Theoretical study of the spin trapping of hydroxyl radical by cyclic nitrones: a density functional theory approach

Electron Paramagnetic Resonance Tracks Condition-Sensitive Water Radical Cation

Oxidizing species or radicals generated in water are of vital importance in catalysis, the environment, and biology. In addition to several related reactive oxygen species, using electron paramagnetic resonance (EPR), we present a nontrapping chemical transformation pathway to track water radical cation (H2O+•) species, whose formation is very sensitive to the conditioning environments, such as light irradiation, mechanical action, and gas/chemical introduction. We reveal that H2O+• can oxidize the 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to the crucial epoxy hydroxylamine (HDMP=O) intermediate, which further reacts with the hydroxyl radical (•OH) for the formation of the EPR-active sextet radical (DMPO=O•). Interestingly, we uncover that H2O+• can react with dimethyl methylphosphonate (DMMP), 2-methyl-2-nitrosopropane (MNP), 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide (BMPO), and α-phenyl-N-tert-butylnitrone (PBN) which contain a double-bond structure to produce corresponding derivatives as well. It is thus expected that both H2O+• and •OH are ubiquitous in nature and in various water-containing experimental systems. These findings provide a novel perspective on radicals for water redox chemistry.

The effect of DMPO on the formation of hydroxyl radicals on the rutile TiO2(110) surface

The detected OH˙ signals by EPR on the rutile TiO₂(110) surface are proposed to primarily originate from water upon illumination rather than the free OH˙ radical.

Amidinoquinoxaline N-oxides: spin trapping of O- and C-centered radicals

Amidinoquinoxaline N-oxides represent a novel family of heterocyclic spin traps. In this work, their ability to trap O- and C-centered radicals was tested using selected derivatives with different structural modifications. All the studied nitrones were able to trap radicals forming persistent spin adducts, also in the case of OH and OOH radicals which are of wide biological interest as examples of ROS. The stability of the adducts was mainly attributed to the wide delocalization of the unpaired electron over the whole quinoxaline moiety. The nitroxide spectral parameters (hfccs and g-factors) were analyzed and the results were supported by DFT calculations. The N-19 hfccs and g-factors were characteristic of each aminoxyl and could aid in the identification of the trapped radical. The enhanced stability of the OH adducts under the employed reaction conditions could be ascribed to their possible stabilization by IHBs with two different acceptors: the N-O˙ moiety or the amidine functionality. DFT calculations indicate that the preferred IHB is strongly conditioned by the amidine ring size. While five membered homologues show a clear preference for the IHB with the N-O˙ group, in six membered derivatives this stabilizing interaction is preferentially established with the amidine nitrogen as an IHB acceptor.

Mechanistic Study of the Validity of Using Hydroxyl Radical Probes To Characterize Electrochemical Advanced Oxidation Processes

The detection of hydroxyl radicals (OH^•) is typically accomplished by using reactive probe molecules, but prior studies have not thoroughly investigated the suitability of these probes for use in electrochemical advanced oxidation processes (EAOPs), due to the neglect of alternative reaction mechanisms. In this study, we investigated the suitability of four OH^• probes (coumarin, p-chlorobenzoic acid, terephthalic acid, and p-benzoquinone) for use in EAOPs. Experimental results indicated that both coumarin and p-chlorobenzoic acid are oxidized via direct electron transfer reactions, while p-benzoquinone and terephthalic acid are not. Coumarin oxidation to form the OH^• adduct product 7-hydroxycoumarin was found at anodic potentials lower than that necessary for OH^• formation. Density functional theory (DFT) simulations found a thermodynamically favorable and non-OH^• mediated pathway for 7-hydroxycoumarin formation, which is activationless at anodic potentials > 2.10 V/SHE. DFT simulations also provided estimates of E° values for a series of OH^• probe compounds, which agreed with voltammetry results. Results from this study indicated that terephthalic acid is the most appropriate OH^• probe compound for the characterization of electrochemical and catalytic systems.

On the vasoprotective mechanisms underlying novel β-phosphorylated nitrones: Focus on free radical characterization, scavenging and NO-donation in a biological model of oxidative stress

A series of new hybrid 2-(diethoxyphosphoryl)-N-(benzylidene)propan-2-amine oxide derivatives with different aromatic substitution (PPNs) were synthesized. These molecules were evaluated for their EPR spin trapping potential on eleven different radicals and NO-donation properties in vitro, cytotoxicity and vasoprotective effect on precontracted rat aortic rings. A subfamily of the new PPNs featured an antioxidant moiety occurring in natural phenolic acids. From the experimental screening of these hydroxyphenyl- and methoxyphenyl-substituted PPNs, biocompatible nitrones 4d, and 4g-4i deriving from caffeic, gallic, ferulic and sinapic acids, which combined improved EPR probing of ROS formation, vasorelaxant action and antioxidant potency, might be potential drug candidate alternatives to PBN and its analogues.

