HOO Radical Research Articles

Is lecanoric acid a good antioxidant?

Lecanoric acid (LA), an abundant chemical found in lichens, has demonstrated a wide range of biological activities, including anti-cancer cytotoxic, antibiotic, antimycobacterial, antiviral, and anti-hepatocarcinoma properties. The antioxidant capacity of this molecule, while inferred from certain experimental findings, is doubtful based on structural characteristics and therefore remains to be established. DFT calculations are used in this work to conduct a comprehensive evaluation of the mechanism and kinetics governing the antiradical activity of LA in lipidic and aqueous solvent environments. Although the DPPH/ABTS+ assays revealed good antioxidant activity in vitro, the modeling yielded mixed results. The data suggests that LA is an efficient scavenger of the HO radical with rate constants of 2.01 × 1010 and 2.80 × 108 M−1 s−1 in polar and lipid media, respectively, by the FHT and RAF mechanisms. However, the data also suggests that LA exhibits only weak activity against the HOO radical in all physiological environments. This is consistent with structural features that predict low activity.

The hydroperoxyl antiradical activity of natural chalcones in physiological environments: Theoretical insights into the mechanism, kinetics and pH effects

Chalcones (CHA), based on the 1,3-diaryl-2-propen-1-one structure, are flavonoids with an open chain. Wide-ranging biological effects of CHA have been observed, including tyrosinase inhibition, anticancer, anti-inflammatory, and antioxidant activities. However, the physical chemistry of the antiradical activity of these compounds has not been explored in depth. We report an investigation on the mechanisms and kinetics of CHA's activity in scavenging hydroperoxyl radicals using calculations based on density functional theory (DFT). It was found that the studied CHA possess powerful HOO radical scavenging properties. In a lipid-like medium, CHA react with HOO via the hydrogen transfer reaction with k = 103 –105 M−1 s−1, while in an aqueous solution, the activity is defined by the anion states according to the single electron transfer (SET) pathway, yielding a rate constant of k = 106 –108 M−1 s−1. The rate constants are sensitive to the protonation state and hence vary with pH showing the highest HOO radical scavenging activity at pH = 7.3–7.5. In the examined media, calculations suggest that CHA can have stronger HOO antiradical activity than Trolox, with kinetics comparable to ascorbic acid or resveratrol.

Intracluster Reaction Dynamics of Ionized Micro-Hydrated Hydrogen Peroxide (H2O2): A Direct Ab Initio Molecular Dynamics Study

Hydrogen peroxide (H2O2) is a unique molecule that is applied in various fields, including energy chemistry, astrophysics, and medicine. H2O2 readily forms clusters with water molecules. In the present study, the reactions of ionized H2O2–water clusters, H2O2+(H2O)n, after vertical ionization of the parent neutral cluster were investigated using the direct ab initio molecular dynamics (AIMD) method to elucidate the reaction mechanism. Clusters with one to five water molecules, H2O2–(H2O)n (n = 1–5), were examined, and the reaction of [H2O2+(H2O)n]ver was tracked from the vertical ionization point to the product state, where [H2O2+(H2O)n]ver is the vertical ionization state (hole is localized on H2O2). After ionization, fast proton transfer (PT) from H2O2+ to the water cluster (H2O)n was observed in all clusters. The HOO radical and H3O+(H2O)n−1 were formed as products. The PT reaction proceeds directly without an activation barrier. The PT times for n = 1–5 were calculated to be 36.0, 9.8, 8.3, 7.7, and 7.1 fs, respectively, at the MP2/6-311++G(d,p) level, indicating that PT in these clusters is a very fast process, and the PT time is not dependent on the cluster size (n), except in the case of n = 1, where the PT time was slightly longer because the bond distance and angle of the hydrogen bond in n = 1 were deformed from the standard structure. The reaction mechanism was discussed based on these results.

Combined Experimental and Computational Kinetics Studies for the Atmospherically Important BrHg Radical Reacting with NO and O2.

