An oxidation of alcohols by oxygen with the enzyme laccase and mediation by TEMPO

Fabbrini, Maura; Galli, Carlo; Gentili, Patrizia; Macchitella, Daniele

doi:10.1016/s0040-4039(01)01463-0

Cited by 230 publications

(128 citation statements)

References 8 publications

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“…Herein, we report the reactivity of 1 and 2 towards a variety of alcohols and activated alkanes, including xanthene. 10,[16][17][18][19][20][21][22][23][24] The latter substrate is significant, because its reactivity confirms that these oxidations can proceed via a concerted proton coupled electron transfer (CPET) step, as was previously surmised. 14 To test the role of the Lewis acid in activating the TEMPO moiety, we also explored the reactivity of TEMPO with FeBr 3 in Et 2 O.…”

Section: Introductionmentioning

confidence: 90%

Oxidation of Alcohols and Activated Alkanes with Lewis Acid-Activated TEMPO

et al. 2014

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The reactivity of MCl 3 (η 1 -TEMPO) (M = Fe, 1; Al, 2; TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl) with a variety of alcohols, including 3,4-dimethoxybenzyl alcohol, 1phenyl-2-phenoxyethanol, and 1,2-diphenyl-2-methoxyethanol, was investigated using NMR spectroscopy and mass spectrometry. Complex 1 was effective in cleanly converting these substrates to the corresponding aldehyde or ketone. Complex 2 was also able to oxidize these substrates; however, in a few instances the products of overoxidation were also observed. Oxidation of activated alkanes, such as xanthene, by 1 or 2 suggests that the reactions proceed via an initial 1-electron concerted proton−electron transfer (CPET) event. Finally, reaction of TEMPO with FeBr 3 in Et 2 O results in the formation of a mixture of FeBr 3 (η 1 -TEMPOH) ( 23) and [FeBr 2 (η 1 -TEMPOH)] 2 (μ-O) (24), via oxidation of the solvent, Et 2 O.

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Section: Introductionmentioning

confidence: 90%

Oxidation of Alcohols and Activated Alkanes with Lewis Acid-Activated TEMPO

et al. 2014

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“…), preventing their reduction to the original state by the nonphenolic substrate they are supposed to oxidize (Fig. 1, catalytic cycle).As an example, dimerization of the phenolic compound, HAA, once oxidized by laccase, is so fast and quantitative that we exploited the formation rate of this dimer in a new spectrophotometric assay of laccase activity in mixed solvents [11].In the context of our studies [12][13][14][15] on laccase-mediator systems, and of the speculations on the natural role of phenolic mediators [1], we were intrigued by the results of Li et al[9] on the violuric acid-mediated oxidation of phenol red by laccase. It was reported, in fact, that some fungal laccases oxidize phenol red ( Fig.…”

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First evidence of catalytic mediation by phenolic compounds in the laccase‐induced oxidation of lignin models

