Cleavage Of H2O2 Research Articles

AuCu/CeO2 bimetallic catalysts for the selective oxidation of fatty alcohol ethoxylates to alkyl ether carboxylic acids

We describe in this work the preparation of a series of bimetallic Au-Cu catalysts supported over nanoceria for the direct oxidation of C12-C14-alcohol polyethyleneglycol ether with on average 7 ethyl oxide units (AEO7) to polyoxyethylene lauryl ether carboxylic acid (AECA6), using H2O2 as an oxidant, under basic conditions. Different preparation methods have been used, including deposition-precipitation, incipient wetness impregnation and wet impregnation for engineering the interaction between Au, Cu and nanoceria. The structure of the bimetallic catalysts is discussed in detail on the basis of refined characterization by XRD, HR-TEM, STEM-EDX-SDD, XPS and ICP-AES. The formation of a AuCu alloy over nanoceria at a Cu/Au molar ratio of 0.11 on Au allows a significant enhancement of the catalytic activity, resulting in an AECA6 yield up to 80% with a selectivity of 90%. The catalyst can be recycled and reused for at least 10 consecutive runs without apparent loss of activity. Detailed DFT calculations on Au, Cu and Au-Cu model alloys reveal a positive role of Cu on Au by favoring the adsorption of AEO7, H2O2 and OH− on the catalyst surface compared to pure Au, as well as by reducing the energy barrier for H2O2 cleavage. Isolated Cu sites on the Au-Cu alloy appear as crucial for enhancing the catalytic properties for AEO7 oxidation.

Grasping periodic trend and rate-determining step for S-modified metals of metal sulfides deployable to produce [rad]OH via H2O2 cleavage

Iron sulfides are fascinating catalytic phases because these include S-modified Feδ+ (δ ≤ 2) species functioning as H2O2 activators to form OH used for oxidatively degrading aqueous contaminants (e.g., phenol). As an initial step for locating S-modified metal species (Mδ+) that outperform Feδ+ in catalytic H2O2 cleavage, hexagonal metal sulfides (MS) were synthesized using Mn, Fe, Co, Ni, and Cu to understand electric potential-assisted H2O2 scission kinetics on Mδ+ species. Niδ+ species were found to show the greatest OH productivity among all Mδ+ species studied, mainly resulting from the Lewis acidic nature of Niδ+ species adequate to expedite the liberation of OH species. This was partially evidenced by H2O2 activation/phenol degradation runs on Mδ+ species, wherein initial H2O2 activation rate (-rH2O2,0) or initial phenol degradation rate (-rPHENOL,0) of Niδ+ species was 3–9 times those of the other Mδ+ species. Niδ+ species, therefore, were located in the middle of the volcano-shaped curve plotting -rH2O2,0 (or -rPHENOL,0) versus the type of Mδ+. Kinetic assessment of Mδ+ species under fine-tuned reaction environments also showed that regardless of varying H2O2 concentrations, Mδ+ species were found to retain their -rH2O2,0 values in the absence of electric potentials. Conversely, Mδ+ species could enhance -rPHENOL,0 values at larger electric potentials, where greater energies were likely exerted on Mδ+ species. This indeed corroborated that OH desorption from Mδ+ species was the rate-determining step to direct catalytic H2O2 scission. In addition to heterogeneous catalytic nature of Niδ+ species in fragmenting H2O2, outstanding H2O2 scission ability provided by Niδ+ species could also compensate for their moderate catalytic stability at pH-neutral condition.

How Proximal Nucleobases Regulate the Catalytic Activity of G-Quadruplex/Hemin DNAzymes

G-quadruplexes (G4s) are versatile catalytic DNAs when combined with hemin. Despite the repertoire of catalytically competent G4/hemin complexes studied so far, little is known about the detailed catalytic mechanism of these biocatalysts. Herein, we have carried out an in-depth analysis of the hemin binding site within the G4/hemin catalysts, providing the porphyrinic cofactor with a controlled nucleotidic environment. We intensively assessed the position-dependent catalytic enhancement in model reactions and found that proximal nucleobases enhance the catalytic ability of the G4/hemin complexes. Our results allow for revisiting the mechanism of the G4/hemin-based catalysis, especially gaining insights into the rate-limiting step, demonstrating how both the G4 core and the proximal nucleotides dA and/or dC concomitantly activate the Compound 0 → 0* prototropic cleavage of H2O2 to foster Compound 1 formation. These results provide mechanistic clues as to how the properties of G4-based catalysts can be impr...

