Hydroperoxide Adduct Research Articles

Cover Feature: Versatile Activity of a Copper(II) Complex Bearing a N4‐Tetradentate Schiff Base Ligand with Reduced Oxygen Species (Eur. J. Inorg. Chem. 8/2022)

The Cover Feature shows the versatility of a copper complex formed with a N4-tetradentate Schiff-base ligand: it reacts with hydrogen peroxide and superoxide, and it catalyzes phenol oxidation. The complex scavenges superoxide by employing a Cu(II)/Cu(I) redox cycle for superoxide dismutase activity. In basic medium and at low temperature, it reacts with HO2− to form a stable end-on Cu(II)-hydroperoxide adduct, which either reacts with a second complex molecule to yield the redox products, through an intermediate peroxo-bridged dimer (catalase activity), or mediates phenol oxidation, through a ternary transition state (peroxidase activity), under suitable conditions. Kinetic, spectroscopic, and theoretical studies indicate that structural constraints imposed by the ligand distort the coordination geometry of the metal and control reactivity. More information can be found in the Research Article by S. R. Signorella and co-workers.

Kinetics of the benzyl + HO2 and benzoxyl + OH barrierless association reactions: fate of the benzyl hydroperoxide adduct under combustion and atmospheric conditions.

Radical-radical association reactions are challenging to address theoretically due to difficulties finding the bottleneck that variationally minimizes the reactive flux. For this purpose, the variable reaction coordinate (VRC) formulation of the variational transition state theory (VTST) represents an appropriate tool. In this work, we revisited the kinetics of two radical-radical association reactions of importance in combustion modelling and poly-aromatic hydrocarbon (PAH) chemistry by performing VRC calculations: benzyl + HO2 and benzoxyl + OH, both forming the adduct benzyl hydroperoxide. Our calculated rate constants are significantly lower than those previously reported based on VTST calculations, which results from a more efficient minimization of the reactive flux through the bottleneck achieved by the VRC formulation. Both reactions show different trends in the variation of their rate constants with temperature. We observed that if the pair of single occupied molecular orbitals (SOMOs) of the associating radicals show a similar nature, i.e. similar character, and thereby a small energy gap, a highly stabilized transition state structure is formed as the result of a very efficient SOMO-SOMO overlap, which may cancel out the free energy bottleneck for the formation of the adduct and result in large rate constants with a negative temperature dependence. This is the case of the benzoxyl and OH radical pair, whose SOMOs show O2p nature with an energy gap of 20.2 kcal mol-1. On the other hand, the benzyl and HO2 radical pair shows lower rate constants with a positive temperature dependence due to the larger difference between both SOMOs (a 28.9 kcal mol-1 energy gap) as a consequence of the contribution of the multiple resonance structures of the benzyl radical. The reverse dissociation rate constants were also calculated using multi-structural torsional anharmonicity partition functions, which were not included in previous work, and the results show a much slower dissociation of benzyl hydroperoxide. Our work may help to improve kinetic models of interest in combustion and PAH formation, as well as to gain further understanding of radical-radical association reactions, which are ubiquitous in different environments.

The mechanism of electrophilic addition of singlet oxygen to pyrrolic ring

Pyrrolic compounds assume an important role in the chemistry of living organisms, coal surrogates and novel drugs. However, literature reports a few studies on their reactivity towards prominent oxidising agents. This contribution presents a comprehensive mechanistic study of the oxidation of unsubstituted pyrrole with singlet oxygen (O2 1∆g) by deploying a quantum chemical framework leading to the production of succinimide, as the major products, through a Diels–Alder addition of O2 1∆g to the aromatic ring. Other products such as maleimide, hydroperoxide, formamide and epoxide adducts appear to form via insignificant channels. The primary Diels–Alder channel encompasses a barrier of 41 kJ/mol with a fitted rate constant of k(T) = 1.87 × 10−13 exp(− 48,000/RT) cm3 mol−1 s−1. Furthermore, a kinetic study has been undertaken to investigate the influence of substituents on reaction rate of the Diels–Alder addition of singlet oxygen to a pyrrolic ring. The results clarify that electropositive substituents such as BeH and BH2 operate as π-acceptors and thus deactivate the ring towards electrophilic attack of singlet oxygen. However, substituents comprising of strong π-donors, e.g., NH2 and OH, destabilise the ring structure, increasing its oxidation reactivity.