Rate constants of hydroxyl radical oxidation of polychlorinated biphenyls in the gas phase: A single-descriptor based QSAR and DFT study

The second-order rate constants (k) of hydroxyl radical (·OH) with polychlorinated biphenyls (PCBs) in the gas phase are of scientific and regulatory importance for assessing their global distribution and fate in the atmosphere. Due to the limited number of measured k values, there is a need to model the k values for unknown PCBs congeners. In the present study, we developed a quantitative structure-activity relationship (QSAR) model with quantum chemical descriptors using a sequential approach, including correlation analysis, principal component analysis, multi-linear regression, validation, and estimation of applicability domain. The result indicates that the single descriptor, polarizability (α), plays an important role in determining the reactivity with a global standardized function of lnk = -0.054 × α ‒ 19.49 at 298 K. In order to validate the QSAR predicted k values and expand the current k value database for PCBs congeners, an independent method, density functional theory (DFT), was employed to calculate the kinetics and thermodynamics of the gas-phase ·OH oxidation of 2,4',5-trichlorobiphenyl (PCB31), 2,2',4,4'-tetrachlorobiphenyl (PCB47), 2,3,4,5,6-pentachlorobiphenyl (PCB116), 3,3',4,4',5,5'-hexachlorobiphenyl (PCB169), and 2,3,3',4,5,5',6-heptachlorobiphenyl (PCB192) at 298 K at B3LYP/6-311++G**//B3LYP/6-31 + G** level of theory. The QSAR predicted and DFT calculated k values for ·OH oxidation of these PCB congeners exhibit excellent agreement with the experimental k values, indicating the robustness and predictive power of the single-descriptor based QSAR model we developed.

Theoretical studies on the spin trapping of the 2-chloro-5-hydroxy-1,4-benzoquinone radical by 5,5-dimethyl-1-pyrroline N-oxide (DMPO): the identification of the C–O bonding spin adduct

The C–O bonding spin adduct has been identified in the spin trapping of 2-chloro-5-hydroxy-1,4-benzoquinone radical by DMPO.

Amidinoquinoxaline N-oxides as novel spin traps

A series of nitrones were synthesized and tested as novel spin traps. The adducts generated by CH₃ addition showed remarkably persistent signals. Their EPR features and kinetics were rationalised by DFT and MP2 calculations.

Potential implication of the chemical properties and bioactivity of nitrone spin traps for therapeutics

Nitrone therapeutics has been employed in the treatment of oxidative stress-related diseases such as neurodegeneration, cardiovascular disease and cancer. The nitrone-based compound NXY-059, which is the first drug to reach clinical trials for the treatment of acute ischemic stroke, has provided promise for the development of more robust pharmacological agents. However, the specific mechanism of nitrone bioactivity remains unclear. In this review, we present a variety of nitrone chemistry and biological activity that could be implicated for the nitrone's pharmacological activity. The chemistries of spin trapping and spin adduct reveal insights on the possible roles of nitrones for altering cellular redox status through radical scavenging or nitric oxide donation, and their biological effects are presented. An interdisciplinary approach towards the development of novel synthetic antioxidants with improved pharmacological properties encompassing theoretical, synthetic, biochemical and in vitro/in vivo studies is covered.

Spin trapping of hydroperoxyl radical by a cyclic nitrone conjugated to β-cyclodextrin: a computational study

Spin trapping of hydroperoxyl radical (HOO^.) by the amide-linked conjugate of 5-carbamoyl-5-methyl-1-pyrroline N-oxide (AMPO) to β-cyclodextrin (β-CD) was studied computationally using a two-layered ONIOM method. From a conformational perspective, the "internal" conformation of 5R-β-CD-AMPO is more favored than the "external" conformation in which the nitrone is located outside of the cavity of the β-CD. When the HOO^. addition product is formed, the most stable isomer has the nitroxyl (N₁-O₁) moiety pointing inside the cavity of the β-CD. Thus, this "internal" conformation might protect the N₁-O₁ moiety of the resulting spin adduct from access by reducing agents, thereby improving the lifetime of the radical adduct. The computed energetic barrier for HOO^. addition to the 5R-β-CD-AMPO is 8.7 kcal/mol, which is marginally smaller than spin trapping by the non-conjugated AMPO (that is, without the β-CD). To optimize the reactivity of the β-CD-AMPO conjugate, the effect of a spacer unit between the AMPO segment and the β-CD moiety with varying methylene units, (CH₂) _n (n = 1, 2, 3), on the energetics of HOO^. addition was evaluated. The structure with only one methylene spacer (n = 1) appears to be optimal as determined by the smaller activation barrier (6.2 kcal/mol) for HOO^. addition to the nitrone moiety. Compared with very time-consuming quantum mechanical methods, the ONIOM method appears to offer significant advantages for evaluation of the best β-CD-AMPO conjugate for trapping of such reactive oxygen species and providing for the rational design of novel nitrones as spin traps.