Self-Catalyzing Chemiluminescence of Luminol-Diazonium Ion and Its Application for Catalyst-Free Hydrogen Peroxide Detection and Rat Arthritis Imaging.

We report the unique self-catalyzing chemiluminescence (CL) of luminol-diazonium ion (N2+-luminol) and its analytical potential. Visual CL emission was initially observed when N2+-luminol was subjected to alkaline aqueous H2O2 without the aid of any catalysts. Further experimental investigations found peroxidase-like activity of N2+-luminol on the cleavage of H2O2 into OH• radical. Together with other experimental evidence, the CL mechanism is suggested as the activation of N2+-luminol and its dediazotization product 3-hydroxyl luminol by OH• radical into corresponding intermediate radicals, and then further oxidation to excited-state 3-N2+-phthalic acid and 3-hydroxyphthalic acid, which finally produce 415 nm CL. The self-catalyzing CL of N2+-luminol provides us an opportunity to achieve the attractive catalyst-free CL detection of H2O2. Experiments demonstrated the 10-8 M level detection sensitivity to H2O2 as well as to glucose or uric acid if presubjected to glucose oxidase or uricase. With the exampled determination of serum glucose and uric acid, N2+-luminol shows its analytical potential for other analytes linking the production or consumption of H2O2. Under physiological condition, N2+-luminol exhibits highly selective and sensitive CL toward 1O2 among the common reactive oxygen species. This capacity supports the significant application of N2+-luminol for detecting 1O2 in live animals. By imaging the arthritis in LEW rats, N2+-luminol CL is demonstrated as a potential tool for mapping the inflammation-relevant biological events in a live body.

Reactions of Ferrous Coproheme Decarboxylase (HemQ) with O2 and H2O2 Yield Ferric Heme b.

A recently discovered pathway for the biosynthesis of heme b ends in an unusual reaction catalyzed by coproheme decarboxylase (HemQ), where the Fe(II)-containing coproheme acts as both substrate and cofactor. Because both O2 and H2O2 are available as cellular oxidants, pathways for the reaction involving either can be proposed. Analysis of reaction kinetics and products showed that, under aerobic conditions, the ferrous coproheme-decarboxylase complex is rapidly and selectively oxidized by O2 to the ferric state. The subsequent second-order reaction between the ferric complex and H2O2 is slow, pH-dependent, and further decelerated by D2O2 (average kinetic isotope effect of 2.2). The observation of rapid reactivity with peracetic acid suggested the possible involvement of Compound I (ferryl porphyrin cation radical), consistent with coproheme and harderoheme reduction potentials in the range of heme proteins that heterolytically cleave H2O2. Resonance Raman spectroscopy nonetheless indicated a remarkably weak Fe-His interaction; how the active site structure may support heterolytic H2O2 cleavage is therefore unclear. From a cellular perspective, the use of H2O2 as an oxidant in a catalase-positive organism is intriguing, as is the unusual generation of heme b in the Fe(III) rather than Fe(II) state as the end product of heme synthesis.

Tertiary treatment of a municipal wastewater toward pharmaceuticals removal by chemical and electrochemical advanced oxidation processes

