action. In this review, we aim to review basic sulfur chemistry in the context of sulfur signaling and various chemical strategies used for designing SO2 donors. We will also discuss potential pharmacological applications of SO2 donors, lay out desirable features for such donors and possibly prodrugs, analyze existing problems, and give our thoughts on research needs.

Cu2O-promoted degradation of sulfamethoxazole by α-Fe2O3-catalyzed peroxymonosulfate under circumneutral conditions: synergistic effect, Cu/Fe ratios, and mechanisms

To promote the application of iron oxides in sulfate radical-based advanced oxidation processes, a convenient approach using Cu₂O as a catalyst additive was proposed. Composite catalysts based on α-Fe₂O₃ (CTX%Cu₂O, X = 1, 2.5, 5, and 10) were prepared for peroxymonosulfate (PMS) activation, and sulfamethoxazole was used as a model pollutant to probe the catalytic reactivity. The results show that a synergistic catalytic effect exists between Cu₂O and α-Fe₂O₃, which was explained by the promoted reduction of Fe(III) by Cu(I). Iron K-edge X-ray absorption spectroscopy investigations indicated that the promoted reduction probably occurred with PMS acting as a ligand that bridges the redox centers of Cu(I) and Fe(III). The weight ratio between Cu₂O and α-Fe₂O₃ influenced the degradation of sulfamethoxazole, and the optimal ratio depended on the dosage of PMS and catalysts. With 40 mg L^-1 PMS and 0.6 g L^-1 catalyst, a pseudo-first-order constant of ∼0.019 min^-1 was achieved for CT2.5%Cu₂O, whereas only 0.004 min^-1 was realized for α-Fe₂O₃. Nearly complete degradation of the sulfamethoxazole was achieved within 180 min under the conditions of 40 mg L^-1 PMS, 0.4 g L^-1 CT2.5%Cu₂O, and pH 6.8. In contrast, less than 20% degradation was realized with α-Fe₂O₃ under similar conditions. The CT2.5%Cu₂O catalyst had the best stoichiometric efficiency of PMS (0.317), which was 4.5 and 5.8 times higher than those of Cu₂O (0.070) and α-Fe₂O₃ (0.054), respectively. On the basis of the products identified, the cleavage of the S-N bond was proposed as a major pathway for the degradation of sulfamethoxazole.

Hole delocalization over adenine tracts in single stranded DNA oligonucleotides

Adiabatic ionization energies of single stranded DNA oligonucleotides containing adenine tracts of different sizes have been computed at the DFT level and compared with the oxidation potentials determined by differential pulse voltammetry. Geometry optimizations have been performed at the full quantum mechanical level, including the sugar phosphate backbone and solvent effects. The observed progressive lowering of the ionization energy upon increasing the number of consecutive adenines is well predicted, the computed ionization potential shifts being in very good agreement with the experimental outcomes, both by using pure and hybrid functionals. The spin density of the oligonucleotide radical cations is distributed almost over the whole adenine tract, forming delocalized polarons.

Synergetic Transformations of Multiple Pollutants Driven by Cr(VI)-Sulfite Reactions

Reduction of Cr(VI) is often deemed necessary to detoxify chromium contaminants; however, few investigations utilized this reaction for the purpose of treating other industrial wastewaters. Here a widely used Cr(VI)-sulfite reaction system was upgraded to simultaneously transform multiple pollutants, namely, the reduction of Cr(VI) and oxidation of sulfite and other organic/inorganic pollutants in an acidic solution. As(III) was selected as a probe pollutant to examine the oxidation capacity of a Cr(VI)-sulfite system. Both (•)OH and SO4(•-) were considered as the primary oxidants for As(III) oxidation, based on the results of electron spin resonance, fluorescence spectroscopy, and specific radicals quenching. As(III)-scavenging, oxidative radicals greatly accelerated Cr(VI) reduction and simultaneously consumed less sulfite. In comparison with a Cr(VI)-H2O2 system with 50 μM Cr(VI), Cr(VI), the sulfite system had excellent performance for both As(III) oxidation and Cr(VI) reduction at pH 3.5. Moreover, in this escalated process, less sulfite was required to reduce Cr(VI) than the traditional Cr(VI) reduction by sulfite process. This effectively improves the environmental compatibility of this Cr(VI) detoxification process, alleviating the potential for SO2 release and sulfate ion production in water. Generally, this study provides an excellent example of a "waste control by waste" strategy for the detoxification of multiple industrial pollutants.