We report on the first experimental determination of the absolute rate constant of the reaction of BrHg + NO in N2 bath gas using a laser photolysis-laser-induced fluorescence (LP-LIF) system. The rate constant of the reaction of BrHg + NO is determined to be 7.0-0.9+1.3 × 10-12 cm3 molecule-1 s-1 over 50-700 Torr and 315-353 K. The absence of a pressure or temperature dependence suggests that this reaction leads mainly to mercury reduction (Hg + BrNO) rather than mercury oxidation (BrHgNO). Our theoretical calculations using NEVPT2 energies on density functional theory (DFT) geometries are consistent with a barrierless reaction to form Hg + BrNO. The equilibrium constants and the rate constants of the reaction BrHg + O2 ⇌ BrHgOO are computed theoretically because they are too low to be measured in the LP-LIF system. Molecular oxygen quenches the LIF signal of BrHg with a large rate constant of (1.7 ± 0.1) × 10-10 cm3 molecule-1 s-1. Thus, different techniques that capture the absolute [BrHg(X̃)] would be advantageous for kinetics studies of BrHg reactions in the presence of O2. The computed equilibrium constant suggests a non-negligible upper limit of the fraction of BrHg stored as BrHgOO (up to 0.5) at low-temperature conditions, e.g., in the upper troposphere and in polar winters at ground level. Preliminary results indicate that BrHgOO behaves like HOO or organic peroxy radicals in reactions with atmospheric radicals.

Mechanistic and kinetic studies of the radical scavenging activity of natural abietanes: A theoretical insight

Abietanes are natural products that were isolated from Salvia clinopodioides. In this work, the HOO scavenging activity of ten abietanes was thoroughly evaluated by using computational methods for the first time. It was found that the studied abietanes could exhibit moderate HOO scavenging activity. Considerable antioxidant activity of the abietanes was only predicted in the aqueous solution and not in lipidic environments. Among the studied compounds, aldehyde (10) exhibited the highest HOO radical scavenging activity in the aqueous solution (koverall = 3.72 × 108 M−1s−1) that is higher than the reference natural antioxidants Trolox, trans-resveratrol, and ascorbic acid.

Are thymol, rosefuran, terpinolene and umbelliferone good scavengers of peroxyl radicals?

DFT-based computational calculations have been used to investigate the hydroperoxyl radical scavenging activity of four essential oil constituents namely thymol (Thy), rosefuran (Ros), terpinolene (Ter), and umbelliferone (Umb). Different reaction mechanisms including formal hydrogen transfer (FHT), radical adduct formation (RAF), sequential proton loss electron transfer (SPLET), and sequential electron transfer proton transfer (SETPT) have been examined in the gas phase and physiological environments. It was found that the HOO radical scavenging activity of these compounds is strongly influenced by the environment, which becomes more important in water than pentyl ethanoate. According to the overall reaction rate constants, the phenolic compounds Thy and Umb are predicted to exhibit excellent activity in aqueous solution. Umb with an overall rate constant of 1.44 × 108M−1s−1 at physiological pH is among the best HOO radical scavengers in water with activity comparable to that of caffeic acid, higher than those of ascorbic acid, guaiacol and eugenol, and much higher than that of Trolox.

Atomic Perspective about the Reaction Mechanism and H2 Production during the Combustion of Al Nanoparticles/H2O2 Bipropellants.

The combination of Al nanoparticles (ANPs) and hydrogen peroxide (H2O2) can serve as environmentally friendly bipropellants and maximize the energetic benefits through harnessing heat release and chemical energy stored in H2. This work presents an atomic insight into the combustion mechanism of ANPs/H2O2. Two main paths, including the ANPs oxidation by H2O2 to produce H2 and Al oxides, and the catalytic decomposition of H2O2 on ANP surface to generate O2 and H2O, are confirmed to maintain the combustion. OH and HOO radicals as well as H2O, O2, H2, and Al oxides are detected as dominant intermediates and products therein. It is evidenced that higher temperature, smaller ANP size, and higher H2O2 concentration enhance the combustion. Moreover, atomic details show that the H desorption from ANPs/Al clusters is a critical step for both H2 production and ANP oxidation. In addition, microexplosion that has been confirmed in hot and dense O2 is not observed in H2O2, even with a high concentration, possibly due to a slower heat release. Besides, the observed excellent specific impulse of the ANP/H2O2 bipropellants could be attributed to the considerable H2 production, instead of heat release. This work is expected to present an overall atomic perspective about the combustion mechanism of ANP/H2O2 bipropellants.

The radical scavenging activity of natural ramalin: A mechanistic and kinetic study

Ramalin, which is isolated from the Antarctic lichen (Ramalina terebrata), has potential anti-inflammatory, anti-cancer and antioxidant activities. In this study, the HO and HOO radical scavenging of ramalin was evaluated in water and pentyl ethanoate (lipid mimetic) environments in silico by using kinetic calculations. It was found that the radicals scavenging in aqueous solution is faster than that in apolar media. The formal hydrogen transfer mechanism was favored for the radical scavenging in lipid media. Compared with Trolox, ramalin has higher hydroperoxyl radical scavenging in the studied environments. Thus ramalin was shown to be a promising natural antioxidant.

