New mediators for the enzyme laccase: mechanistic features and selectivity in the oxidation of non-phenolic substrates
New mediators for the enzyme laccase: mechanistic features and selectivity in the oxidation of non-phenolic substrates
复制标题
DOI：
10.1039/b507657a
复制
发表时间：
2005-10-01
影响因子：
3.3
通讯作者：
Lanzalunga, O
中科院分区：
文献类型：
--
作者：
Astolfi, P;Brandi, P;Lanzalunga, O
对漆酶的新介质进行了比较评价，并对对甲基苯甲醇的基准有氧氧化进行了排序。每种介质氧化这种非酚类底物的机制，最初由漆酶氧化为Med（ox）形式，已经在三种替代方案中进行了评估。后者使酚氧化酶能够间接氧化非酚类（因此是“非自然的”）底物。用分光光度法、电化学法和热化学法对这些介质进行了实验表征。通过诱捕实验，获得了在漆酶和N-OH介质氧化特定苯甲醇时形成苄基中间体的明确证据。漆酶催化氧化两种相互竞争的木质素和多糖模型化合物的选择性通过使用高度精通的4-MeO-HPI介质进行了评估，并发现前者模型非常有利。这一证据与自由基抽氢过程的操作一致，该过程有效地分裂了两种模型的苯基而不是脂肪族的碳氢键。重要的是发现儿茶酚，即木质素中反复出现的酚结构模型，一旦被漆酶氧化为芳氧基自由基，就能够介导非酚类化合物的自由基氧化。这支持了漆酶作为一种脱木质素酶在自然界中完全成熟的作用，没有其他介质，而不是木质素的酚类。最后，利用热化学循环和理论计算，对HBT的NO-H键的离解能进行了评估，这是实验无法获得的。
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 Med(ox) 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 thermochernical 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.