Synthesis, structure, theoretical and experimental in vitro antioxidant/pharmacological properties of α-aryl, N-alkyl nitrones, as potential agents for the treatment of cerebral ischemia

The synthesis, structure, theoretical and experimental in vitro antioxidant properties using the DPPH, ORAC, and benzoic acid, as well as preliminary in vitro pharmacological activities of (Z)-α-aryl and heteroaryl N-alkyl-nitrones 6-15, 18, 19, 21, and 23, is reported. In the in vitro antioxidant activity, for the DPPH radical test, only nitrones bearing free phenol groups gave the best RSA (%) values, nitrones 13 and 14 showing the highest values in this assay. In the ORAC analysis, the most potent radical scavenger was nitrone indole 21, followed by the N-benzyl benzene-type nitrones 10 and 15. Interestingly enough, the archetypal nitrone 7 (PBN) gave a low RSA value (1.4%) in the DPPH test, or was inactive in the ORAC assay. Concerning the ability to scavenge the hydroxyl radical, all the nitrones studied proved active in this experiment, showing high values in the 94-97% range, the most potent being nitrone 14. The theoretical calculations for the prediction of the antioxidant power, and the potential of ionization confirm that nitrones 9 and 10 are among the best compounds in electron transfer processes, a result that is also in good agreement with the experimental values in the DPPH assay. The calculated energy values for the reaction of ROS (hydroxyl, peroxyl) with the nitrones predict that the most favourable adduct-spin will take place between nitrones 9, 10, and 21, a fact that would be in agreement with their experimentally observed scavenger ability. The in vitro pharmacological analysis showed that the neuroprotective profile of the target molecules was in general low, with values ranging from 0% to 18.7%, in human neuroblastoma cells stressed with a mixture of rotenone/oligomycin-A, being nitrones 18, and 6-8 the most potent, as they show values in the range 24-18.4%.

Effect of 2, 5-substituents on the stability of cyclic nitrone superoxide spin adducts: A density functional theory approach

In the present study, five cyclic nitrone superoxide spin adducts, i.e. DMPO-OOH, M(3)PO-OOH, EMPO-OOH, DEPMPO-OOH and DEPDMPO-OOH, were chosen as model compounds to investigate the effect of 2,5-subsitituents on their stability, through structural analysis and decay thermodynamics using density functional theory (DFT) calculations. Analysis of the optimized geometries reveals that none of the previously proposed stabilizing factors, including intramolecular H-bonds, intramolecular non-bonding interactions, bulky steric protection nor the C(2)-N(1) bond distance can be used to clearly explain the effect of 2,5-substituents on the stability of the spin adducts. Subsequent study found that spin densities on the nitroxyl nitrogen and oxygen are well correlated with the half-lives of the spin adducts and consequently are the proper parameters to characterize the effect of 2,5-substituents on their stability. Examination of the decomposition thermodynamics further supports the effect of the substituents on the persistence of cyclic nitrone superoxide spin adducts.

Reactions of superoxide with the myoglobin tyrosyl radical

The contribution of superoxide-mediated injury to oxidative stress is not fully understood. A potential mechanism is the reaction of superoxide with tyrosyl radicals, which either results in repair of the tyrosine or formation of tyrosine hydroperoxide by addition. Whether these reactions occur with protein tyrosyl radicals is of interest because they could alter protein structure or modulate enzyme activity. Here, we have used a xanthine oxidase/acetaldehyde system to generate tyrosyl radicals on sperm whale myoglobin in the presence of superoxide. Using mass spectrometry we found that superoxide prevented myoglobin dimer formation by repairing the protein tyrosyl radical. An addition product of superoxide at Tyr151 was also identified, and exogenous lysine promoted the formation of this product. In our system, reaction of tyrosyl radicals with superoxide was favored over dimer formation with the ratio of repair to addition being approximately 10:1. Our results demonstrate that reaction of superoxide with protein tyrosyl radicals occurs and may play a role in free radical-mediated protein injury.

A Kinetic Study on the Nitric Oxide Trapping by the Fe(III)-Dithiocarbamate-Nitroxyl Complex: Influence of the Ligand Structure and External Pressure on the Trapping Rate

The Fe(III)-dithiocarbamate-nitroxyl (Fe-dtc-nitroxyl) complex, where nitroxyl represents nitroxyl free radical (R-N(O)-R’), has been shown to trap nitric oxide (NO) in aqueous solution, replacing the nitroxyl ligand. As a result, non-complexed nitroxyl radical was released to the solution. Fe(III)-dithiocarbamate (Fe-dtc) complexes without the nitroxyl ligand also traps NO, producing NO-Fe-dtc complex. These two reactions have been used for the purpose of NO detection. We investigated how the ligand structure and external pressure influence NO trapping rates in these complexes. The ratios of NO trapping rates (k 1/k 2) between various Fe-dtc-nitroxyl complexes and the Fe-dtc complex were determined by using a competitive NO trapping method. The ratio k 1/k 2 was 29.8±2.8 for Fe-dtc-nitroxyl and Fe-dtc, where dtc = N-(dithiocarboxy) sarcosine (DTCS) and nitroxyl = 2,2,6,6-tetramethyl- piperidine-1-oxyl (TEMPO). For another combination, dtc = N-methyl-D-glucamine dithiocarbamate (MGD)/nitroxyl = TEMPO, k 1/k 2 was 7.19±0.25. Overall, NO trapping rate of the Fe-dtc-nitroxyl complex was faster than that of the Fe-dtc complex, and k 1/k 2 for Fe-MGD complex was dependent on the electron-withdrawing or -repelling nature of the functional group in the ligand. Based on pressure dependence experiments for the competitive reaction, we obtained large negative activation volumes for NO trapping of the Fe-dtc-nitroxyl complex as well as the difference in activation volumes (–45 to –26 cm3 mol–1) between the NO trapping reactions by the two Fe complexes. These data sets with different nature allowed us to speculate the reaction mechanism for NO trapping of the Fe-dtc-nitroxyl complex.