This study focuses on the degradation of pharmaceuticals from a municipal wastewater after secondary treatment by applying various advanced oxidation processes (AOPs) and electrochemical AOPs (EAOPs) like UVC, H2O2/UVC, anodic oxidation (AO), AO with electrogenerated H2O2 (AO-H2O2), AO-H2O2/UVC and photoelectro-Fenton (PEF) using either UVC radiation (PEF-UVC) or UVA radiation (PEF-UVA). The municipal wastewater after secondary treatment was spiked with 5.0 mg L−1 of trimethoprim (TMP) antibiotic. The efficiency of processes to remove TMP followed the order UVC < AO-H2O2 < PEF-UVA << AO ≈ PEF-UVC < AO-H2O2/UVC < PEF-UVA (pH = 2.8) < H2O2/UVC ≈ PEF-UVC (pH = 2.8), using neutral pH, except when identified. While the UVC radiation alone led to a very low TMP removal, the H2O2/UVC process promoted a very high TMP degradation due to the production of hydroxyl radicals (OH) by H2O2 cleavage. In the AO-H2O2/UVC process, the electrogeneration of H2O2 can avoid the risks associated with the transportation, storage and manipulation of this oxidant and, furthermore, OH at the anode surface are also formed. Nevertheless, low contents of H2O2 were detected mainly at the beginning of the reaction, leading to a lower initial reaction rate when compared with the H2O2/UVC system. In the PEF-UVC, the addition of iron at neutral pH led to the visible formation of insoluble iron oxides that can filter the light. At pH 2.8, the iron remained dissolved, thereby promoting the Fenton's reaction and increasing the organics removal. The UVA-driven processes showed limited efficiency when compared with those using UVC light. For all processes with H2O2 electrogeneration, the active chlorine species can be scavenged by the H2O2, diminishing the efficiency of the processes. This can explain the lower efficiency of AO-H2O2 when compared with AO. Moreover, the degradation of the MWWTP effluent spiked with 18 pharmaceuticals in μg L−1 during AO process was assessed as well as the influence of the following operational variables on the process efficiency: (i) H2O2 concentration on H2O2/UVC, (ii) current density on AO, AO-H2O2, AO-H2O2/UVC, PEF-UVC and PEF-UVA, and (iii) pH on PEF-UVA.

Emergence of Function in P450-Proteins: A Combined Quantum Mechanical/Molecular Mechanical and Molecular Dynamics Study of the Reactive Species in the H2O2-Dependent Cytochrome P450SPα and Its Regio- and Enantioselective Hydroxylation of Fatty Acids.

This work uses combined quantum mechanical/molecular mechanical and molecular dynamics simulations to investigate the mechanism and selectivity of H2O2-dependent hydroxylation of fatty acids by the P450SPα class of enzymes. H2O2 is found to serve as the surrogate oxidant for generating the principal oxidant, Compound I (Cpd I), in a mechanism that involves homolytic O-O bond cleavage followed by H-abstraction from the Fe-OH moiety. Our results rule out a substrate-assisted heterolytic cleavage of H2O2 en route to Cpd I. We show, however, that substrate binding stabilizes the resultant Fe-H2O2 complex, which is crucial for the formation of Cpd I in the homolytic pathway. A network of hydrogen bonds locks the HO· radical, formed by the O-O homolysis, thus directing it to exclusively abstract the hydrogen atom from Fe-OH, thereby forming Cpd I, while preventing the autoxoidative reaction, with the porphyrin ligand, and the substrate oxidation. The so formed Cpd I subsequently hydroxylates fatty acids at their α-position with S-enantioselectivity. These selectivity patterns are controlled by the active site: substrate's binding by Arg241 determines the α-regioselectivity, while the Pro242 residue locks the prochiral α-CH2, thereby leading to hydroxylation of the pro-S C-H bond. Our study of the mutant Pro242Ala sheds light on potential modifications of the enzyme's active site in order to modify reaction selectivity. Comparisons of P450SPα to P450BM3 and to P450BSβ reveal that function has evolved in these related metalloenzymes by strategically placing very few residues in the active site.

Structural basis for cytochrome c Y67H mutant to function as a peroxidase.

The catalytic activity of cytochrome c (cyt c) to peroxidize cardiolipin to its oxidized form is required for the release of pro-apoptotic factors from mitochondria, and for execution of the subsequent apoptotic steps. However, the structural basis for this peroxidation reaction remains unclear. In this paper, we determined the three-dimensional NMR solution structure of yeast cyt c Y67H variant with high peroxidase activity, which is almost similar to that of its native form. The structure reveals that the hydrogen bond between Met80 and residue 67 is disrupted. This change destabilizes the sixth coordination bond between heme Fe3+ ion and Met80 sulfur atom in the Y67H variant, and further makes it more easily be broken at low pH conditions. The steady-state studies indicate that the Y67H variant has the highest peroxidase activities when pH condition is between 4.0 and 5.2. Finally, a mechanism is suggested for the peroxidation of cardiolipin catalyzed by the Y67H variant, where the residue His67 acts as a distal histidine, its protonation facilitates O-O bond cleavage of H2O2 by functioning as an acidic catalyst.