Thiol activated prodrugs of sulfur dioxide (SO2) as MRSA inhibitors

Drug resistant infections are becoming common worldwide and new strategies for drug development are necessary. Here, we report the synthesis and evaluation of 2,4-dinitrophenylsulfonamides, which are donors of sulfur dioxide (SO2), a reactive sulfur species, as methicillin-resistant Staphylococcus aureus (MRSA) inhibitors. N-(3-Methoxyphenyl)-2,4-dinitro-N-(prop-2-yn-1-yl)benzenesulfonamide (5e) was found to have excellent in vitro MRSA inhibitory potency. This compound is cell permeable and treatment of MRSA cells with 5e depleted intracellular thiols and enhanced oxidative species both results consistent with a mechanism involving thiol activation to produce SO2.

Delocalized hole domains in Guanine-rich DNA oligonucleotides

Differential pulse voltammetries of guanine-rich single- and double-stranded oligonucleotides containing up to six consecutive guanines are reported. The observed progressive lowering of the first voltammetric peak potential as the number of adjacent guanines increases unambiguously points toward the establishment of delocalized hole domains; the hole stabilization energy is ca. 0.1 eV per GG step, significantly lower than that observed for AA steps.

Vibronic couplings and coherent electron transfer in bridged systems

A computational strategy to analyze the dynamics of coherent electron transfer processes in bridged systems, involving three or more electronic states, is presented.

Kinetics of the autoxidation of sulfur(IV) co-catalyzed by peroxodisulfate and silver(I) ions

The kinetics and mechanism of the reaction between dissolved oxygen and sulfur(iv) was studied in aqueous acidic medium using co-catalysts peroxodisulfate and silver(i) ions. The presence of both catalysts was required to observe measurable rates in the studied process. The reaction rate was determined through following the UV-absorption of hydrated sulfur dioxide, and the trends were determined as a function of pH, reactant and catalyst concentrations. Individual kinetic curves under conditions where dissolved oxygen was the limiting reagent were close to zeroth-order. A chain mechanism with four chain carriers, sulfite, sulfate, peroxomonosulfate ion radical and silver(ii) ion, is proposed to interpret all the kinetic and stoichiometric findings, and an explicit formula was obtained for the rate law. The role of the co-catalysts is to produce chain carriers, whereas silver(i) and silver(ii) ions also participate in chain propagation steps. Further supporting evidence for the proposed mechanism was gained in laser flash photolysis studies, which showed that sulfate ion radical reacts quite rapidly with silver(i) ion.

Mechanistic aspects of hydration of guanine radical cations in DNA

The mechanistic aspects of hydration of guanine radical cations, G(•+) in double- and single-stranded oligonucleotides were investigated by direct time-resolved spectroscopic monitoring methods. The G(•+) radical one-electron oxidation products were generated by SO4(•-) radical anions derived from the photolysis of S2O8(2-) anions by 308 nm laser pulses. In neutral aqueous solutions (pH 7.0), after the complete decay of SO4(•-) radicals (∼5 μs after the actinic laser flash) the transient absorbance of neutral guanine radicals, G(-H)(•) with maximum at 312 nm, is dominant. The kinetics of decay of G(-H)(•) radicals depend strongly on the DNA secondary structure. In double-stranded DNA, the G(-H)(•) decay is biphasic with one component decaying with a lifetime of ∼2.2 ms and the other with a lifetime of ∼0.18 s. By contrast, in single-stranded DNA the G(-H)(•) radicals decay monophasically with a ∼ 0.28 s lifetime. The ms decay component in double-stranded DNA is correlated with the enhancement of 8-oxo-7,8-dihydroguanine (8-oxoG) yields which are ∼7 greater than in single-stranded DNA. In double-stranded DNA, it is proposed that the G(-H)(•) radicals retain radical cation character by sharing the N1-proton with the N3-site of C in the [G(•+):C] base pair. This [G(-H)(•):H(+)C ⇆ G(•+):C] equilibrium allows for the hydration of G(•+) followed by formation of 8-oxoG. By contrast, in single-stranded DNA, deprotonation of G(•+) and the irreversible escape of the proton into the aqueous phase competes more effectively with the hydration mechanism, thus diminishing the yield of 8-oxoG, as observed experimentally.