Experimental and computational studies of Criegee intermediate reactions with NH3 and CH3NH2.

Ammonia and amines are emitted into the troposphere by various natural and anthropogenic sources, where they have a significant role in aerosol formation. Here, we explore the significance of their removal by reaction with Criegee intermediates, which are produced in the troposphere by ozonolysis of alkenes. Rate coefficients for the reactions of two representative Criegee intermediates, formaldehyde oxide (CH2OO) and acetone oxide ((CH3)2COO) with NH3 and CH3NH2 were measured using cavity ring-down spectroscopy. Temperature-dependent rate coefficients, k(CH2OO + NH3) = (3.1 ± 0.5) × 10-20T2 exp(1011 ± 48/T) cm3 s-1 and k(CH2OO + CH3NH2) = (5 ± 2) × 10-19T2 exp(1384 ± 96/T) cm3 s-1 were obtained in the 240 to 320 K range. Both the reactions of CH2OO were found to be independent of pressure in the 10 to 100 Torr (N2) range, and average rate coefficients k(CH2OO + NH3) = (8.4 ± 1.2) × 10-14 cm3 s-1 and k(CH2OO + CH3NH2) = (5.6 ± 0.4) × 10-12 cm3 s-1 were deduced at 293 K. An upper limit of ≤2.7 × 10-15 cm3 s-1 was estimated for the rate coefficient of the (CH3)2COO + NH3 reaction. Complementary measurements were performed with mass spectrometry using synchrotron radiation photoionization giving k(CH2OO + CH3NH2) = (4.3 ± 0.5) × 10-12 cm3 s-1 at 298 K and 4 Torr (He). Photoionization mass spectra indicated production of NH2CH2OOH and CH3N(H)CH2OOH functionalized organic hydroperoxide adducts from CH2OO + NH3 and CH2OO + CH3NH2 reactions, respectively. Ab initio calculations performed at the CCSD(T)(F12*)/cc-pVQZ-F12//CCSD(T)(F12*)/cc-pVDZ-F12 level of theory predicted pre-reactive complex formation, consistent with previous studies. Master equation simulations of the experimental data using the ab initio computed structures identified submerged barrier heights of -2.1 ± 0.1 kJ mol-1 and -22.4 ± 0.2 kJ mol-1 for the CH2OO + NH3 and CH2OO + CH3NH2 reactions, respectively. The reactions of NH3 and CH3NH2 with CH2OO are not expected to compete with its removal by reaction with (H2O)2 in the troposphere. Similarly, losses of NH3 and CH3NH2 by reaction with Criegee intermediates will be insignificant compared with reactions with OH radicals.

The N-Hydroxyphthalimide catalyzed oxidation of cyclohexene to cyclohexenyl hydroperoxide: Reasons for deactivation and stability of the catalyst

Selective oxidation of cyclohexene to cyclohexenyl hydroperoxide catalyzed by N-hydroxyphthalimide (NHPI) was studied. Kinetic observations and 13C and 1H NMR spectroscopy showed that accumulation of cyclohexenyl hydroperoxide was accompanied by deactivation of the catalyst. A mechanism of the deactivation through phthalimido N-oxyl radical (PINO) addition to vinyl position of olefin was suggested. Responsible for the deactivation, PINO-C6H10-OOH hydroperoxide adducts were detected in the reaction conditions, and their molecular structure was elucidated by DFT technique. The mechanism explained an unexpected experimental effect, that is, lowering the oxygen partial pressure improved the stability of NHPI without decrease in production of cyclohexenyl hydroperoxide.