Lipophilic beta-cyclodextrin cyclic-nitrone conjugate: synthesis and spin trapping studies

Nitrone spin traps are commonly employed as probes for the identification of transient radicals in chemical and biological systems using electron paramagnetic resonance (EPR) spectroscopy. Nitrones have also found applications as therapeutic agent in the treatment of radical-mediated diseases. Therefore, a spin trap that incorporates high reactivity to superoxide radical anion (O2(*-)), more persistent superoxide adduct, enhanced bioavailability, and selective targeting in one molecular design is desirable. In this work, the synthesis of a nitrone spin trap, 4, that is tethered via amide bonds to a beta-cyclodextrin (beta-CD) and a dodecyl chain was achieved with the expectation that the beta-cyclodextrin would lead to increased reactivity to O2(*-) and persistent O2(*-) adduct while the lipophilic chain would impart membrane targeting property. The two constitutional racemic isomers, 4a and 4b, were separated using preparative HPLC, and structural analysis and self-aggregation properties were carried out using NMR, induced circular dichroism, dynamic light scattering, transmission electron microscopy, and computational approach. EPR spin trapping of O2(*-) by 4a and 4b was only successful in DMSO and not in an aqueous system, due most likely to the amphiphilic character of 4 that can favor conformations (or aggregation) hindering radical addition to nitrone. Kinetics of formation and decay of the 4a-O2H adduct in polar aprotic solvents show faster reactivity to O2(*-) and more persistent O2(*-) adduct compared to nitrones not conjugated to beta-CD. Computational analysis of 4a and 4b as well as 4a-OOH and 4b-OOH adducts were carried out, and results show that isomerism, both constitutional and stereochemical, affects the orientations of aminoxyl-NO and/or hydroperoxyl groups relative to the beta-CD annulus for optimal H-bond interaction and stability.

Superoxide radical anion adduct of 5,5-dimethyl-1-pyrroline N-oxide. 4. Conformational effects on the EPR hyperfine splitting constants

Spin trapping has been commonly employed in the detection of superoxide radical anion in chemical and biological systems; hence, accurate interpretation of the hyperfine splitting constants (hfsc's) arising from the O(2)(*-) adducts (also referred to as hydroperoxyl (HO(2)(*)) radical adducts) of various nitrones is important. In this work, the nature of the relevant hfsc's was investigated by examining the effect of conformational changes in the hydroperoxyl moiety of the O(2)(*-) adducts of 5,5-dimethyl-1-pyrroline N-oxide (DMPO), 5-ethoxycarbonyl-5-methyl-1-pyrroline N-oxide (EMPO), 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO), 5-carbamoyl-5-methyl-1-pyrroline N-oxide (AMPO), and 7-oxa-1-azaspiro[4.4]non-1-en-6-one N-oxide, (CPCOMPO) on the magnitude of a(N), a(beta-H), and a(gamma-H). Conformational change around the substituents and their effect on the hfsc's were also explored. Results indicate that a(beta-H) is most sensitive to conformational changes of the hydroperoxyl and substituent groups relative to hfsc's of other nuclei. The orbital overlap between the C-H sigma-orbital and the SOMO of the nitroxyl nitrogen plays a crucial factor in determining the magnitude of the a(beta-H). The hfsc values for the O(2)(*-) adducts were predicted with high accuracy by using a low-cost computational method at the PCM(water)/BHandHLYP/EPR-III//B3LYP/6-31G* level of theory without taking into account the explicit water interaction.

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.

Improved spin trapping properties by beta-cyclodextrin-cyclic nitrone conjugate

Spin trapping using a nitrone and electron paramagnetic resonance (EPR) spectroscopy is commonly employed in the identification of transient radicals in chemical and biological systems. There has also been a growing interest in the pharmacological activity of nitrones, and there is, therefore, a pressing need to develop nitrones with improved spin trapping properties and controlled delivery in cellular systems. The beta-cyclodextrin (beta-CD)-cyclic nitrone conjugate, 5-N-beta-cyclodextrin-carboxamide-5-methyl-1-pyrroline N-oxide (CDNMPO) was synthesized and characterized. 1-D and 2-D NMR show two stereoisomeric forms (i.e., 5S- and 5R-) for CDNMPO. Spin trapping using CDNMPO shows distinctive EPR spectra for superoxide radical anion (O2(*-)) compared to other biologically relevant free radicals. Kinetic analysis of O2(*-) adduct formation and decay using singular value decomposition and pseudoinverse deconvolution methods gave an average bimolecular rate constant of k = 58 +/- 1 M(-1) s(-1) and a maximum half-life of t(1/2) = 27.5 min at pH 7.0. Molecular modeling was used to rationalize the long-range coupling between the nitrone and the beta-CD, as well as the stability of the O2(*-) adducts. This study demonstrates how a computational approach can aid in the design of spin traps with a relatively high rate of reactivity to O2(*-), and how beta-CD can improve adduct stability via intramolecular interaction with the O2(*-) adduct.