Kinetic and UV/Vis spectroscopic studies of the solvent influence on the cobalt(II)-acetylacetonate catalyzed cleavage of hydrogen peroxide

The kinetics of H2O2 cleavage catalyzed by cobalt(II)-acetylacetonate has been studied at 40 °C in water, acetic acid, acetonitrile, tetrahydrofuran, 2-propanol, t-butanol, morpholine, and 1,4-dioxane by kinetic and spectroscopic techniques. As revealed, the nature of the solvent has a decisive influence on process. Medium polarizability and polarity have been found to be the rate determining factors.

The influence of the reaction medium on the cleavage of hydrogen peroxide catalyzed with cobalt ions

The peculiarity of H2O2 cleavage catalyzed with cobalt(II)acetylacetonate has been studied by the method of inhibitors in different hydrophilic solvents. As established, physical-chemical properties of reaction medium can significantly affect the rate of peroxide decomposition and catalyst oxidation as well.

Zeolite NaY-mediated oxidation of dyes with H2O2: unique heterogeneous non-transition metal center cleavage of H2O2 under visible light irradiation

This study investigated the visible-light catalysis mediated by zeolite NaY on the oxidation of dyes with H2O2. The results demonstrated that zeolite NaY acts as a sink for the electron from the photo-excited dye in the heterogeneous catalysis. Furthermore, the electron can effectively activate H2O2 to produce ·OH radical that is a powerful oxidant for the oxidation of dye at room temperature. The effects of the framework topology, Si/Al ratio, and exchangeable cation of the zeolite on the oxidation of various dyes were also shown.

New Insights into the Mechanisms of O−O Bond Cleavage of Hydrogen Peroxide andtert-Alkyl Hydroperoxides by Iron(III) Porphyrin Complexes

The mechanisms of heterolytic versus homolytic O−O bond cleavage of H2O2, tert-butyl hydroperoxide (t-BuOOH), 2-methyl-1-phenyl-2-propyl hydroperoxide (MPPH), and m-chloroperoxybenzoic acid (m-CPBA) by iron(III) porphyrin complexes have been studied by carrying out catalytic epoxidations of cyclohexene in protic solvent. In these reactions, various iron(III) porphyrin complexes containing electron-withdrawing and -donating substituents on phenyl groups at the meso position of the porphyrin ring were employed to study the electronic effect of porphyrin ligands on the heterolytic versus homolytic O−O bond cleavage of the hydroperoxides. In addition, various imidazoles were introduced as axial ligands to investigate the electronic effect of axial ligands on the pathways of hydroperoxide O−O bond cleavage. Unlike the previous suggestions by Traylor, Bruice, and co-workers, the hydroperoxide O−O bonds were found to be cleaved both heterolytically and homolytically and partitioning between heterolysis and homolysis was significantly affected by the electronic nature of the iron porphyrin complexes (i.e., electronic properties of porphyrin and axial ligands). Electron-deficient iron porphyrin complexes show a tendency to cleave the hydroperoxide O−O bonds heterolytically, whereas electron-rich iron porphyrin complexes cleave the hydroperoxide O−O bonds homolytically. The heterolytic versus homolytic O−O bond cleavage of the hydroperoxides was also found to be significantly affected by the substituent of the hydroperoxides, ROOH (R = C(O)R‘, H, C(CH3)3, and C(CH3)2CH2Ph for m-CPBA, H2O2, t-BuOOH, and MPPH, respectively), in which the tendency of O−O bond heterolysis was in the order of m-CPBA > H2O2 > t-BuOOH > MPPH. This result indicates that the O−O bond of hydroperoxides containing electron-donating tert-alkyl groups such as t-BuOOH and MPPH tends to be cleaved homolytically, whereas electron-withdrawing substituents such as an acyl group in m-CPBA facilitates O−O bond heterolysis. Since we have observed that the homolytic O−O bond cleavage of hydroperoxides prevails in the reactions performed with electron-rich iron porphyrin complexes and with hydroperoxides containing electron-donating substituents such as the tert-alkyl group, we suggest that the homolytic O−O bond cleavage is facilitated when more electron density resides on the O−O bond of (Porp)Fe(III)-OOR intermediates. We also present convincing evidence that the previous assertion that the reactions of iron(III) porphyrin complexes with hydrogen peroxide and tert-alkyl hydroperoxides invariably proceed by heterolytic O−O bond cleavage in protic solvent and that the failure to obtain high epoxide yields in iron porphyrin complex-catalyzed epoxidation of olefins by hydroperoxides is due to the mechanism of heterolytic O−O bond cleavage followed by a fast hydroperoxide oxidation is highly unlike.

