Catalytic Efficiency of some Mediators in Laccase-Catalyzed Alcohol Oxidation

An Organic Chemist's Guide to Mediated Laccase Oxidation

Laccases are oxidases that only require O₂ as a terminal oxidant. Thus, they provide an attractive green alternative to established alcohol oxidation protocols. However, laccases typically require catalytic amounts of mediator molecules to serve as electron shuttles between the enzyme and desired substrate. Consequently, laccase-mediator systems are defined by a multitude of parameters such as, e. g., the choice of laccase and mediator, the respective concentrations, pH, and the oxygen source. This complexity and a perceived lack of comparable data throughout literature represent an entry burden into this field. To provide a solid starting point, particularly for organic chemists, we herein provide a time-resolved, quantitative laccase and mediator screening based on the oxidation of anis alcohol as model reaction. We measured the redox potentials of mediators under the reaction conditions to relate them to their performance. Lastly, for particularly efficient laccase-mediator pairs, we screened important reaction parameters, resulting in an optimized setup for mediator-assisted laccase catalyzed oxidations.

Depolymerization of biorefinery lignin by improved laccases of the white-rot fungus Obba rivulosa

Fungal laccases are attracting enzymes for sustainable valorization of biorefinery lignins. To improve the lignin oxidation capacity of two previously characterized laccase isoenzymes from the white-rot fungus Obba rivulosa, we mutated their substrate-binding site at T1. As a result, the pH optimum of the recombinantly produced laccase variant rOrLcc2-D206N shifted by three units towards neutral pH. O. rivulosa laccase variants with redox mediators oxidized both the dimeric lignin model compound and biorefinery poplar lignin. Significant structural changes, such as selective benzylic α-oxidation, were detected by nuclear magnetic resonance analysis, although no polymerization of lignin was observed by gel permeation chromatography. This suggests that especially rOrLcc2-D206N is a promising candidate for lignin-related applications.

Applicability of Recombinant Laccases From the White-Rot Fungus Obba rivulosa for Mediator-Promoted Oxidation of Biorefinery Lignin at Low pH

Utilization of lignin-rich side streams has been a focus of intensive studies recently. Combining biocatalytic methods with chemical treatments is a promising approach for sustainable modification of lignocellulosic waste streams. Laccases are catalysts in lignin biodegradation with proven applicability in industrial scale. Laccases directly oxidize lignin phenolic components, and their functional range can be expanded using low-molecular-weight compounds as mediators to include non-phenolic lignin structures. In this work, we studied in detail recombinant laccases from the selectively lignin-degrading white-rot fungus Obba rivulosa for their properties and evaluated their potential as industrial biocatalysts for the modification of wood lignin and lignin-like compounds. We screened and optimized various laccase mediator systems (LMSs) using lignin model compounds and applied the optimized reaction conditions to biorefinery-sourced technical lignin. In the presence of both N-OH-type and phenolic mediators, the O. rivulosa laccases were shown to selectively oxidize lignin in acidic reaction conditions, where a cosolvent is needed to enhance lignin solubility. In comparison to catalytic iron(III)-(2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) oxidation systems, the syringyl-type lignin units were preferred in mediated biocatalytic oxidation systems.

Laccase complex with polyvinylamine bearing grafted TEMPO is a cellulose adhesion primer

Polyelectrolyte complexes formed between laccase and polyvinylamine with grafted TEMPO moieties, PVAm-T, adsorb onto cellulose, causing oxidation. All evidence supports the view that aldehyde groups on oxidized cellulose condense with primary amine groups, giving a grafted layer of PVAm-T complexed with laccase. The grafted PVAm-T serves as a primer layer promoting wet cellulose-to-cellulose adhesion in the presence of PVAm adhesive. The cellulose modification occurs at ambient temperatures and pH 5. The adhesion improvements with mixtures of PVAm-T and laccase are remarkable because both components are macromolecular, which should inhibit the ability of the TEMPO to act as a shuttle between the enzyme and the primary hydroxyl groups on cellulose. It is proposed that PVAm-bound oxoammonium ions exchange with neighboring TEMPO moieties, providing a mechanism for the transfer of oxidation activity from immobilized enzyme to the cellulose surfaces.