A theoretical investigation on the mechanism and kinetics of the gas-phase reaction of naphthalene with OH radical

The oxidation of naphthalene initiated by OH radical is studied by using density functional theory methods, M06-2X, ωB97XD and MPWB1K with 6-311G(d,p) basis set. The relative energy of the reactive species is also calculated at CCSD(T)/6-311G(d,p), CCSD(T)/cc-pVDZ, M06-2X/aug-cc-pVTZ and ωB97XD/aug-cc-pVTZ level of theories with the geometry optimized at M06-2X/6-311G(d,p) level of theory. The reactions were found to proceed via electrophilic addition of OH radical to the aromatic ring of naphthalene and H-atom abstraction from the aromatic ring of naphthalene by OH radical. The OH addition and H-atom abstraction reactions are occurring favorably at C1 position of naphthalene. The H-atom abstraction is dominant than OH addition reaction at high temperature (400–873 K). The reactions subsequent to the initial OH radical addition and H-atom abstraction reactions are studied in detail. The reaction of the initially formed intermediates with O2 results in the formation of peroxy radical and 1-naphthol. The isomerization of peroxy radical and reaction of peroxy radical with HO2, NO and NO2 leads to the formation of epoxynaphthalene, hydroperoxide adducts and nitro naphthalene. We have calculated the rate constant for favorable initial and secondary reactions. This study provides thermochemical and kinetic data for the atmospheric oxidation of naphthalene by OH radical and demonstrates the secondary reactions of naphthalene derivatives.

Chemiluminogenic acridinium salts: A comparison study. Detection of intermediate entities appearing upon light generation

The nine derivatives of acridine-9-carboxylic acid (CMADs) capable for chemiluminescence (CL), representing various classes of compounds were isolated in a chemically pure state (assessed by RP-HPLC) and identified using high resolution mass spectrometry (ESI-QTOF) and magnetic resonance (1H NMR) techniques. Among them are aryl acridinium esters, containing certainly selected and located substituents in both aromatic systems, an acridinium perfluorinated aliphatic ester and an acridinium sulfonamide. The main aim of the study was to compare, under the conditions of their optimal performance, the luminogenic properties of the above molecular systems, such as efficiency and kinetics of emission, limits of detection/quantification as well as stability in aqueous solutions. All of the investigated acridinium salts can serve as CL indicators or fragments of CL labels in modern medical diagnostics (e.g. chemiluminescence and electrochemiluminescence immunoassays (CLIA, ECLIA)), hybridization protection assays of nucleic acids (HPA) as well as other ultra-sensitive luminescence tests. Taking advantage of the flow injection mass analysis, the appearance of unstable cationic intermediates produced upon chemiluminogenic oxidation of selected acridinium salts in an alkaline environment was confirmed for the first time. The observed entities include hydroxyl and hydroperoxide adducts (as radical cations) as well as the acridan spirodioxetanone derivative, making a high energetic and direct precursor of the target emitters, an electronically excited 10-methylacridan-9-ones. The results are supported with quantum chemistry calculations at the DFT level of theory. Theoretical studies have helped to determine the unstable intermediates appearing during in CL in ESI-QTOF experiments. In the group of acridinium ester they allowed to observe the positive linear trend among between the kinetics of CL decay and calculated coefficients of pz LUMO orbitals on the carbon atoms being the reaction centers responsible for generation of light.

ChemInform Abstract: Osmium(III) and Osmium(V) Complexes Bearing a Macrocyclic Ligand: A Simple and Efficient Catalytic System for cis‐Dihydroxylation of Alkenes with Hydrogen Peroxide.