A molecular dynamics investigation of the resting, hydrogen peroxide-bound and compound II forms of cytochrome C peroxidase and Artromyces ramosus peroxidase

Molecular Dynamics (MD) simulations have been used to study structural and dynamic properties of the resting, hydrogen peroxide adduct and compound II forms of cytochrome C peroxidase (CCP) and Artromyces ramosus peroxidase (ARP). MD simulations of CCP show that: (i) hydrogen peroxide might form an outer sphere complex within the active site of the enzyme before the coordination to the iron centre takes place; (ii) Trp51 and His52 residues play a crucial role in the recognition and binding of hydrogen peroxide, while Arg48 is not directly involved; (iii) distal histidine (His52) allows an easy proton 1,2 shift within the H2O2 molecule, while Arg48 is not expected to play a role as crucial as His52 in promoting the heterolytic O–O bond breaking; (iv) the large mobility (about 2Å) of the side chain of Arg48 in the compound II form allows the formation of a hydrogen bond (H-bond) with the ferryl oxygen, which contributes to the stabilisation of such an intermediate. The active site of the ARP enzyme is characterised by structural and dynamic features slightly different from the CCP active site. In particular, (i) the outer sphere complex with hydrogen peroxide occurring in CCP is not observed in ARP because of the substitution of Trp51 of CCP with the more hydrophobic residue Phe55 of ARP; (ii) His56 and the carbonyl group of Arg52 are determinant in controlling the hydrogen peroxide binding and its orientation in the active site. In ARP, both H2O2 and His56 have orientation different than in CCP, but still suited for an easy 1,2 proton shift. (iii) Arg52 in ARP is on average more distant from the heme-iron than in CCP, but its relative orientation is suited to promote an easy cleavage of H2O2. (iv) In compound II form of ARP, the Arg52 side chain is too far from the oxy-ferryl group to form a hydrogen bond and therefore ARP looses a stabilising factor, which is present in the corresponding form of CCP.

The exogenous NADH dehydrogenase of heart mitochondria is the key enzyme responsible for selective cardiotoxicity of anthracyclines.

The molecular mechanism of the anthracycline-dependent development of cardiotoxicity is still far from being clear. However, it is generally accepted, that mitochondria play a significant role in triggering this organ specific injury. The results presented in this study demonstrate that, in contrast to liver mitochondria, isolated heart mitochondria shuttle single electrons to adriamycin, giving rise to oxygen radical formation via autoxidation of adriamycin semiquinones. This one electron reduction of anthracyclines is catalyzed by the exogenous NADH dehydrogenase associated with complex I of heart mitochondria, an enzyme which is lacking in liver mitochondria. Upon addition of NADH heart mitochondria generate significant amounts of adriamycin semiquinones while liver mitochondria were ineffective. Adriamycin semiquinones undergo both autoxidation leading to superoxide radical release and complex reactions under formation of adriamycin aglycone. Due to the high lipophilicity adriamycin aglycones accumulate in the inner mitochondrial membrane where they interfere with electron carriers of the respiratory chain. Adriamycin aglycone semiquinones emerging from an interaction with complex I were found to trigger homolytic cleavage of H2O2 which results in the formation of hydroxyl radicals. As demonstrated in this study the activation of adriamycin by the exogenous NADH dehydrogenase of cardiac mitochondria initiates a cascade of reaction steps leading to the establishment of oxidative stress. Our experiments suggest the exogenous NADH dehydrogenase of heart mitochondria to play a key role in the cardiotoxicity of adriamycin. This organ-specific enzyme initiates a sequence of one electron transfer reactions ending up in the establishment of oxidative stress.