Identification of two aliphatic position isomers between α- and β-ketoglutaric acid by using a Briggs-Rauscher oscillating system.

This paper reports a novel method for identification of two aliphatic position isomers between α-ketoglutaric acid (α-KA) and β-ketoglutaric acid (β-KA) by their different perturbation effects on a Briggs-Rauscher oscillating system, in which tetraaza-macrocyclic complex [NiL](ClO4)2 is used as the catalyst. The ligand L in the complex is 5,7,7,12,14,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-4,11-diene. The experimental results have shown that addition of α-KA into the system does not affect the oscillating patterns, while the presence of β-KA in a dynamic system influences the oscillatory amplitude. A more interesting feature is that, in the presence of a higher concentration of β-KA, there are damped oscillations after the initial spike, followed by quenching (more exactly: very small oscillations) of the oscillations before the subsequent regeneration of oscillations. A qualitative approach was thus established by employing a Briggs-Rauscher system for identification of these two isomers. The concentrations of these two isomers that can be distinguished lie over the range between 5.0 × 10(-6) and 2.5 × 10(-3) mol/L. A reaction mechanism based on the FCA model has been proposed. An explanation is that β-KA reacts with HOO(•) radicals to form acetone, whereas the α-KA does not.

Lipoic acid and dihydrolipoic acid. A comprehensive theoretical study of their antioxidant activity supported by available experimental kinetic data.

The free radical scavenging activity of lipoic acid (LA) and dihydrolipoic acid (DHLA) has been studied in nonpolar and aqueous solutions, using the density functional theory and several oxygen centered radicals. It was found that lipoic acid is capable of scavenging only very reactive radicals, while the dehydrogenated form is an excellent scavenger via a hydrogen transfer mechanism. The environment plays an important role in the free radical scavenging activity of DHLA because in water it is deprotonated, and this enhances its activity. In particular, the reaction rate constant of DHLA in water with an HOO(•) radical is close to the diffusion limit. This has been explained on the basis of the strong H-bonding interactions found in the transition state, which involve the carboxylate moiety, and it might have implications for other biological systems in which this group is present.

Molecular mechanism of plasma sterilization in solution with the reduced pH method: importance of permeation of HOO radicals into the cell membrane

Sterilization of certain infected areas of the human body surface is necessary for dental and surgical therapies. Because the blood is filled with body fluid, sterilization in solution is essential. In vitro solution sterilization has been successively carried out using a combination of low-temperature atmospheric-pressure plasma and the reduced pH method, where the solution is sufficiently acidic. Here, we show the molecular mechanism of such plasma sterilization in solution based on microbiology. Three kinds of bacteria were inactivated by plasma treatment under various pH conditions. The theoretical and experimental models revealed that the sterilization was characterized by the concentration of hydroperoxy radicals (HOO·), which were dependent on the pH value. Bacterial inactivation rates were proportional to the HOO· concentrations calculated by the theoretical model. To evaluate the penetration of radicals into the cell membrane, a bacterial model using dye-included micelles was used. Decolouration rates of the model were also in proportion with the calculated HOO· concentrations. These results indicate that the key species for plasma sterilization were hydroperoxy radicals. More importantly, the high permeation of hydroperoxy radicals into the cell membrane plays a key role for efficient bactericidal inactivation using the reduced pH method.

Communication: Theoretical prediction of the structure and spectroscopic properties of the $\tilde{\mathrm{X}}$X̃ and $\tilde{\mathrm{A}}$Ã states of hydroxymethyl peroxy (HOCH2OO) radical

The hydroxymethyl peroxy (HMOO) radical is a radical product from the oxidation of non-methane hydrocarbons. The present study provides theoretical prediction of critical spectroscopic features of this radical that should aid in its experimental characterization. Structure, rotational constants, and harmonic frequencies are presented for the ground and first excited electronic states of HMOO. The adiabatic transition energy for the Ã←X̃ process is 7360 cm(-1), suggesting that this transition, occurring in the mid to near infrared, is the most promising candidate for observing the radical spectroscopically. The band origin of the Ã←X̃ transition of HMOO is calibrated and benchmarked with the corresponding state of the HOO radical, which is experimentally and theoretically well characterized.