D’Acunzo

Galli

2003

European Journal of Biochemistry

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The sulfonephthalein indicator, phenol red, exhibits an unusually slow rate of oxidation by laccase from Poliporus pinsitus, in spite of the fact that it is a phenol and therefore a natural substrate for this phenoloxidase enzyme. Nevertheless, after prolonged exposure to laccase (24 h) phenol red is oxidized by more than 90%. We found that phenol red, which can be oxidatively converted into a resonance-stabilized phenoxy radical, performs as a mediator in the laccase-catalyzed oxidation of a nonphenolic substrate (4-methoxybenzyl alcohol) and also of a hindered phenol (2,4,6-tri-tert-butylphenol). In particular, phenol red was found to be at least 10 times more efficient than 3-hydroxyanthranilate (a reported natural phenolic mediator of laccase) in the oxidation of 4-methoxybenzyl alcohol. Other phenols, which do not bear structural analogies to phenol red, underwent rapid degradation and did not perform as laccase mediators. On the other hand, several variously substituted sulfonephthaleins, of different pK 2 values, mediated the laccase catalysis, the most efficient being dichlorophenol red, which has the lowest pK 2 of the series. The mediating efficiency of phenol red and dichlorophenol red was found to be pH dependent, as was their oxidation E p value (determined by cyclic voltammetry). We argue that the relative abundance of the phenoxy anion, which is easier to oxidize than the protonated phenol, may be one of the factors determining the efficiency of a phenolic mediator, together with its ability to form relatively stable oxidized intermediates that react with the desired substrate before being depleted in undesired routes.Keywords: laccase; phenolic mediators; lignin models; radicals; acidity.Laccases (EC 1.10.3.2) are multicopper oxidases, produced by micro-organisms and plants, which participate in nature in both the biosynthesis and degradation of lignin [1]. In the latter case, laccase operates in conjunction with other ligninolitic enzymes, such as lignin peroxidase and manganese peroxidase. The latter two enzymes, which are stronger oxidants than laccase, are able to oxidize most aromatic constituents of lignin, whereas laccase oxidizes directly only the phenolic subunits, which are easier to oxidize but relatively less abundant (15%). It has been speculated [1], however, that laccase may react indirectly with nonphenolic lignin components through mediation by phenolic species present in its natural environment. These Ônatural mediatorsÕ could be metabolites [2], or even lignin fragments [3] generated by the other ligninolitic enzymes, and would open up alternative, possibly radical, routes to the oxidation of nonphenolic components [1]. It is quite reasonable to argue that natural mediators may participate in the laccase-catalyzed oxidation of nonphenolic lignin subunits in those micro-organisms that only rely on laccase for their ligninolitic action [4,5], and a number of substances, such as phenolic acids [6,7] and 3-hydroxyanthranilate (HAA) [8], have indeed been proposed as natural laccase ...

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“…In all these cases the oxoammonium form of TEMPO is involved. In our particular case, laccase would be the catalytic oxidant of TEMPO [17]. Following a nucleophilic attack of the lone-pair of the alcohol onto the TEMPO-oxoammonium ion, the intermediate adduct is deprotonated at the a-C-H benzylic bond by the base-form of the buffer B [17,[25][26][27].…”

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Oxidation of phenols by laccase and laccase‐mediator systems

D’Acunzo

Galli

Masci

2002

European Journal of Biochemistry

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To investigate how solubility and steric issues affect the laccase-catalysed oxidation of phenols, a series of oligomeric polyphenol compounds, having increasing size and decreasing solubility in water, was incubated with laccase. The extent of substrate conversion, and the nature of the products formed in buffered aqueous solutions, were compared to those obtained in the presence of an organic cosolvent, and also in the presence of two mediating species, i.e. N-hydroxyphthalimide (HPI) and 2,2,6,6-tetramethylpiperidin-1-yloxy (TEMPO). This approach showed not only an obvious role of solubility, but also a significant role of the dimension of the substrate upon the enzymatic reactivity. In fact, reactivity decreases as substrate size increases even when solubility is enhanced by a cosolvent. This effect may be ascribed to limited accessibility of encumbered substrates to the enzyme active site, and can be compensated through the use of the appropriate mediator. While TEMPO was highly efficient at enhancing the reactivity of large, less soluble substrates, HPI proved less effective. In addition, whereas the laccase/HPI system afforded the same products as laccase alone, the use of TEMPO provided a different product with high specificity. These results offer the first evidence of the role of Ôoxidation shuttlesÕ that the mediators of laccase may have, but also suggest two promising routes towards an environmentally friendly process for kraft pulp bleaching: (a) the identification of mediators which, once oxidized by laccase, are able to target strategic functional groups present in lignin, and (b) the introduction of those strategic functional groups in an appropriate pretreatment.

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An oxidation of alcohols by oxygen with the enzyme laccase and mediation by TEMPO

Cited by 230 publications

References 8 publications

Oxidation of Alcohols and Activated Alkanes with Lewis Acid-Activated TEMPO

Oxidation of Alcohols and Activated Alkanes with Lewis Acid-Activated TEMPO

First evidence of catalytic mediation by phenolic compounds in the laccase‐induced oxidation of lignin models

Oxidation of phenols by laccase and laccase‐mediator systems

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