A simple protocol that uses [Os(III)(OH)(H2O)(L-N4Me2)](PF6)2 (1; L-N4Me2 = N,N'-dimethyl-2,11-diaza[3.3](2,6)pyridinophane) as a catalyst and H2O2 as a terminal oxidant for efficient cis-1,2-dihydroxylation of alkenes is presented. Unfunctionalized (or aliphatic) alkenes and alkenes/styrenes containing electron-withdrawing groups are selectively oxidized to the corresponding vicinal diols in good to excellent yields (46-99 %). In the catalytic reactions, the stoichiometry of alkene:H2O2 is 1:1, and thus the oxidant efficiency is very high. For the dihydroxylation of cyclohexene, the catalytic amount of 1 can be reduced to 0.01 mol % to achieve a very high turnover number of 5500. The active oxidant is identified as the Os(V)(O)(OH) species (2), which is formed via the hydroperoxide adduct, an Os(III)(OOH) species. The active oxidant 2 is successfully isolated and crystallographically characterized.

Mechanism and Kinetics of the Atmospheric Oxidative Degradation of Dimethylphenol Isomers Initiated by OH Radical

Dimethylphenols are highly reactive in the atmosphere, and their oxidation plays a vital role in the autoignition and combustion processes. The dominant oxidation process for dimethylphenols is by gas-phase reaction with OH radical. In the present study, the reaction of OH radical with dimethylphenol isomers is studied using density functional theory methods, B3LYP, M06-2X, and MPW1K, and also at the MP2 level of theory using 6-31G(d,p) and 6-31+G(d,p) basis sets. The activation energy values have also been calculated using the CCSD(T) method with 6-31G(d,p) and 6-311+G(d,p) basis sets using the geometries optimized at the M06-2X/6-31G(d,p) level of theory. The reactions subsequent to the principal oxidation steps are studied, and the different reaction pathways are modeled. The positions of the OH and CH3 substituents in the aromatic ring have a great influence on the reactivity of dimethylphenol toward OH radical. Accordingly, the reaction is initiated in four different ways: H-atom abstraction from the phenol group, H-atom abstraction from a methyl group, H-atom abstraction from the aromatic ring by OH radical, or electrophilic addition of OH radical to the aromatic ring. Aromatic peroxy radicals arising from initial H-atom abstraction and subsequent O2 addition lead to the formation of hydroperoxide adducts and alkoxy radicals. The O2 additions to dimethylphenol-OH adduct results in the formation of epoxide and bicyclic radicals. The rate constants for the most favorable reaction pathways are calculated using canonical variational transition state theory with small curvature tunneling corrections. This study provides thermochemical and kinetic data for the oxidation of dimethylphenol in the atmosphere and demonstrates the mechanism for the conversion of peroxy radical into aldehydes, hydroperoxides, epoxides, and bicyclic radicals, and their lifetimes in the atmosphere.

Osmium(III) and Osmium(V) Complexes Bearing a Macrocyclic Ligand: A Simple and Efficient Catalytic System for cis‐Dihydroxylation of Alkenes with Hydrogen Peroxide

A simple protocol that uses [Os(III)(OH)(H2O)(L-N4Me2)](PF6)2 (1; L-N4Me2 = N,N'-dimethyl-2,11-diaza[3.3](2,6)pyridinophane) as a catalyst and H2O2 as a terminal oxidant for efficient cis-1,2-dihydroxylation of alkenes is presented. Unfunctionalized (or aliphatic) alkenes and alkenes/styrenes containing electron-withdrawing groups are selectively oxidized to the corresponding vicinal diols in good to excellent yields (46-99 %). In the catalytic reactions, the stoichiometry of alkene:H2O2 is 1:1, and thus the oxidant efficiency is very high. For the dihydroxylation of cyclohexene, the catalytic amount of 1 can be reduced to 0.01 mol % to achieve a very high turnover number of 5500. The active oxidant is identified as the Os(V)(O)(OH) species (2), which is formed via the hydroperoxide adduct, an Os(III)(OOH) species. The active oxidant 2 is successfully isolated and crystallographically characterized.