Antioxidant and Electron Donating Function of Hypothalamic Polypeptides: Galarmin and Gx-NH2

Chemical mechanisms of antioxidant and electron donating function of the hypothalamic proline-rich polypeptides have been clarified on the molecular level. The antioxidant-chelating property of Galarmin and Gx-NH(2) was established by their capability to inhibit copper(II) dichloride catalyzed H(2)O(2) decomposition, thus preventing formation of HO(*) and HOO(*) radicals. The antiradical activity of Galarmin and Gx-NH(2) was determined by their ability to react with 2,2-diphenyl-1-picrylhydrazyl radical applying differential pulse voltammetry and UV-Vis spectrophotometry methods. Galarmin manifest antiradical activity towards 2,2-diphenyl-1-picrylhydrazyl radical, depending on the existence of phenolic OH group in tyrosine residue at the end of the molecule. The presence of antiradical activity and reduction properties of Galarmin are confirmed by the existence of an oxidation specific peak in voltammograms made by differential pulse voltammetry at E ( composite function) = 0.795 V vs. Ag/Ag(+) aq.

Direct Ab Initio MD Study on the Electron Capture Dynamics of Hydroperoxy Radical (HOO)-Water Complexes

The electron capture dynamics of hydroperoxy radical-water complexes have been investigated by means of direct density functional theory (DFT) molecular dynamics (MD) method to elucidate the solvation (hydration) effect on the reaction mechanism. The complexes composed of HOO and one to four water molecules, HOO(H(2)O)(n) (n = 1-4), were considered to be the hydrated HOO system. After the electron capture of n = 1, only solvent reorientation of H(2)O around HOO(-) occurred, and a stable complex (HOO(-)-H(2)O) was formed within 100-300 fs. In the case of n = 2-4, a proton of H(2)O was transferred from H(2)O to OOH(-), whereas H(2)O(2) and OH(-)(H(2)O)(n-1) were found as products. It was suggested that the HOO radical adsorbed on water cluster is efficiently converted in the H(2)O(2) without activation barrier after the electron capture of HOO. Time scales of proton transfer were calculated to be 200-300 fs. The mechanism of electron capture of HOO in polar stratospheric cloud was discussed on the basis of theoretical results.

Coupling Interactions between Sulfurous Acid and the Hydroperoxyl Radical

Radical-molecule complexes associated with the hydroperoxyl radical (HOO) play an important role in atmospheric chemistry. Herein, the nature of the coupling interactions between sulfurous acid (H(2)SO(3)) and the HOO radical is systematically investigated at the B3LYP/6-311++G(3df,3pd) level of theory in combination with the atoms in molecules (AIM) theory, the natural bond orbital (NBO) method, and energy decomposition analyses (EDA). Eight stable stationary points possessing double H-bonding features were located on the H(2)SO(3)...HOO potential energy surface. The largest binding energies of -12.27 and -11.72 kcal mol(-1) are observed for the two most stable complexes, where both of them possess strong double intermolecular H-bonds of partially covalence. Moreover, the characteristics of the IR spectra for the two most stable complexes are discussed to provide some help for their possible experimental identification.

Direct ab initio MD study on the interaction of hydroperoxy radical (HOO) with water molecules

Structures and electronic states of the HOO radical interacting with water molecules, expressed by HOO(H2O)n (n = 1 and 2), have been investigated by means of a direct ab initio molecular dynamics (MD) method. From the static ab initio calculation of HOO-H2O complex, two types of HOO radical were found: i.e., the HOO radical acts as a hydrogen donor or acceptor in the complex (n = 1). The binding energies of former and latter complexes were calculated to be 8.7 and 3.3 kcal mol(-1), respectively, at the QCISD/6-311++G(2d,2p) level. In the case of 1:2 complex HOO(H2O)2, a cyclic structure with a hydrogen donor of HOO was obtained as the stable form. Effects of zero point vibration on the structures and hyperfine coupling constants of the HOO radical were also investigated. The structures and electronic states of HOO(H2O)n (n = 1 and 2) were discussed on the basis of theoretical results.