Laccases and their natural mediators: biotechnological tools for sustainable eco-friendly processes

Laccases are oxidoreductases which oxidize a variety of aromatic compounds using oxygen as the electron acceptor and producing water as by-product. The interest for these old enzymes (first described in 19th century) has progressively increased due to their outstanding biotechnological applicability. The presence of redox mediators is required for a number of biotechnological applications, providing the oxidation of complex substrates not oxidized by the enzyme alone. The efficiency of laccase-mediator systems to degrade recalcitrant compounds has been demonstrated, but still the high cost and possible toxicity of artificial mediators hamper their application at the industrial scale. Here, we present a general outlook of how alternative mediators can change this tendency. We focus on phenolic compounds related to lignin polymer that promotes the in vitro transformation of recalcitrant non-phenolic structures by laccase and are seemingly the natural mediators of laccases. The use of eco-friendly mediators easily available from lignocellulose, could contribute to the industrial implementation of laccases and the development of the 21th century biorefineries.

Hydrogen atom transfer reactions of a ruthenium imidazole complex: hydrogen tunneling and the applicability of the Marcus cross relation

The reaction of Ru(II)(acac)2(py-imH) (Ru(II)imH) with TEMPO(*) (2,2,6,6-tetramethylpiperidine-1-oxyl radical) in MeCN quantitatively gives Ru(III)(acac)2(py-im) (Ru(III)im) and the hydroxylamine TEMPO-H by transfer of H(*) (H(+) + e(-)) (acac = 2,4-pentanedionato, py-imH = 2-(2'-pyridyl)imidazole). Kinetic measurements of this reaction by UV-vis stopped-flow techniques indicate a bimolecular rate constant k(3H) = 1400 +/- 100 M(-1) s(-1) at 298 K. The reaction proceeds via a concerted hydrogen atom transfer (HAT) mechanism, as shown by ruling out the stepwise pathways of initial proton or electron transfer due to their very unfavorable thermochemistry (Delta G(o)). Deuterium transfer from Ru(II)(acac)2(py-imD) (Ru(II)imD) to TEMPO(*) is surprisingly much slower at k(3D) = 60 +/- 7 M(-1) s(-1), with k(3H)/k(3D) = 23 +/- 3 at 298 K. Temperature-dependent measurements of this deuterium kinetic isotope effect (KIE) show a large difference between the apparent activation energies, E(a3D) - E(a3H) = 1.9 +/- 0.8 kcal mol(-1). The large k(3H)/k(3D) and DeltaE(a) values appear to be greater than the semiclassical limits and thus suggest a tunneling mechanism. The self-exchange HAT reaction between Ru(II)imH and Ru(III)im, measured by (1)H NMR line broadening, occurs with k(4H) = (3.2 +/- 0.3) x 10(5) M(-1) s(-1) at 298 K and k(4H)/k(4D) = 1.5 +/- 0.2. Despite the small KIE, tunneling is suggested by the ratio of Arrhenius pre-exponential factors, log(A(4H)/A(4D)) = -0.5 +/- 0.3. These data provide a test of the applicability of the Marcus cross relation for H and D transfers, over a range of temperatures, for a reaction that involves substantial tunneling. The cross relation calculates rate constants for Ru(II)imH(D) + TEMPO(*) that are greater than those observed: k(3H,calc)/k(3H) = 31 +/- 4 and k(3D,calc)/k(3D) = 140 +/- 20 at 298 K. In these rate constants and in the activation parameters, there is a better agreement with the Marcus cross relation for H than for D transfer, despite the greater prevalence of tunneling for H. The cross relation does not explicitly include tunneling, so close agreement should not be expected. In light of these results, the strengths and weaknesses of applying the cross relation to HAT reactions are discussed.

Kinetic Study of the Hydrogen Abstraction Reaction of the Benzotriazole-N-oxyl Radical (BTNO) with H-Donor Substrates