Theoretical and experimental investigation of the oxidized and oxygenated forms of pyrocatechuic acid (2,3-dihydroxybenzoic acid)

The catecholic derivative 2,3-dihydroxybenzoic acid (2,3-DHBA or pyrocatechuic acid) represents a diversity of actions in enzymatic processes. In the present study DFT calculations (at the B3LYP/TZVP level of theory) have been performed for neutral 2,3DHBA and its dimer (models 1–1a), several semiquinone forms of 2,3-DHBA, namely the neutral (models 2–4), the monoionized (models 5–7), the di-ionized (models 8) and the dimer 7a. The more stable species in each case are those with the carboxyl group protonated. Oxygenated adducts were also constructed (models 10–15) in which the dioxygen is either H-bonded to the catecholic or carboxylic hydrogen or it is concerned to act on the intradiol or extradiol carbon atoms. The side-on placement of O2 on C(2) facilitates the intradiol C–C cleavage. Protonation of the oxygenated on C(1) adduct leads to decarboxylation of 2,3-DHBA. Isolation in the solid state and characterization by ESMS, IR, NMR, electronic spectra of the blue-green oxidized product of 2,3-DHBA (solid 1) supports the possibility of the existence of the semiquinone form or the hydroperoxide–adduct. Experimental spectroscopic data are correlated to the calculated spectroscopic parameters. In the ESMS the decarboxylation and degradation products as well a peroxo-adduct have been detected. Oxygenated species may also account for the plethora of redox signals in the cyclic voltammograms of solid 1 (in DMSO solutions).

Oxidation of the Benzyl Radical: Mechanism, Thermochemistry, and Kinetics for the Reactions of Benzyl Hydroperoxide.

Oxidation of the benzyl radical plays a key role in the autoignition, combustion, and atmospheric degradation of toluene and other alkylated aromatic hydrocarbons. Under relevant autoignition conditions of moderate temperature and high pressure, and in the atmosphere, benzyl reacts with O2 to form the benzylperoxy radical, and the further oxidation reactions of this radical are not yet fully characterized. In this contribution, we further develop the reaction chemistry, thermodynamics, and kinetics of benzyl radical oxidation, highlighting the important role of benzyl hydroperoxide and the benzoxyl (benzyloxyl) radical. The benzylperoxy + H reaction mechanism is studied using computational chemistry and statistical reaction rate theory. High-pressure limit rate constants in the barrierless benzylperoxy + H association are obtained from variational transition state theory calculations, with internal rotor contributions. The benzylperoxy + H reaction is seen to produce an activated benzyl hydroperoxide adduct that has 87 kcal mol(-1) excess energy over the ground state. We show that this activated adduct proceeds almost exclusively to the benzoxyl radical + OH across a wide range of temperature and pressure conditions. Minor reaction paths include benzyl + HO2, α-hydroxylbenzyl + OH, and benzaldehyde + H2O, each constituting around 1% of the total reaction rate at higher temperatures. Thermal decomposition of benzyl hydroperoxide, formed by hydrogen abstraction reactions in the benzylperoxy radical and at low temperatures in the benzylperoxy + H and benzyl + HO2 reactions, is also investigated. Decomposition to benzoxyl + OH is fast at temperatures of 900 K and above. The contribution of benzyl hydroperoxide chemistry to the ignition and oxidation of alkylated aromatics is discussed. Benzyl radical oxidation chemistry achieves the conversion of toluene to benzaldehyde, aiding autoignition via processes that either release large amounts of energy or form reactive free radicals through chain-branching.

Theoretical and Experimental Studies of Tyrosyl Hydroperoxide Formation in the Presence of H-Bond Donors