Supercritical water oxidation of ion exchange resins: Degradation mechanisms

Spent ion exchange resins are radioactive process wastes for which there is no satisfactory industrial treatment. Supercritical water oxidation could offer a viable treatment alternative to destroy the organic structure of resins and contain radioactivity. IER degradation experiments were carried out in a continuous supercritical water reactor. Total organic carbon degradation rates in the range of 95–98% were obtained depending on operating conditions. GC–MS chromatography analyses were carried out to determine intermediate products formed during the reaction. Around 50 species were identified for cationic and anionic resins. Degradation of polystyrenic structure leads to the formation of low molecular weight compounds. Benzoic acid, phenol and acetic acid are the main compounds. However, other products are detected in appreciable yields such as phenolic species or heterocycles, for anionic IERs degradation. Intermediates produced by intramolecular rearrangements are also obtained. A radical degradation mechanism is proposed for each resin. In this overall mechanism, several hypotheses are foreseen, according to HOO radical attack sites.

Hydrogen bond of radicals: Interaction of HNO with HCO, HNO, and HOO

AbstractAb initio quantum mechanics methods are employed to investigate hydrogen bonding interactions between HNO and HCO, HOO radicals, and closed‐shell HNO. The systems were calculated at MP2/6‐311++G (2d, 2p) level and G2MP2 level. The topological and NBO analysis were investigated the origin of hydrogen bonds red‐ or blue‐shifts. In addition, the comparisons were performed between HNO‐opened‐shell radical (HCO, HOO) complexes and HNO‐corresponding closed‐shell molecule (H2CO, HOOH) complexes. It is found that the stabilities of complexes increase from HNO‐HCO to HNO‐HOO. There are blue‐shifts of NH, CH stretching vibrational frequencies and a red‐shift of OH stretching vibrational frequency in the complexes. Rehybridization and electron density redistribution contribute to the blue‐shifts of CH and NH stretching vibrational frequencies. Compared with the closed‐shell H2CO, HCO is weaker proton donor and weaker proton acceptor. For the HOO, it is stronger proton donor and weaker proton acceptor than the HOOH is. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010

Mechanism of oxidations with H 2O 2 catalyzed by vanadate anion or oxovanadium(V) triethanolaminate (vanadatrane) in combination with pyrazine-2-carboxylic acid (PCA): Kinetic and DFT studies

The oxovanadium(V) triethanolaminate complex (vanadatrane, 2), in the presence of pyrazine-2-carboxylic acid (PCA) and H 2O 2, is shown to act as a catalyst for the efficient oxidation of isopropanol to acetone (without other solvents) and of cyclohexane (in MeCN) to cyclohexyl hydroperoxide at low temperature (40–50 °C). This catalytic system, that is similar to the previously discovered “vanadate ion ( 1)–PCA–H 2O 2” reagent, is investigated by detailed spectroscopic, kinetic, and DFT methods which allow us to establish the mechanism of the oxidations. The kinetic and spectroscopic studies reveal that both catalytic systems activate C–H bonds via a common reaction mechanism. The 51V NMR data show that catalytic cycles involve the same intermediate species. Calculations by the DFT method allow us to explore intimate details of the mechanism of free radical generation and to introduce modifications to the mechanistic proposals published previously. Thus, a “water-assisted” mechanism of H-transfer steps is suggested and shown to be more effective than the “PCA-assisted” or “robot’s arm” mechanism. The formation of both HOO and HO radicals is more favorable along the cycle based on complexes containing only one peroxo fragment V(OO) (“cycle I”) than in the cycle based on diperoxo species containing V(OO) 2 fragments (“cycle III”). The generation of the reactive HO radicals occurs via the addition of H 2O 2 to the V IV complex [V(OO)(OH)(pca)] (pca = anionic basic form of PCA) to produce [V IV(OO)(OH)(pca)(H 2O 2)], followed by water-assisted H-transfer to the HO-ligand (the rate-limiting stage of the overall process), isomerization, and O–OH bond cleavage. The pca ligand is found to play a key role as a stabilizer of the transition state for the H-transfer.

Complexes pairing aliphatic amines with hydroxyl and hydroperoxyl radicals: A computational study

Quantum calculations are used to analyze the nature and strength of binding of complexes pairing aliphatic amines with the HO and HOO radicals, with particular emphasis on the comparison of HO with HOO. Complexes are stabilized by N⋯HO(O) H-bonds involving the N lone pair. The OOH radical binds more strongly in all cases than does OH, and shows a longer stretch of its OH bond, as well as a greater red shift of this bond’s stretching frequency. MP2/6-311++G(3df, 3pd) binding energies vary from 28 kJ/mol for CH 3NH 2⋯HO to a maximum of 56 kJ/mol for (CH 3) 2NH⋯HOO. The reliability of a new hybrid meta exchange-correlation functional, M05-2X, compares favorably with MP2, generally better than B3LYP.