Investigation of the generation of hydroxyl radicals and their oxidative role in the presence of heterogeneous copper catalysts

The presence and impact of hydroxyl radicals generated via the catalytic decomposition of H(2)O(2) over heterogeneous copper catalysts were investigated by using two detection methods, an electron spin resonance-spin trapping method and a chemical probe method. Detection of the (5,5-dimethyl-1-pyrroline-N-oxide)-OH adduct signal and formation of 4-chlorocatechol during the oxidation of a 4-chlorophenol substrate demonstrated that the three heterogeneous copper catalysts employed here (CuO, Cu/Al(2)O(3) and CuO.ZnO/Al(2)O(3)) were capable of generating hydroxyl radicals in combination with H(2)O(2). The oxidative mechanism of the hydroxyl radical in the presence of heterogeneous copper catalysts is discussed with regard to the further oxidation of the (5,5-dimethyl-1-pyrroline-N-oxide)-OH adduct and hydroxylated products of 4-chlorophenol oxidation. Interestingly, integration of the 5,5-dimethyl-1-pyrroline-N-oxide-OH adduct signal could not be used to reliably measure the total amount of hydroxyl radicals generated as a result of oxidative attack on the adduct. This may be as a result of locally higher hydroxyl radical concentrations in the presence of a heterogeneous catalyst leading to further unwanted oxidation of the (5,5-dimethyl-1-pyrroline-N-oxide)-OH.

Reactivity of superoxide radical anion with cyclic nitrones: role of intramolecular H-bond and electrostatic effects

Limitations exist among the commonly used cyclic nitrone spin traps for biological free radical detection using electron paramagnetic resonance (EPR) spectroscopy. The design of new spin traps for biological free radical detection and identification using EPR spectroscopy has been a major challenge due to the lack of systematic and rational approaches to their design. In this work, density functional theory (DFT) calculations and stopped-flow kinetics were employed to predict the reactivity of functionalized spin traps with superoxide radical anion (O2*-). Functional groups provide versatility and can potentially improve spin-trap reactivity, adduct stability, and target specificity. The effect of functional group substitution at the C-5 position of pyrroline N-oxides on spin-trap reactivity toward O2*- was computationally rationalized at the PCM/B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) and PCM/mPW1K/6-31+G(d,p) levels of theory. Calculated free energies and rate constants for the reactivity of O2*- with model nitrones were found to correlate with the experimentally obtained rate constants using stopped-flow and EPR spectroscopic methods. New insights into the nucleophilic nature of O2*- addition to nitrones as well as the role of intramolecular hydrogen bonding of O2*- in facilitating this reaction are discussed. This study shows that using an N-monoalkylsubstituted amide or an ester as attached groups on the nitrone can be ideal in molecular tethering for improved spin-trapping properties and could pave the way for improved in vivo radical detection at the site of superoxide formation.

Comparative DFT study of the spin trapping of methyl, mercapto, hydroperoxy, superoxide, and nitric oxide radicals by various substituted cyclic nitrones

The thermodynamics of the spin trapping of various cyclic nitrones with biologically relevant radicals such as methyl, mercapto, hydroperoxy, superoxide anion, and nitric oxide was investigated using computational methods. A density functional theory (DFT) approach was employed in this study at the B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) level. The order of increasing favorability for Delta G(rxn) (kcal/mol) of the radical reaction with various nitrones, in general, follows a trend similar to their respective experimental reduction potentials as well as their experimental second-order rate constants in aqueous solution: NO (14.57) < O2*- (-7.51) < *O2H (-13.92) < *SH (-16.55) < *CH3 (-32.17) < *OH (-43.66). The same qualitative trend is predicted upon considering the effect of solvation using the polarizable continuum model (PCM): i.e., NO (14.12) < O2*- (9.95) < *O2H (-6.95) < *SH (-13.57) < *CH3 (-32.88) < *OH (-38.91). All radical reactions with these nitrones are exoergic, except for NO (and O2*- in the aqueous phase), which is endoergic, and the free energy of activation (Delta G) for the NO additions ranges from 17.7 to 20.3 kcal/mol. This study also predicts the favorable formation of certain adducts that exhibit intramolecular H-bonding interactions, nucleophilic addition, or H-atom transfer reactions. The spin density on the nitronyl N of the superoxide adducts reveals conformational dependences. The failure of nitrones to trap NO at normal conditions was theoretically rationalized due to the endoergic reaction parameters.