Novel complexes of Co(III) and Ni(II) containing peptide ligands: synthesis, DNA binding and photonuclease activity

The new cobalt(III) and nickel(II) complexes of the type [M(L)2(H2O)2](n)(+) (where M = Co(III) or Ni(II) ion, n = 3 for Co and 2 for Ni, L = peptides Fmoc. Ala-val-OH (F-AVOH), Fmoc-Phe-Leu-Ome (F-PLOMe) and Z-Ala-Phe-CONH2 (Z-APCONH2)) were synthesized and structurally characterized by FTIR, (1)H NMR, elemental analysis and electronic spectral data. An octahedral geometry has been proposed for all the synthesized Co(III) and Ni(II) metal complexes. The binding property of the complexes with CT-DNA was studied by absorption spectral analysis, followed by viscosity measurement and thermal denaturation studies. Detailed analysis revealed that the metal complexes intercalates into the DNA base stack as intercalator. The photo induced cleavage studies shows that the complexes possess photonuclease property against pUC19 DNA under UV-Visible irradiation.

Ferrous-activated persulfate oxidation of arsenic(III) and diuron in aquatic system

In situ chemical oxidation (ISCO) can be an effective technology for the remediation of soil and groundwater polluted by organic and inorganic contaminants. This study investigated the oxidation of arsenic(III) (As(III)) and diuron using ferrous activated persulfate-based ISCO. The results indicated that Fe(II)/persulfate oxidation could be an effective method to oxidize As(III) and diuron. Effects of pH, S2O8(2-) and Fe(II) amounts on the destruction of As(III) and diuron were examined in batch experiments. Acidic conditions favored the removal of As(III) and diuron. Four chelating agents, citric acid (CA), Na2S2O3, diethylene triamine pentaacetic acid (DTPA) and ethylene diamine tetraacetic acid disodium (EDTA-Na2) were used in attempt to maintain the quantity of ferrous ion in solution. In our experiments, CA and Na2S2O3 were found to be more effective than DTPA and EDTA-Na2. Our results also revealed a widely practical prospect of inorganic chelating agent Na2S2O3. Hydroxyl and sulfate radical were determined to play key roles in the oxidation process by using ethanol and tertiary butanol as molecular probes. Oxidation of As(III) yielded As(V) via the electron-transfer reaction. In the oxidation process of diuron, a stepwise nucleophilic substitution of chlorine by hydroxyl and a stepwise oxidation process of the methyl on the dimethylurea group by hydroxyl and sulfate radical were proposed.

Novel photo-sulfite system: toward simultaneous transformations of inorganic and organic pollutants