Oxidative damage to biomolecules such as lipids, proteins, nucleotides, and sugars has been implicated in the pathogenesis of various diseases. Superoxide radical anion (O 2 (*-)) addition to nitrones bearing an amide N-H has been shown to be more favored as compared to other nitrones [ Villamena, F. A. , ( 2007) J. Am. Chem. Soc. 129, 8177- 8191 ]. It has also been demonstrated by others [ Winterbourn, C. C. , ( 2004) Biochem. J. 381, 241- 248 ] that O 2 (*-) addition to tyrosine to form hydroperoxide is favored in the presence of basic amino groups, but the mechanism for this observation remains obscure. We, therefore, hypothesized that the alpha-effect resulting from the interaction of O 2 (*-) with N-H can play a crucial role in the enhancement of hydroperoxide formation. Understanding this phenomenon is important in the elucidation of mechanisms leading to oxidative stress in cellular systems. Computational (at the PCM/B3LYP/6-31+G**//B3LYP/6-31G level of theory) as well as experimental studies were carried out to shed insights into the effect of amide or amino N-H on the enhancement (or stabilization) of hydroperoxide formation in tyrosine. H-bond interaction of amino acid group with O 2 (*-) results in the perturbation of the spin and charge densities of O 2 (*-). A similar phenomenon has been predicted for non-amino acids bearing H-bond donor groups. Using the FOX assay, tyrosyl hydroperoxide formation was enhanced in the presence of H-bond donors from amino acids and non-amino acids. The role of H-bonding in either stabilizing the hydroperoxide adduct or facilitating O 2 (*-) addition via an alpha-effect was further theoretically investigated, and results show that the latter mechanism is more thermodynamically preferred. This study provides new mechanistic insights in the initiation of oxidative modification to tyrosyl radical.

Thermochemical Similarities Among Three Reaction Systems: Vinyl + O2 – Phenyl + O2 – Dibenzofuranyl + O2

It is interesting to study and compare the thermochemical parameters in the vinyl + O2, phenyl + O2 and dibenzofuranyl + O2 chemical activation reaction systems as well as the corresponding peroxy radical and hydroperoxide adduct dissociations. The analogy in C–O and C–H bond energies, well depths, bond type environment and reaction paths in these three reaction systems suggest a corresponding similarity in the initial reaction kinetics of these unsaturated R• + O2 reactions. In this study we identify the similarities in thermochemistry, bond energies and reaction kinetics of paths. The similar structures and parameters suggest that calculations on the vinyl system can often serve as surrogate or model for reaction paths and kinetic barriers in the larger phenyl system and the large dibenzofuranyl system for which high-level calculations are difficult. Here the vinyl + O2 and the phenyl + O2 systems can be used as model for the dibenzofuranyl + O2 system.

Olefin Epoxidation by Alkyl Hydroperoxide with a Novel Cross-Bridged Cyclam Manganese Complex: Demonstration of Oxygenation by Two Distinct Reactive Intermediates

Olefin epoxidation provides an operative protocol to investigate the oxygen transfer process in nature. A novel manganese complex with a cross-bridged cyclam ligand, MnIV(Me2EBC)(OH)2(2+) (Me2EBC = 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane), was used to study the epoxidation mechanism with biologically important oxidants, alkyl hydroperoxides. Results from direct reaction of the freshly synthesized manganese(IV) complex, [Mn(Me2EBC)(OH)2](PF6)2, with various olefins in neutral or basic solution, and from catalytic epoxidation with oxygen-labeled solvent, H2 18O, eliminate the manganese oxo moiety, Mn(IV)=O, as the reactive intermediate and obviate an oxygen rebound mechanism. Epoxidations of norbornylene under different conditions indicate multiple mechanisms for epoxidation, and cis-stilbene epoxidation under atmospheric 18O2 reveals a product distribution indicating at least two distinctive intermediates serving as the reactive species for epoxidation. In addition to alkyl peroxide radicals as dominant intermediates, an alkyl hydroperoxide adduct of high oxidation state manganese(IV) is suggested as the third kind of active intermediate responsible for epoxidation. This third intermediate functions by the Lewis acid pathway, a process best known for hydrogen peroxide adducts. Furthermore, the tert-butyl peroxide adduct of this manganese(IV) complex was detected by mass spectroscopy under catalytic oxidation conditions.