Theoretical and experimental studies of the spin trapping of inorganic radicals by 5,5-dimethyl-1-pyrroline N-oxide (DMPO). 2. Carbonate radical anion

Previous studies have shown that the enzyme-mediated generation of carbonate radical anion (CO(3)(.-)) may play an important role in the initiation of oxidative damage in cells. This study explored the thermodynamics of CO(3)(.-) addition to 5,5-dimethyl-1-pyrroline N-oxide (DMPO) using density functional theory at the B3LYP/6-31+G(**)//B3LYP/6-31G* and B3LYP/6-311+G* levels with the polarizable continuum model to simulate the effect of the bulk dielectric effect of water on the calculated energetics. Theoretical data reveal that the addition of CO(3)(.-) to DMPO yields an O-centered radical adduct (DMPO-OCO2) as governed by the spin (density) population on the CO(3)(.-). Electron paramagnetic resonance spin trapping with the commonly used spin trap, DMPO, has been employed in the detection of CO(3)(.-). UV photolysis of H(2)O(2) and DMPO in the presence of sodium carbonate (Na(2)CO(3)) or sodium bicarbonate (NaHCO(3)) gave two species (i.e., DMPO-OCO(2) and DMPO-OH) in which the former has hyperfine splitting constant values of a(N) = 14.32 G, a(beta)-Eta = 10.68 G, and a(gamma-H) = 1.37 G and with a shorter half-life compared to DMPO-OH. The origin of the DMPO-OH formed was experimentally confirmed using isotopically enriched H(2)(17)O(2) that indicates direct addition of HO(.) to DMPO. Theoretical studies on other possible pathways for the formation of DMPO-OH from DMPO-OCO(2) in aqueous solution and in the absence of free HO(.) such as in the case of enzymatically generated CO(3)(.-), show that the preferred pathway is via nucleophilc substitution of the carbonate moiety by H(2)O or HO(-). Nitrite formation has been observed as the end product of CO(3)(.-) trapping by DMPO and is partly dependent on the basicity of solution. The thermodynamic behavior of CO(3)(.-) in the aqueous phase is predicted to be similar to that of the hydroperoxyl (HO(2)(.)) radical.

Combined ESR and thermodynamic studies of the superoxide adduct of 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO): hindered rotation around the O-O bond evidenced by two-dimensional simulation of temperature-dependent spectra

Experiments were performed to elucidate the origin of the superhyperfine structure and line width alternation (LWA) seen in the ESR spectrum of the major diastereoisomer (1) of DEPMPO-OOH, the remarkably persistent superoxide adduct of 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO). Using selectively deuterated DEPMPO derivatives, we demonstrated that the superhyperfine pattern can be unambiguously attributed to long-range couplings. The recording in pyridine of highly resolved spectra in a wide temperature range, combined with two-dimensional simulation, allowed us to characterize an inverted LWA in 1 and revealed a uniform line broadening in the spectrum of the minor DEPMPO-OOH diastereoisomer (2), with both effects originating from a chemical exchange between conformers. When the individual spectra of 1 presenting LWA in the fast-exchange regime were simulated, four equally good fits were obtained and this ambiguity could be resolved by using a two-dimensional simulation technique. The thermodynamic and kinetic constants of this exchange were consistent with a rotation around the O-O bond. We propose that line broadening effects in 1 and 2 result from this O-O rotation concerted with the pseudo-rotation of the pyrrolidine ring.

Theoretical and Experimental Studies of the Spin Trapping of Inorganic Radicals by 5,5-Dimethyl-1-Pyrroline N-Oxide (DMPO). 1. Carbon Dioxide Radical Anion

The carbon dioxide radical anion (CO2*-) is known to be generated in vivo through various chemical and biochemical pathways. Electron paramagnetic resonance (EPR) spin trapping with the commonly used spin trap, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), has been employed in the detection of CO2*-. The thermodynamics of CO2*- addition to DMPO was predicted using density functional theory (DFT) at the B3LYP/6-31++G**//B3LYP/6-31G* and B3LYP/6-311+G* levels with the polarizable continuum model (PCM) to simulate the effect of the bulk dielectric effect of water on the calculated energetics. Three possible products of CO2*- addition to DMPO were predicted: (1) a carboxylate adduct, (2) pyrroline-alcohol and (3) DMPO-OH. Experimentally, UV photolysis of H2O2 in the presence of sodium formate (NaHCO2) and DMPO gave an EPR spectrum characteristic of a C-centered carboxylate adduct and is consistent with the theoretically derived hyperfine coupling constants (hfcc). The pKa of the carboxylate adduct was estimated computationally to be 6.4. The mode of CO2*- addition to DMPO is predicted to be governed predominantly by the spin (density) population on the radical, whereas electrostatic effects are not the dominant factor for the formation of the persistent adduct. The thermodynamic behavior of CO2*- in the aqueous phase is predicted to be similar to that of mercapto radical (*SH), indicating that formation of CO2*- in biological systems may have an important role in the initiation of oxidative damage in cells.

Effect of the Phosphoryl Substituent in the Linear Nitrone on the Spin Trapping of Superoxide Radical and the Stability of the Superoxide Adduct: Combined Experimental and Theoretical Studies

A new phosphorylated linear nitrone N-(4-hydroxybenzyliene)-1-diethoxyphosphoryl-1-methylethylamine N-oxide (4-HOPPN) was synthesized, and its X-ray structure was determined. The spin trapping ability of various kinds of free radicals by 4-HOPPN was evaluated. Kinetic study of decay of the superoxide spin adduct (4-HOPPN-OOH) shows the half-life time of 8.8 min. On the basis of the X-ray structural coordinates, theoretical analyses using density functional theory (DFT) calculations at the B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) level were performed on spin-trapping reactions of superoxide radical with 4-HOPPN and PBN and three possible decay routes for their corresponding superoxide adducts. The comparative calculations on the spin-trapping reactions with superoxide radical predicted that both spin traps share an identical reaction type and have comparable potency when spin trapping superoxide radical. Analysis of the optimized geometries of 4-HOPPN-OOH and PBN-OOH reveals that an introduction of the phosphoryl group can efficiently stabilize the spin adduct through the intramolecular H-bonds, the intramolecular nonbonding attractive interactions, as well as the bulky steric protection. Examination of the decomposition thermodynamics of 4-HOPPN-OOH and PBN-OOH further supports the stabilizing role of the phosphoryl group to a linear phosphorylated spin adduct.