An efficient and green advanced oxidation process (i.e., photo-sulfite reaction) for the simultaneous oxidation of sulfite and organic pollutants in water is reported. The photo-sulfite system (UV-Fe(III)-sulfite) is based on the Fe-catalyzed sulfite oxidation and photochemistry of Fe(III) species. SO4(•-) and (•)OH radicals were identified in the photo-sulfite system with radical scavenging experiments using specific alcohols. This novel technology was consistently proven to be more favorable than the alternative Fe(III)-sulfite systems for the degradation of 2,4,6-trichlorophenol (2,4,6-TCP) and other organic pollutants at all conditions tested. The reactivity of photo-sulfite system was sustained due to the spontaneous switch of photoactive species from Fe(III)-sulfito to Fe(III)-hydroxo complexes with the depletion of sulfite and the decrease in pH. In contrast, in the absence of light the performance of the Fe(III)-sulfite system was greatly diminished after the consumption of sulfite. The formation of the Fe(III)-sulfito complex is a necessary step for initiating the photo-sulfite reaction. Inhibition of the oxidation of 2,4,6-TCP and methyl orange (MO) was observed in the presence of ligands that can stabilize one or more of the reactants: Fe(III), Fe(II), or sulfite. Our study provides a new facile route for the generation of SO4(•-) and simultaneous removal of organic and inorganic pollutants.

Stacking interactions between adenines in oxidized oligonucleotides

The effects of stacking interactions on the oxidation potentials of single strand oligonucleotides containing up to four consecutive adenines, alternated with thymines and cytosines in different sequences and ratios, have been determined by means of differential pulse voltammetry. Voltammetric measurements point toward the establishment in solution of structured oligonucleotide conformations, in which the nucleobases are well stacked altogether. Molecular dynamics simulations confirm that finding, indicating that single strands assume geometrical parameters characteristic of the B-DNA form. The analysis of the voltammetric signals in terms of a simple effective tight binding quantum model leads one to infer a robust set of parameters for treating hole transfer in one-electron-oxidized DNA containing adenines and thymines.

Generation of DNA-damaging reactive oxygen species via the autoxidation of hydrogen sulfide under physiologically relevant conditions: chemistry relevant to both the genotoxic and cell signaling properties of H(2)S

Hydrogen sulfide (H(2)S) has long been known for its toxic properties; however, in recent years, evidence has emerged that this small, gaseous molecule may serve as an endogenous cell-signaling agent. Though perhaps surprising in light of its potential role as an endogenous signaling agent, a number of studies have provided evidence that H(2)S is a DNA-damaging mutagen. In the work reported here, the chemical mechanisms of DNA damage by H(2)S were examined. Using a plasmid-based DNA strand cleavage assay, we found that micromolar concentrations of H(2)S generated single-strand DNA cleavage. Mechanistic studies indicate that this process involved autoxidation of H(2)S to generate superoxide, hydrogen peroxide, and, ultimately, the well-known DNA-damaging agent hydroxyl radical via a trace metal-mediated Fenton-type reaction. Strand cleavage by H(2)S proceeded in the presence of physiological thiol concentrations, and the known byproducts of H(2)S oxidation such as thiosulfate, sulfite, and sulfate do not contribute to the strand cleavage process. However, initially generated oxidation products such as persulfide (S(2)(2-)) likely undergo rapid autoxidation reactions that contribute to the generation of superoxide. The potential relevance of autoxidation processes to the genotoxic and cell signaling properties of H(2)S is discussed.

Comparison of Transition Metal-Mediated Oxidation Reactions of Guanine in Nucleoside and Single-Stranded Oligodeoxynucleotide Contexts

As the most readily oxidized of DNA's four natural bases, guanine is a prime target for attack by reactive oxygen species (ROS) and transition metal-mediated oxidants. The oxidation products of a modified guanosine nucleoside and of a single-stranded oligodeoxynucleotide, 5'-d(TTTTTTTGTTTTTTT)-3' have been studied using oxidants that include Co(II), Ni(II), and Ir(IV) compounds as well as photochemically generated oxidants such as sulphate radical, electron-transfer agents (riboflavin) and singlet oxygen. The oxidized lesions formed include spiroiminodihydantoin (Sp), guanidinohydantoin (Gh), imidazolone (Iz), oxazolone (Z) and 5-carboxamido-5-formamido-2-iminohydantion (2-Ih) nucleosides with a high degree of dependence on the exact oxidation system employed. Interestingly, a nickel(II) macrocyclic complex in conjunction with KHSO(5) leads to the recently reported 2-Ih heterocycle as the major product in both the nucleoside and oligonucleotide contexts.

