Oxidation catechols

A. Naidja, and P. M. Huang, Significance of the Henri-MichaeUs-Menten theory in abiotic catalysis Catechol oxidation by 5-Mn02, Surf. Sci. 506(1-2), L243-L249 (2002). 

While only tyrosinase catalyzes the ortho-hydroxylation of phenol moieties, both tyrosinase and catechol oxidase mediate the subsequent oxidation of the resulting catechols to the corresponding quinones. Various mono- and dinu-clear copper coordination compounds have been investigated as biomimetic catalysts for catechol oxidation [21,194], in most cases using 3,5-di-tert-butylcatechol (DTBC) as the substrate (Eq. 16). The low redox potential of DTBC makes it easy to oxidize, and its bulky tert-butyl groups prevent un-... 

Particularly high efficiency in catechol oxidation was achieved with some Cu complexes based on aminocarbohydrate ligands [222,223]. 

With tyrosinase, on the contrary, a two-electron oxidation occurs, as no EPR signal was detected in the catechol oxidation at pH 5.3 Melanins are polymerization products of tyrosine, whereby tyrosinase catalyses the first steps the formation of dopa (3,4-dihydroxyphenylalanine) and of dopaquinone, leading to an indolequi-none polymer The peroxidase mechanism for the conversion of tyrosine into dopa in melanogenesis was not substantiated In natural and synthetic melanins free radicals of a semiquinone type were detected by EPR 4-10 x 10 spins g of a hydrated suspension (the material was modified on drying and the number of free spins increased). The fairly symmetrical EPR signal had a g-value of 2.004 and a line-width of 4-10 G The melanins seem to be natural radical scavengers. 

Harayama, S., Rekik, M., Wasserfallen, A. Bairoch, A. (1987). Evolutionary relationship between catabolic pathways for aromatics conservation of gene order and nucleotide sequences for catechol oxidation genes of pWWO and NAH7 plasmids. Molecular General Genetics, 210, 241-7. 

The positive E° values of the overall redox reactions indicate that the reactions are thermodynamically feasible, and catechol oxidation can thus be accelerated by Fe... 

Figure 2.21. Mechanisms of the oxidative polymerization of catechol to melanins (humic polymers) in the presence of tyrosinase or birnessite. Reprinted with permission from Naidja, A., Huang, P. M., Dec, J., and Bollag, J.-M. (1999). Kinetics of catechol oxidation catalyzed by tyrosinase or 8-Mn02. In Effect of Mineral-Organic-Microorganism Interactions on Soil and Freshwater Environments, Berthelin, J., Huang, P. M., Bollag, J.-M., and Andreux, F., eds., Kluwer Academic/Plenum Publishers, New York, 181-188.

Naidja, A., and Huang, P. M. (2002). Significance of the Henri-Michaelis-Menten theory in abiotic catalysis catechol oxidation by 8-Mn02. Surface Sci. 506, L243-L249. 

Scheme 5.2 The possible reaction pathways in the catalytic cycle of catechol oxidation by dicopper(ll) complexes, as proposed by Casella and co-workers. Redrawn after Casella etal. [38],...

Figure 5.15 The proposed mechanism for the catalytic catechol oxidation by [Cu2([22]pr4pz)(C03)(H20)]2+[48].

In conclusion, the mechanism of catechol oxidation by the model compounds is very intricate, which obviously explains often contradictory literature reports on the catalytic behavior of copper(II) complexes. However, despite being sometimes controversial, studies on model compounds offer stimulating results, which improve our knowledge of the structure-activity relationships in natural systems. There is little doubt that the combination of distinct but complementary disci-... 

Catechol oxidation catalyzed by peroxidases can be used not only for the synthesis of sulfur-substituted catechols but also for the preparation of synthetic compounds related to pheomelanins, which contain benzothiazine units. In fact, the quinone undergoes an extremely easy nucleophilic addition by thiols. For example, treating the neurotransmitter dopamine with cysteine, in the presence of HRP/H2O2, gives rise to 2-S- and 5-5-cysteinyl-catecholamine and a smaller amount of the 2-S,5-S,-di-cysteinyl-catecholamme conjugate [48, 49] (Fig. 6.3e). 

Because of its bioisosteric similarity to the normal physiological substrate L-dopa (4), L-mimosine (5) inhibits catechol oxidation by the enzyme tyrosinase (7). These compounds exemplify a situation in which bioisosteresdis-play opposite pharmacologic effects at the same receptor. 

The decomposition of the intermediate in the catechol oxidation is first-order and is subject to catalysis by hydrogen ions, viz. 

Results for these catechols taken from Reference 139, using inhibited oxidation of styrene initiated by AIBN. Earlier n factors for catechols of 2-3 were attributed to reactions of the initial catechol-oxidation products with peroxyl radical. ... 

Various kinetic experiments and product analyses indicate that the catechol oxidation catalyzed by the bispidine-copper(ll) complexes proceeds via the mechanism shown in Scheme 19. There is one quinone product per cycle and dioxygen is reduced to hydrogen peroxide. Interestingly, all individual steps (1 2 3 1) are relatively fast... 

The hydroxylation of DPQ (steps F-H in Figure 7) is mimicked by the copper(II)-catalyzed aqueous autooxidation of 4-alkylcatechols to 2-alkyl-5-hydroxybenzoquinones (Figure 10). The role of the catalyst here is to promote oxidation of the catechol precursor the subsequent hydrolysis step is promoted by base, but not by copper. Interestingly, the hydrolysis is in fact a result of 1,4-addition of H2O2 to C5 of the catechol, rather than H2O (Figure 10) " H2O2 is produced in the reaction mixture as a by-product of catechol oxidation. Hence, this apparently simple synthetic reaction does not accurately model the biogenesis of TPQ, in which the C2 O atom in the final product is derived from water. 

Scheme 9. Reversible oxygenation, and phenol hydroxylation and catechol oxidation promoted by the complex [Cu2(L66)] at low temperatures.

Scheme 13. Oxygenation, phenol hydroxylation, and catechol oxidation by the complex [Cu(DBED)] at —80°C.

The initial step in the catechol oxidation is fast, basically for the high reactivity of the copper(II) complex toward DTBCH2. Since quinone formation occurs after a pre-equilibrium binding of the substrate, the reaction... 

---