The line asymmetry of electron spin resonance spectra as a tool to determine the cis:trans ratio for spin-trapping adducts of chiral pyrrolines N-oxides: the mechanism of formation of hydroxyl radical adducts of EMPO, DEPMPO, and DIPPMPO in the ischemic-reperfused rat liver

Nonstereospecific addition of free radicals to chiral nitrones yields cis/trans diastereoisomeric nitroxides often displaying different electron spin resonance (ESR) characteristics. Glutathione peroxidase-glutathione (GPx-GSH) reaction was applied to reduce the superoxide adducts (nitrone/*OOH) to the corresponding hydroxyl radical (HO*) adducts (nitrone/*OH) of two nitrones increasingly used in biological spin trapping, namely 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO) and 5-ethoxycarbonyl-5-methyl-1-pyrroline N-oxide, and of 5-diisopropoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DIPPMPO), a sterically hindered DEPMPO analogue. The method offered improved conditions to record highly resolved ESR spectra and by accurate simulation of line asymmetry we obtained clear evidence for the existence of previously unrecognized isomer pairs of cis- and trans-[DEPMPO/*OH] and [DIPPMPO/*OH]. Additional nitrone/*OH generation methods were used, i.e. photolysis of hydrogen peroxide and the Fenton reaction. We developed a kinetic model involving first- and second-order decay and a secondary conversion of trans to cis isomer to fully account for the strongly configuration-dependent behavior of nitrone/*OH. In the reductive system and, to a lower extent, in the Fenton or photolytic systems cis-nitrone/*OH was the more stable diastereoisomer. In various biologically relevant milieu, we found that the cis:trans-nitrone/*OH ratio determined right after the spin adduct formation significantly differed upon the GPx-GSH vs (Fenton or photolytic) systems of formation. This new mechanistic ESR index consistently showed for all nitrones that nitrone/*OH signals detected in the postischemic effluents of ischemic isolated rat livers are the reduction products of primary nitrone/*OOH. Thus, ESR deconvolution of cis/trans diastereoisomers is of great interest in the study of HO* formation in biological systems.

High static pressure alters spin trapping rates in solution. Dependence on the structure of nitrone spin traps

Using a competitive spin trapping method, relative spin trapping rates were quantified for various short-lived radicals (methyl, ethyl, and phenyl radicals). High static pressure was applied to the competitive spin-trapping system by employing high-pressure electron spin resonance (ESR) equipment. Under high pressure (490 bar), spin trapping rate constants for alkyl and phenyl radicals increased by 10 to 40%, and the increase was dependent on the structure of nitrone spin traps. A maximum increase was obtained when tert-butyl(4-pyridinylmethylene)amine N-oxide (4-POBN) was used as a spin trap. Activation volumes (DeltaDeltaV(double dagger)) for the two spin trapping reactions were calculated to be -17-(-9) cm(3) mol(-1) for the 4-POBN system.

Is There Stereoselectivity in Spin Trapping Superoxide by 5-tert-Butoxycarbonyl-5-methyl-1-pyrroline N-Oxide?

[reaction: see text] Ester-containing nitrones, including 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide 5, have been reported to be robust spin traps for superoxide (O2*-). Using a chiral column, we have been able to isolate the two enantiomers of nitrone 5. With enantiomerically pure nitrone 5a and 5b we explored whether one of these isomers was solely responsible for the EPR spectrum of aminoxyl 6. Data obtained demonstrate that the spin trapping of O2*- by nitrone 5a and nitrone 5b affords the identical EPR spectra and lifetimes in homogeneous aqueous solution and exhibits the same ratio of cis and trans isomers. Quantum chemical modeling in vacuo also finds no difference, aside from the expected optical activity, arising from the difference in stereochemistry.

Nitric oxide release from the unimolecular decomposition of the superoxide radical anion adduct of cyclic nitrones in aqueous medium