Rotating ring-disk voltammetric investigations on the degradation rate of the nickel(III)-glycylglycyl-L-histidine complex

The voltammetric behavior of the Ni(II)/glycylglycyl-L-histidine complex and the degradation of the electrogenerated Ni(III) species were investigated by cyclic voltammetry. Results of electrochemical and spectrophotometric experiments indicated some differences in the nature and reactivity of Ni(III) complexes formed by two independent routes, i.e., anodic oxidation of Ni(II) and homogeneous oxidation by an autocatalytic reaction in a medium containing oxygen and S(IV) (H2SO3, HSO[Formula: see text], and SO[Formula: see text]). The Ni(III) species formed electrochemically reacts in a fast chemical process (EC mechanism) and the observed rate constant of the degradation (k = 0.17 s–1) was determined by a rotating ring-disk electrode technique measuring kinetic collection efficiency values as a function of the rotation rate. Key words: nickel, glycylglycyl-L-histidine, sulfite, degradation reaction, rotating ring-disk voltammetry.

DNA damage and 2′-deoxyguanosine oxidation induced by S(IV) autoxidation catalyzed by copper(II) tetraglycine complexes: Synergistic effect of a second metal ion

S(IV) (SO(2),HSO(3)(-)andSO(3)(2-)) autoxidation catalyzed by Cu(II)/tetraglycine complexes in the presence of DNA or 2'-deoxyguanosine (dGuo) resulted in DNA strand breaks and formation of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo), respectively. Ni(II), Co(II) or Mn(II) (1.0x10(-4)M) complexes had much smaller effects. Cu(II)/tetraglycine (1.0x10(-4)M) in the presence of Ni(II) or Mn(II) (10(-7)-10(-6)M) and S(IV) showed remarkable synergistic effect with these metal ions producing a higher yield of 8-oxodGuo. Oxidation of dGuo and DNA damage were attributed to oxysulfur radicals formed as intermediates in S(IV) autoxidation catalyzed by transition metal ions. SO*(3)(-) and HO* radicals were detected by EPR-spin trapping experiments with DMPO (5,5-dimethyl-1-pyrroline-N-oxide).

Kinetics and Mechanism of the Photoinitiated Autoxidation of Sulfur(IV) in the Presence of Iodide Ion

The kinetics and mechanism of the photoinitiated and iodide ion-catalyzed aqueous autoxidation of sulfur(IV) has been studied in a diode-array spectrophotometer using the same light beam for excitation and detection. Light absorption of both the iodide ion and sulfur(IV) contribute to the initiation of a highly efficient radical chain reaction, the overall rate of which depends on the reactant and catalyst concentrations, the pH, and the light intensity in a complex manner. To interpret all the experimental findings, an elaborate scheme is proposed, in which the chain carriers are SO3-*, SO4-*, SO5-*, I*, and I2-*. There are three termination steps, each of them is second-order with respect to the chain carriers. Model calculations and nonlinear fitting have been used to show that the proposed scheme gives an excellent quantitative interpretation of the experimental results.

Hydrogen-Atom Abstraction from the Adenine−Uracil Base Pair†

The hydrogen-abstracted radicals from the adenine-uracil (AU) base pair have been studied at the B3LYP/DZP++ level of theory. The A(N9)-U and A-U(N1) radicals, which correspond to hydrogen-atom abstraction at the adenine N9 and uracil N1 atoms, respectively, were predicted to be the two lowest-lying among the nine (AU-H) radicals studied in this study. The removal of the amino hydrogen of the adenine moiety that forms a hydrogen bond with the uracil O4 atom in the AU pair resulted in radical A(N6a)-U, which has the smallest base-pair dissociation energy, 5.9 kcal mol(-1). This radical is more likely to dissociate into the two isolated bases than to recover the hydrogen bond with the O4 atom through N6-H bond rotation along the C6-N6 bond. In general, the radicals generated by C-H bond breaking were higher in energy than those arising from N-H bond cleavage, because the unpaired electrons in the carbon-centered radicals were mainly localized on the carbon atom from which the hydrogen atom was removed. However, the highest-lying radical was found to arise from removal of the N3 hydrogen of uracil. The most remarkable structural feature of this radical is a very short C-H...O distance of 2.094 A, consistent with a substantial hydrogen bond. Although this radical lost the N1...H-N3 hydrogen bond between the two bases, its dissociation energy was predicted to be 12.9 kcal mol(-1), similar to that of the intact AU base pair. This is due to the transfer of electron density from the adenine N1 atom to the uracil N3 atom.

