New mediators for the enzyme laccase: mechanistic features and selectivity in the oxidation of non-phenolic substrates

Abstract

New mediators of laccase have been comparatively evaluated and ranked towards the benchmark aerobic oxidation of p-MeO-benzyl alcohol. The mechanism of oxidation of this non-phenolic substrate by each mediator, which is initially oxidised by laccase to the Medox form, has been assessed among three alternatives. The latter make the phenoloxidise laccase competent for the indirect oxidation of non-phenolic (and thus ‘unnatural’) substrates. Experimental characterisation of the mediators, by means of spectrophotometric, electrochemical and thermochemical survey, is reported. Clear-cut evidence for the formation of a benzyl radical intermediate in the oxidation of a particular benzyl alcohol with laccase and a {N–OH mediator is attained by means of a trapping experiment. The selectivity of the laccase-catalysed oxidation of two competing lignin and polysaccharide model compounds has been assessed by using the highly proficient 4-MeO-HPI mediator, and found very high in favour of the former model. This evidence is in keeping with the operation of a radical hydrogen-abstraction process that efficiently cleaves the benzylic rather than the aliphatic C–H bond of the two models. Significant is the finding that catechol, i.e., a model of recurring phenolic structures in lignin, once oxidised to aryloxyl radical by laccase is capable to mediate a radical oxidation of non-phenolic compounds. This supports a fully-fledged role of laccase as a delignifying enzyme in nature by way of no other mediators than the very phenolic groups of lignin. Finally, an evaluation of the dissociation energy of the NO–H bond of HBT, which is not accessible experimentally, is provided by the use of a thermochemical cycle and theoretical calculations

Hydrogen abstraction from H‐donor substrates by the 6‐CF3‐benzotriazol‐N‐oxyl radical (TFNO)

“…In fact, there is no doubt that PINO is more reactive than BTFN, in keeping with the higher energy of the NO-H bond it forms on Habstraction (88 vs 84 kcal/mol). [10] The BDE NO-H of TFBT and HBT are instead intermediate in value between the two above cases and indeed very close together, [10,22] so that it is a little puzzling that TFNO comes out three-to-ten fold more reactive than BTNO. However, because the >N-O .…”

“…[21] Because of the limited stability of TFNO, we have not been able to carry out the determination of the energy value of the NO-H bond of TFBT by EPR equilibration methods, [22] but nevertheless have attained an evaluation of that BDE NO-H as 86±1 kcal/mol from resorting to a thermochemical cycle. [23] By and large, all the evidence collected on TFNO is in keeping with the formerly reported characterization of the parent BTNO species from HBT (Table 1).…”

“…The intermolecular kinetic isotope effect for bis α-deuteriated-Xsubstituted benzyl alcohols ( Products study with probe substrate (1). As described previously, [18,22,33] 60 mmol of substrate 1, 20 mmol of TFBT and 20 mmol of CAN were let to react at room temperature in 3 mL MeCN. After a 5 h reaction time and conventional workup with ethyl acetate, GC-MS analysis enabled us to detect the formation of ketone 3 and the absence of aldehyde 2; no other products besides unreacted 1 were detected.…”

“…The BDE NO-H of TFBT has been obtained from the following thermochemical cycle (pertinent data in Table 1, to be converted into kcal/mol), the value of E 2° being 54 kcal/mol: [22,23] [9,23] BDE NO-H = pK a (R 2 NOH) + E 1° (R 2 NO . / R 2 NO -)…”

Nitroxide: Anwendungen in der Synthese und in der Polymerchemie

“…So wurde beispielsweise die Reaktion von 4-Hydroxy-TEMPO (45) [136] Aufgrund der eher schwachen O-H-Bindung in TEMPOH (69 kcal mol À1 ) [137] erscheint die H-Abstraktion durch TEMPO jedoch nicht effektiv genug, und die Reaktion muss mit einem großen Überschuss an erhitztem Cyclohexen durchgeführt werden, das in diesem Fall als Lösungsmittel vorlag. Durch photochemische Anregung kann TEMPO in ein weitaus reaktiveres O-zentriertes Radikal überführt werden, das unter diesen Bedingungen in der Lage ist, HAtome aus Acetonitril und Toluol zu abstrahieren.…”