Baciocchi Enrico, Fabbri Claudia, Lanzalunga Osvaldo
Dipartimento di Chimica, Università La Sapienza, Piazzale A. Moro 5, I-00185 Rome, Italy.
J Org Chem. 2003 Nov 14;68(23):9061-9. doi: 10.1021/jo035052w.
The H(2)O(2)-promoted oxidations of the two nonphenolic beta-O-aryl lignin model trimers 1 and 2, catalyzed by lignin peroxidase (LiP) at pH = 3.5, have been studied. The results have been compared with those obtained in the oxidation of 1 and 2 with the genuine one-electron oxidant potassium 12-tungstocobalt(III)ate. These models present a different substitution pattern of the three aromatic rings, and by one-electron oxidation, they form radical cations with the positive charge, which is localized in the dialkoxylated ring as also evidenced by a pulse radiolysis study. Both the oxidations with the enzymatic and with the chemical systems lead to the formation of products deriving from the cleavage of C-C and C-H bonds in a beta position with respect to the radical cation with the charge residing in the dialkoxylated ring (3,4-dimethoxybenzaldehyde (5) and a trimeric ketone 6 in the oxidation of 1 and a dimeric aldehyde 8 and a trimeric ketone 9 in the oxidation of 2). These products are accompanied by a dimeric aldehyde 7 in the oxidation of 1 and 4-methoxybenzaldehyde (10) in the oxidation of 2. The unexpected formation of these two products has been explained by suggesting that 1.+ and 2.+ can also undergo an intramolecular electron transfer leading to the radical cations 1a.+ and 2a.+ with the charge residing in a monoalkoxylated ring. The fast cleavage of a C-C bond beta to this ring, leading to 7 from 1.+ and to 10 from 2.+, is the driving force of the endoergonic electron transfer. A kinetic steady-state investigation of the LiP-catalyzed oxidation of the trimer 2, the dimeric model 1-(3,4-dimethoxyphenyl)-2-phenoxy-1-ethanol (4), and 3,4-dimethoxybenzyl alcohol (3) has indicated that the turnover number (k(cat)) and the affinity for the enzyme decrease significantly by increasing the size of the model compound. In contrast, the three substrates exhibited a very similar reactivity toward a chemical oxidant [Co(III)W]. This suggests a size-dependent interaction of the enzyme with the substrate which may influence the efficiency of the electron transfer.
研究了在pH = 3.5时，木质素过氧化物酶（LiP）催化的H₂O₂促进的两种非酚型β - O - 芳基木质素模型三聚体1和2的氧化反应。将结果与用真正的单电子氧化剂十二钨钴（III）酸钾氧化1和2所得到的结果进行了比较。这些模型呈现出三个芳环不同的取代模式，通过单电子氧化，它们形成带有正电荷的自由基阳离子，脉冲辐解研究也证明该正电荷定域在二烷氧基化环上。酶促氧化体系和化学氧化体系都导致形成了相对于电荷位于二烷氧基化环上的自由基阳离子在β位的C - C和C - H键断裂产生的产物（1氧化时生成3,4 - 二甲氧基苯甲醛（5）和三聚体酮6，2氧化时生成二聚体醛8和三聚体酮9）。在1的氧化反应中这些产物伴有二聚体醛7，在2的氧化反应中伴有4 - 甲氧基苯甲醛（10）。这两种产物的意外形成可以解释为，1⁺和2⁺也可以发生分子内电子转移，导致电荷位于单烷氧基化环上的自由基阳离子1a⁺和2a⁺。该环β位C - C键的快速断裂，从1⁺生成7，从2⁺生成10，是吸能电子转移的驱动力。对LiP催化的三聚体2、二聚体模型1 - (3,4 - 二甲氧基苯基)-2 - 苯氧基 - 1 - 乙醇（4）和3,4 - 二甲氧基苄醇（3）氧化反应的动力学稳态研究表明，通过增加模型化合物的尺寸，周转数（kcat）和对酶的亲和力显著降低。相反，这三种底物对化学氧化剂[Co(III)W]表现出非常相似的反应性。这表明酶与底物之间存在尺寸依赖性相互作用，这可能会影响电子转移效率。