The Manganese Binding Site of Manganese Peroxidase: Characterization of an Asp179Asn Site-Directed Mutant Protein

A site-directed mutant, D179N, in the gene encoding Phanerochaete chrysosporium manganese peroxidase isozyme 1 (mnp1), was created by overlap extension, using polymerase chain reaction. The mutant gene was expressed in P. chrysosporium under the control of the glyceraldehyde-3-phosphate dehydrogenase promoter. The mutant manganese peroxidase (MnP) was purified, and its spectra and MW were very similar to those of the wild-type enzyme. Steady-state kinetic analysis of MnP D179N revealed that the Km for the substrate MnII was approximately 50-fold greater than the corresponding Km for the wild-type recombinant enzyme (3.7 mM versus approximately 70 microM). Likewise, the kcat value for MnII oxidation of the mutant protein was only 1/265 of that for the wild-type enzyme. By comparison, the apparent Km for H2O2 of MnP D179N was similar to the corresponding value of the wild-type MnP. The first-order rate constant for MnP D179N compound II reduction by MnII was approximately 1/200 of that for the wild-type enzyme. The equilibrium dissociation constant (KD) for MnP D179N compound II reduction by MnII was approximately 100-fold greater than the KD for the wild-type compound II. In contrast, the second-order rate constant for p-cresol reduction of the mutant compound II was similar to that of the wild-type enzyme. These results also suggest that the mutation affects the binding of MnII to the enzyme and, consequently, the rate of compound II reduction by MnII. In contrast, the mutation apparently does not have a significant effect on H2O2 cleavage during compound I formation or on p-cresol reduction of compound II.(ABSTRACT TRUNCATED AT 250 WORDS)

Catalytic properties of Phe41?His mutant of horseradish peroxidase expressed inE. coli

The recombinant horseradish peroxidase and its single-point F41H mutant have been reactivated fromE. coli inclusion bodies. The influence of the mutation on the heme entrapment, stability and activity of the enzyme was demonstrated. The catalytic rate constants for H2O2 cleavage and ammonium 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate (ABTS) oxidation decrease by two and one orders of magnitude, respectively. Unlike the wild-type recombinant horseradish peroxidase, the elimination of the ABTS oxidation product is not a rate-determining step for the mutant. The F41H replacement results in significant changes of kinetics of iodide ion oxidation. The reaction rate is linear to the concentrations of iodide, H2O2, and the enzyme. The results suggest the direct interaction of iodide with the porphyrin ring of the heme. The decrease in ABTS oxidation activity accompanied by retention of activity in iodide oxidation in the course of low-dosage radiolysis of the F41H mutant is additional evidence of the direct electron transfer from iodide to the heme, in contrast to ABTS oxidation, in which the electron transfer chain in the protein molecule is involved.

Indirect involvement of ligninolytic enzyme systems in cell wall degradation

Observations by electron microscopy of the micromorphological alterations caused in wood by fungal strains in which the cellulase system had been repressed or inhibited showed a substantial destruction of the cellulose network. The question thus arises which agents, other than enzymes, could be responsible for the modification of polysaccharides during fungal attack. Among the small diffusible agents possibly involved, activated oxygen species generated from hydrogen peroxide have been suggested. In particular the powerful oxidizing hydroxyl radical (· OH), which can be generated by reductive cleavage of H2O2 or by reduction of oxygen, has been reported to take part in degradation of lignin by white- and brown-rot fungi. Manganic ions which are generated by manganese-dependent peroxidase (MnP) represent another diffusible oxidizing agent. To investigate whether these agents, produced by lignin-degrading enzymes, could also participate in wood polysaccharide degradation, the action of hydroxyl radicals and of some Mn(III)-organic acid complexes on carbohydrate and polysaccharide model compounds and also on wood and pulps, was studied. The action of ·OH, generated by the iron-catalyzed Fenton’s reagent, on the glucuronoxylan from birch wood resulted in an extensive degradation of the hemicellulose (about 50% loss of carbohydrate after 180 min). A study of the degraded products suggests that internal cleavage occurs in the xylan chain accompanied by sugar ring degradation, the latter becoming predominant on the soluble oligomeric products. In a parallel study, the susceptibility of carbohydrates to oxidation by manganic ions showed that monosaccharide degradation occurred rapidly in strongly acidic medium. The mechanism which was studied with a Mn(III)-pyrophosphate complex, involves, as an initial step, the oxidative cleavage of C1 with the formation of a lower carbohydrate, and then proceeds by a recurrent series of oxidations. However, the reaction seems to be controlled by the formation of a Mn(III)-pyrophosphate complex with the sugar, which prevents further oxidation. The two oxidizing agents were applied to birch wood wafers. The hydroxyl radicals had only a relatively weak degradative action on the cell walls. However, when the radicals were generated in situ, the extent of degradation as visualized by electron microscopy, gave images comparable to those obtained on fungal-degraded wood. Samples treated with manganic pyrophosphate and manganic organic acid complexes also showed typical degradation indicating that lignin was primarily degraded and also that hemi-celluloses had also been partly damaged.

