Catechol model systems

Scheme 15 Possible reaction pathways in the catalytic cycle of dicopper(II) catechol oxidase model systems [182]...

Breslow s P-cyclodextrine ribonuclease model system represents one of the best examples concerning the construction of small enzyme-like molecules [33]. Breslow functionalized the P-cyclodextrine with two imidazole moieties (Figure 10.1). Selectively, catechol cyclic phosphate carrying a 4-tert-butyl group (Figure 10.1a) binds into the cavity of the catalyst (Figure 10. lb) in water solution, and is then hydrolyzed by the... 

The substrate analog and spectroscopic studies led Solomon et al. to suggest a mechanism for the Tyr cresolase activity (Figure 2) [22], Here, a phenol substrate could bind initially to oxy-Tyr in an axial fashion, a possibility confirmed in model studies [28,29], In this ternary Cu2/02/substrate complex, rearrangement through a trigonal bipyramidal intermediate could be accompanied by ortho-hy-droxylation, followed by loss of water and coordination of the diphenol product. Such a catecholate dicopper(II) complex is known in model systems [30], Intramolecular electron transfer would result in release of product o-quinone and the dicopper(I) produced could react with 02 again to produce oxy-Tyr. 

Model Systems of Catechol Oxidase Historic Overview... 

Figure 37.3. The smallest measurable particle sizes for biogenic silica in the branches of Equisetum arvense [black bars], silica produced in a model system from a silicon catecholate complex [striped bars], and silica produced in a model system in the presence of a biosilica protein containing extract from Equisetum arvense [hatched bars].

Step behind their counterparts, the intradiol-cleaving dioxygenases. In particular, no iron(II) catecholate complexes have been reported to carry out the oxygenation chemistry of the enzymes. The development of model systems that proceed via the same pathway as the enzymes presents itself as the next challenge in the effective development of functional model systems. 

Model studies and much of the developmental research on synthetic catechol oxidation systems has been concentrated on iron and copper catecholate complexes. 

DTBSQ occurs also without an added catalyst. This raises the question of how far this mechanism may be involved in catechol dioxygenase model systems. The semiquinone radical is supposed to be an intermediate of 3,5-DTBC oxidation to the quinone. Once there is a path for its formation the reactions of Scheme 11 may also start to occur and contribute to the overall diol cleavage process. Thus it seems that diagnostic tests are required for reactions of Scheme 11, perhaps via... 

Despite a typical substrate in the enzymatic system, protocatechuic acid was found to be oxygenated by model complexes not in organic solvents but in water. The highly selective and catalytic intradiol oxygenation of 4-fm-butylcatechol indicated that various types of catechols other than 3,5-di-fert-butylcatechol can be used as substrates in the model systems and that oxygenations by nonheme iron systems in water are attractive. The reactivity and selectivity are dependent on the substituent on catechols and pH of the solution. 

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