Olefin Oxygenation by the Hydroperoxide Adduct of a Nonheme Manganese(IV) Complex: Epoxidations by a Metallo−Peracid Produces Gentle Selective Oxidations

The reactive intermediates and mechanisms of oxygenation of olefins by manganese complexes were investigated by treating olefins with newly synthesized [MnIV(Me2EBC)(OH)2](PF6)2 in the presence and absence of peroxide and by studying its catalytic epoxidation reaction in normal aqueous solution and, individually, with isotopically labeled H218O, 18O2, and H218O2. The manganese oxo species is not the reactive intermediate for the oxygen transfer process mediated by this manganese complex. A novel manganese(IV) peroxide intermediate, MnIV(Me2EBC)(O)(OOH)+, was captured by mass spectrometry and is proposed as the intermediate that oxygenates olefins in this catalytic system.

Studies on nitroaromatic compound degradation in modified Fenton reactions by electrospray ionization tandem mass spectrometry (ESI-MS-MS).

Degradation products resulting from modified Fenton reactions with the nitroaromatic compounds trinitrotoluene (TNT) and trinitrobenzene (TNB) were identified by electrospray ionization tandem mass spectrometry (ESI-MS-MS). Several hydroperoxide adducts were tentatively identified as initial, one-electron reduction products of TNT and tandem mass spectrometry confirmed their structure. A transformation pathway of TNT, resulting from reactions with oxygen radical species generated by the modified Fenton reaction, was proposed.

Release of free nucleobases from oligomers by copper(II)–peroxide adduct

The release of nucleobases from oligonucleotides (12-mer, d(5′-CGCTTTAAAGCG) 2, 8-mer, d(5′-AAACGTTT) 2, 7-mer, d(TCGCGCG) 2, and 6-mer, d(CGCGCG) 2) by copper(II) complex and hydrogen peroxide systems was investigated in terms of HPLC. The [Cu(bdpg)Cl]Cl complex gave a large quantity of free nucleobases in the reaction mixture, but the activities of other compounds, such as [Cu(tpa)Cl]ClO 4, were much lower than that of the (bdpg)-complex; where (bdpg) and (tpa) represent a N, N-bis(2-pyridylmethyl) derivative of β-alanineamide and 2-aminomethylpyridine, respectively. The above results and the electrospray mass spectral data on the solutions strongly suggest that the remarkable activity of the (bdpg)-compound is due to the high electrophilic nature of the copper(II)–hydroperoxide adduct, whose formation is greatly promoted through hydrogen bonding with the amide-oxygen atom coordinated to the copper(II) atom, and this electrophilic copper(II)–peroxide adduct attacks the sugar ring of nucleoside, to give the corresponding hydroxylated species, and from which free nucleobases are released.

Important role of proton in activation of oxygen molecules in heme-containing oxygenases

DFT (density functional theory) calculations have revealed that the position of the proton of the hydroperoxide adduct of an Fe(III)–porphyrin compound influences greatly the electron density of the oxygen atoms of the peroxide ion, suggesting that the position of the proton in the hydroperoxo–iron(III) compound plays an important role in controlling the electronic interaction between the hydroperoxide adduct and the porphyrin system (OOH–π interaction).

Mechanistic and Spatial Study of Ultrasonically Induced Luminol Chemiluminescence

Aqueous solutions containing 10-3 M luminol and varying concentrations of hydrogen peroxide are irradiated with 20 kHz ultrasound at 50 °C. The intensity of sonogenerated chemiluminescence (SCL) is shown to increase linearly with ultrasound power and to be strongly pH dependent, reaching a maximum at pH 12. For pH 10 ISCL increases monotonically with H2O2 concentration up to10-4 M but decreases as the concentration is increased further. A mechanism is proposed in which HO2- and the luminol monoanion competitively reduce sonochemically generated HO•, producing O2•- and luminol radical anion, respectively. Luminescence follows the decomposition of a hydroperoxide adduct formed by reaction between O2•- and luminol radical anion. EDTA is shown to suppress the background (silent) chemiluminescence of solutions containing luminol and H2O2 without significantly affecting ISCL. Digital images of SCL emission occurring near the transducer−solu...