[Reaction: see text]. The aminoxyl radical (>N-O*) BTNO (benzotriazole-N-oxyl) has been generated by the oxidation of 1-hydroxybenzotriazole (HBT; >N-OH) with a Ce(IV) salt in MeCN. BTNO presents a broad absorption band with lambda(max) 474 nm and epsilon 1840 M(-1) cm(-1), and spontaneously decays with a first-order rate constant of 6.3 x 10(-3) s(-1) in MeCN at 25 degrees C. Characterization of BTNO radical by EPR, laser flash photolysis, and cyclic voltammetry is provided. The spontaneous decay of BTNO is strongly accelerated in the presence of H-donor substrates such as alkylarenes, benzyl and allyl alcohols, and alkanols, and rate constants of H-abstraction by BTNO from a number of substrates have been spectroscopically investigated at 25 degrees C. The kinetic isotope effect confirms the H-abstraction step as rate-determining. Activation parameters have been measured in the 15-40 degrees C range with selected substrates. A correlation between E(a) and BDE(C-H) (C-H bond dissociation energy) for a small series of H-donors has been obtained according to the Evans-Polanyi equation, giving alpha = 0.44. From this plot, the experimentally unavailable BDE(C-H) of benzyl alcohol can be extrapolated, as ca. 79 kcal/mol. With respect to the H-abstraction step, peculiar differences in the DeltaS++ parameter emerge between an alkylarene, ArC(H)R2, and a benzyl alcohol, ArC(H)(OH)R. The data acquired on the H-abstraction reactivity of BTNO are compared with those recently reported for the aminoxyl radical PINO (phthalimide-N-oxyl), generated from N-hydroxyphthalimide (HPI). The higher reactivity of radical PINO is explained on the basis of the higher energy of the NO-H bond of HPI, as compared with that of HBT (88 vs ca. 85 kcal/mol, respectively), which is formed on H-abstraction from the RH substrate.

Comparison of different fungal enzymes for bleaching high-quality paper pulps

Wild and recombinant hydrolases and oxidoreductases with a potential interest for environmentally sound bleaching of high-quality paper pulp (from flax) were incorporated into a totally chlorine free (TCF) sequence that also included a peroxide stage. The ability of feruloyl esterase (from Aspergillus niger) and Mn2+-oxidizing peroxidases (from Phanerochaete chrysosporium and Pleurotus eryngii) to decrease the final lignin content of flax pulp was shown. Laccase from Pycnoporus cinnabarinus (without mediator) also caused a slight improvement of pulp brightness that was increased in the presence of aryl-alcohol oxidase. However, the best results were obtained when the laccase treatment was performed in the presence of a mediator, 1-hydroxybenzotriazol (HBT), enabling strong delignification of pulps. The enzymatic removal of lignin resulted in high-final brightness values that are difficult to attain by chemical bleaching of this type of pulp. A partial inactivation of laccase by HBT was observed but this negative effect was strongly reduced in the presence of pulp. The good results obtained with the same laccase expressed in A. niger at bioreactor scale, revealed the feasibility of using recombinant laccase for bleaching high-quality non-wood pulps in the presence of a mediator.

Kinetics of oxidation of benzyl alcohols by the dication and radical cation of ABTS. Comparison with laccase–ABTS oxidations: an apparent paradox

Laccase, a blue copper oxidase, in view of its moderate redox potential can oxidise only phenolic compounds by electron-transfer. However, in the presence of ABTS (2,2'-azinobis(3-ethylbenzthiazoline-6-sulfonate) as a redox mediator, laccase reacts with the more difficult to oxidise non-phenolic substrates, such as benzyl alcohols. The role of ABTS in these mediated oxidations is investigated. Redox interaction with laccase could produce in situ two reactive intermediates from ABTS, namely ABTS++ or ABTS*+. These species have been independently generated by oxidation with Ce(iv) or Co(iii) salts, respectively, and their efficiency as monoelectronic oxidants tested in a kinetic study towards a series of non-phenolic substrates; a Marcus treatment is provided in the case of ABTS++. On these grounds, intervention of ABTS++ as a reactive intermediate in laccase-ABTS oxidations appears unlikely, because the experimental conditions under which ABTS++ is unambiguously generated, and survives long enough to serve as a diffusible mediator, are too harsh (2 M H2SO4 solution) and incompatible with the operation of the enzyme. Likewise, ABTS*+ seems an intermediate of limited importance in laccase-ABTS oxidations, because this weaker monoelectronic oxidant is unable to react directly with many of the non-phenolic substrates that laccase-ABTS can oxidise. To solve this paradox, it is alternatively suggested that degradation by-products of either ABTS++ or ABTS*+ are formed in situ by hydrolysis during the laccase-ABTS reactions, and may be responsible for the observed oxidation of non-phenolics.