Nitrones such as 5,5-dimethyl-1-pyrroline N-oxide (DMPO), 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO) and 5-ethoxycarbonyl-5-methyl-1-pyrroline N-oxide (EMPO) have become the spin-traps of choice for the detection of transient radical species in chemical and biological systems using electron paramagnetic resonance (EPR) spectroscopy. The mechanism of decomposition of the superoxide radical anion (O2(.-)) adducts of DMPO, DEPMPO and EMPO in aqueous solutions was investigated. Our findings suggest that nitric oxide (NO) was formed during the decomposition of the O2(.-) adduct as detected by EPR spin trapping using Fe(II)N-methyl-d-glucamine dithiocarbamate (MGD). Nitric oxide release was observed from the O2(.-) adduct formed from hypoxanthine-xanthine oxidase, PMA-activated human neutrophils, and DMSO solution of KO2. Nitric oxide formation was not observed from the independently generated hydroxyl radical adduct. Formation of nitric oxide was also indirectly detected as nitrite (NO2(.-)) utilizing the Griess assay. Nitrite concentration increases with increasing O2(.-) concentration at constant DMPO concentration, while NO2(.-) formation is suppressed at anaerobic conditions. Moreover, large excess of DMPO also inhibits NO2(.-) formation which can be attributed to the oxidation of DMPO to hydroxamic acid nitroxide (DMPO-X) by nitrogen dioxide (NO2), a precursor to NO2(.-). Product analysis was also conducted to further elucidate the mechanism of adduct decay using gas chromatography-mass spectrometry (GC-MS) technique.

On the electrophilicity of hydroxyl radical: a laser flash photolysis and computational study

The rate coefficients for reactions of hydroxyl radical with aromatic hydrocarbons were measured in acetonitrile using a novel laser flash photolysis method. Comparison of kinetic data obtained in acetonitrile with those obtained in aqueous solution demonstrates an unexpected solvent effect on the reactivity of hydroxyl radical. In particular, reactions of hydroxyl radical with benzene were faster in water than in acetonitrile, and by a significant factor of 65. Computational studies, at the B3LYP and CBS-QB3 levels, have confirmed the rate enhancement of hydroxyl radical addition to benzene via calculation of the transition states in the presence of explicit solvent molecules as well as a continuum dielectric field. The origin of the rate enhancement lies entirely in the structures of the transition states and not in the pre-reactive complexes. The calculations reveal that the hydroxyl radical moiety becomes more anionic in the transition state and, therefore, looks more like hydroxide anion. In the transition states, solvation of the incipient hydroxide anion is more effective with water than with acetonitrile and provides the strong energetic advantage for a polar solvent capable of hydrogen bonding. At the same time, the aromatic unit looks more like the radical cation in the transition state. The commonly held view that hydroxyl radical is electrophilic in its reactions with DNA bases is, therefore, strongly dependent on the ability of the organic substrate to stabilize the resulting radical cation.

Superoxide Radical Anion Adduct of 5,5-Dimethyl-1-pyrroline N-Oxide (DMPO). 2. The Thermodynamics of Decay and EPR Spectral Properties

The formation of the superoxide radical anion (O2*-) adduct of the nitrone 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as detected by electron paramagnetic resonance (EPR) spectroscopy is one of the most common techniques for O2*- detection in chemical and biological systems. However, the nature of DMPO-O2H has confounded spin-trapping investigators over the years, since there has been no independently synthesized DMPO-O2H to date. A density functional theory (DFT) approach was used to predict the isotropic hyperfine coupling constants arising from the N, beta-H, and gamma-H nuclei of DMPO-O2H using explicit interactions with water molecules as well as via a bulk dielectric effect employing the polarizable continuum model (PCM). Theoretical calculation on the thermodynamics of DMPO-O2H decay shows favorable intramolecular rearrangement to form a nitrosoaldehyde and a hydroxyl radical as products, consistent with experimental observations. Some pathways for the bimolecular decomposition of DMPO-O2H and DMPO-OH have also been computed.

Spin trapping by 5-carbamoyl-5-methyl-1-pyrroline N-oxide (AMPO): theoretical and experimental studies

The nitrone 5-carbamoyl-5-methyl-1-pyrroline N-oxide (AMPO) was synthesized and characterized. Spin trapping of various radicals by AMPO was demonstrated for the first time by electron paramagnetic resonance (EPR) spectroscopy. The resulting spin adducts for each of these radicals gave unique spectral profiles. The hyperfine splitting constants for the superoxide adduct are as follows: isomer I (80%), a(nitronyl)(-)(N) = 13.0 G and a(beta)(-)(H) = 10.8 G; isomer II (20%), a(nitronyl)(-)(N) = 13.1 G, a(beta)(-)(H) = 12.5 G, and a(gamma)(-)(H) = 1.75 G. The half-life of the AMPO-O(2)H was about 8 min, similar to that observed for EMPO but significantly shorter than that of the DEPMPO-O(2)H with t(1/2) approximately 16 min. However, the spectral profile of AMPO-O(2)H at high S/N ratio is distinguishable from the spectrum of the (*)OH adduct. Theoretical analyses using density functional theory calculations at the B3LYP/6-31+G//B3LYP/6-31G level were performed on AMPO and its corresponding superoxide adduct. Calculations predicted the presence of intramolecular H-bonding in both AMPO and its superoxide adduct. The H-bonding interaction was further confirmed by an X-ray structure of AMPO, and of the novel and analogous amido nitrone 2-amino-5-carbamoyl-5-methyl-1-pyrroline N-oxide (NH(2)-AMPO). The thermodynamic quantities for superoxide radical trapping by various nitrones have been found to predict favorable formation of certain isomers. The measured partition coefficient in an n-octanol/buffer system of AMPO was similar to those of DMPO and DEPMPO. This study demonstrates the suitability of the AMPO nitrone for use as a spin trap to study radical production in aqueous systems.