Spin Trapping by Use ofN-Tert-Butylhydroxylamine. Involvement of Fenton Reactions

Spin trapping of short-lived R. radicals is done by use of N-tert-butylhydroxylamine (1) and H2O2. The hydroxylamine is oxidized to the radical t-BuN(O)H (2) which is converted into the spin trap 2-methyl-2-nitrosopropane (3). Simultaneously, hydroxyl radicals .OH are formed from H2O/. The latter radical species abstracts hydrogen atoms from suitable molecules HR to give R. radicals, which are trapped with the formation of aminooxyl radicals, i.e., t-BuN(O)R (4), detectable by EPR spectroscopy. The reaction is enhanced by the presence of iron ions. The cleavage of H2O2 into .OH radicals is considered to involve both a radical-driven (t-BuN(O)H 2) and an iron-driven Fenton reaction.

Lipid peroxidation during the oxidation of haemoproteins by hydroperoxides. Relation to electronically excited state formation.

The formation of electronically excited states during hydroperoxide metabolism is analysed in terms of recombination reactions involving secondary peroxyl radicals and scission of the O-O bond of peroxides by haemoproteins, mainly myoglobin. Both processes may be sequentially interrelated, for the cleavage of H2O2 by metmyoglobin leads to the formation of a strong oxidizing equivalent with the capability to promote peroxidation of polyunsaturated fatty acids. The decomposition of lipid hydroperoxides by ferryl-hydroxo complexes, as that formed during the oxidation of metmyoglobin by H2O2, is a source of peroxyl radicals, the recombination of which proceeds with elimination of a conjugated triplet carbonyl or singlet oxygen.

Mechanism of iodide-dependent catalatic activity of thyroid peroxidase and lactoperoxidase.

Mechanisms that have been proposed for peroxidase-catalyzed iodination require the utilization of 1 mol of H2O2 for organic binding of 1 mol of iodide. When we measured the stoichiometry of this reaction using thyroid peroxidase or lactoperoxidase at pH 7.0, we consistently obtained a ratio less than 1.0. This was shown to be attributable to catalase-like activity of these enzymes, resulting in unproductive cleavage of H2O2. This catalatic activity was completely iodide-dependent. To elucidate the mechanism of the iodide-dependent catalatic activity, the effects of various agents were investigated. The major observations may be summarized as follows: 1) The catalatic activity was inhibited in the presence of an iodine acceptor such as tyrosine. 2) The pseudohalide, SCN-, could not replace I- as a promoter of catalatic activity. 3) The inhibitory effects of the thioureylene drugs, methimazole and carbimazole, on the iodide-dependent catalatic activity were very similar to those reported previously for thyroid peroxidase-catalyzed iodination. 4) High concentrations of I- inhibited the catalatic activity of thyroid peroxidase and lactoperoxidase in a manner similar to that described previously for peroxidase-catalyzed iodination. On the basis of these observations and other findings, we have proposed a scheme which offers a possible explanation for iodide-dependent catalatic activity of thyroid peroxidase and lactoperoxidase. Compound I of the peroxidases is represented as EO, and oxidation of I- by EO is postulated to form enzyme-bound hypoiodite, represented in our scheme as [EOI]-. We suggest that the latter can react with H2O2 in a catalase-like reaction, with evolution of O2. We postulate further that the same form of oxidized iodine is also involved in iodination of tyrosine, oxidation of thioureylene drugs, and oxidation of I-, and that inhibition of catalatic activity by these agents occurs through competition with H2O2 for oxidized iodine.