Two closely related nickel complexes have different effects on DNA damage and cell viability

Nickel is considered a weak carcinogen. It is known to interact with DNA and DNA-binding proteins. The ability of certain nickel compounds to cleave DNA has been exploited mainly for research purposes and less for developing new anticancer drugs. Here we compare the interactions of two closely related nickel complexes, [NiCR]2+ and [Ni(CR-2H)]2+, with DNA. CR stands for 2,12-dimethyl-3,7,11,17-tetraazabicyclo-[11.3.1]-heptadeca-1(17),2,11,13,15-pentaene. [NiCR]2+ has been used in the past as a structure-specific probe for RNA and DNA oligonucleotides in the presence of oxidizing agent but little is known about the biological effects of either complex. Our results show that [Ni(CR-2H)]2+ can damage DNA in vivo and in vitro in the absence of an added oxidizing agent and has an IC50 of 70 microM in human breast cancer cells whereas [NiCR]2+ and NiCl2 do not exhibit significant cytotoxicity. However, both [NiCR]2+ and [Ni(CR-2H)]2+ bind to the minor groove of double-stranded DNA.

Kinetics of the light-driven aqueous autoxidation of sulfur(iv) in the absence and presence of iron(ii)

The photochemical autoxidation of aqueous, acidic sulfur(IV) solutions was studied in the absence and presence of iron(II) by a newly introduced technique using a diode-array spectrophotometer, in which the same light source is used to drive and detect the reaction. Based on detailed kinetic and stoichiometric data sets, a non-chain mechanism is proposed for the autoxidation of sulfur(IV). In this mechanism, excited hydrated sulfur dioxide, *H2O.SO2, first reacts with O2 to form peroxomonosulfate ion, HSO5-, which rapidly oxidizes another H2O.SO2 to give hydrogensulfate ion as a final product. In the presence of iron(II), the formation of iron(III) was detected, which can be interpreted through the simultaneous contribution of two additional pathways: some of the HSO5- formed oxidizes iron(II) instead of sulfur(iv), and *H2O.SO2 also reacts directly with iron(II) to yield iron(III). This mechanism provides a sufficient quantitative interpretation of all experimental observations.

Highly Efficient Photoinitiation in the Cerium(III)-Catalyzed Aqueous Autoxidation of Sulfur(IV). An Example of Comprehensive Evaluation of Photoinduced Chain Reacions

The photoinitiated and cerium(III)-catalyzed aqueous reaction between sulfite ion and oxygen has been studied in a diode-array spectrophotometer using the same light beam for excitation and detection. Cerium(III) is identified as the photoactive absorbing species, and the production of cerium(IV) initiates a radical chain reaction. To interpret all the experimental findings, a simple scheme is proposed, in which the additional chain carriers are sulfite ion radical (SO3(-.)), sulfate ion radical (SO4(-.), and peroxomonosulfate ion radical (SO5(-.). The overall rate of oxidation is proportional to the square root of the light intensity per unit volume, which is readily interpreted by the second-order termination reaction of the proposed scheme. It is also shown that the reaction proceeds for an extended period of time in the dark following illumination, and a quantitative analysis is presented for this phase as well. The postulated model predicts that cerium(III) should have a cocatalytic or synergistic effect on the autoxidation of sulfite ion in the presence of other catalysts. This prediction was confirmed in the iron(III)-sulfite ion-oxygen system. The experimental method and the mathematical treatment used might be applicable to a wide range of photoinduced chain reactions.