Catechol structural model

Liu, C., and Huang, P. M. (2001). The influence of catechol humification on surface properties of metal oxides. In Humic Substances Structure, Models and Functions, Ghabbour, E. A. and Davies, G, eds., Royal Society of Chemistry, Cambridge, UK, pp. 253-270. 

The complexes obtained can be considered as structural models of two states of catechol oxidase the native met state and the reduced deoocy state. In order to investigate whether the dicopper(II) complex also possesses the functionality of the natural enzyme, for example, if it can perform the catalytic oxidation of cate-... 

Fig. 45. Structural models of adsorbed molecules at Pt(100) and Pt(lll) surfaces, (a) L-Dopa (b) L-Tyrosine (c) L-Cysteine (d) L-Phenylalanine (e) L-Alanine (f) Dopamine (g) Catechol. Reprinted from ref. 84.

As compared to the oxygenation reaction of phenols to catechols (phenolase reaction), dehydrogenation of catechols to the corresponding o-quinones (catecholase reaction) proceeds more readily. Thus, the catalytic activity of several tyrosinase and catechol oxidase models have been examined using 2,4-di-tert-butylcatechol (DTBC) as a substrate.Direct reactions between the (/r-77 77 -peroxo)dicopper(II) complexes and DTBC also have been studied at a low tempera-and a semiquinone-copper(II) complex has been isolated and structurally characterized... 

Such a mechanism, the reduction of a-TOH at Hpid-water interfaces, encompasses the exportation of the radical character from the lipid to the water phase via the rapid elimination of a-tocopheroxyl radical by an amphiphilic co-antioxidant and affords a high antioxidant protection to LDL (Laranjinha et al, 1995). This has been subsequently verified for distinct compounds in several different models (Witting et al, 1996 Carbonneau et al, 1997 Zhu et al, 1999, Pedrielli Skibsted, 2002 Mukai et al, 2005 Zhou et al, 2005), and evidences for such occurrence in vivo have been forwarded. For instance, it has been observed that lipoproteins from caffeic acid-fed rats were markedly resistant to oxidative modification and that dietary supplementation with caffeic acid resulted in a statistically significant increase of a-TOH, both in plasma and in lipoproteins (Nardini et al, 1997 Zhou et al, 2005 Frank et al, 2006). More recently, it was shown that diet supplementation with flavonoids exhibiting a catechol structure (quercetin, epicatechin, and catechin) resulted in a substantial increase in a-TOH concentrations in blood plasma and liver tissue of male Sprague-Dawley rats by a mechanism likely involving the reduction of a-tocopheroxyl radical by the flavonoids (Frank et al, 2006). 

This result indicates that modification of ligands provides good functional models for catechol dioxygenases. Recently, both functional and structural model studies for Fe -enzymes have started. This is a challenge to the model studies for the V 102 systems which seem to have the higher barrier to be overcome than the Fe /02 system. It is expected that catalytic systems for selective extradiol oxygenation of catechols by iron complexes will be developed in future. 

Catechol 1,2-dioxygenases had been extensively studied from viewpoints of structural models, e.g. by using Fe(salen)OAc (vide post). These complexes are inert for... 

Structural models for substrateAron binary intermediates of intradioTcleaving catechol dioxygenases... 

Structural models for substrate-iron binary intermediates of extradiol-cleaving catechol dioxygenases... 

Figure 3. Structural models of adsorbed molecules at Pt(lOO) and Pt(lll) surfaces. (A) L-dopa (LD) (B) L-Tyrosine (TYR) (C) L-cysteine (CYS) (D) L-phenylalanine (PHE) (E) L-alanine (ALA) (F) dopamine (DA) (G) catechol (CT) (H) 3,4-dihydroxyphenyl-acetic acid (DOPAC).

Catechol oxidase and tyrosinase are among the enzymes that have Type 3 dinuclear centres. However, the prototype of this class of proteins is the invertebrate oxygen transport protein, haemocyanin (Figure 14.5), for which structures of the oxy and deoxy forms have been determined at high resolution and confirm, as predicted from model compounds, that the... 

Tyrosinase is a monooxygenase which catalyzes the incorporation of one oxygen atom from dioxygen into phenols and further oxidizes the catechols formed to o-quinones (oxidase action). A comparison of spectral (EPR, electronic absorption, CD, and resonance Raman) properties of oxy-tyrosinase and its derivatives with those of oxy-Hc establishes a close similarity of the active site structures in these proteins (26-29). Thus, it seems likely that there is a close relationship between the binding of dioxygen and the ability to "activate" it for reaction and incoiporation into organic substrates. Other important copper monooxygenases which are however of lesser relevance to the model studies discussed below include dopamine p-hydroxylase (16,30) and a recently described copper-dependent phenylalanine hydroxylase (31). 

Fukuzumi and co-workers described spectroscopic evidence for a ix-rf- ] -peroxo-(Cu )2 species stabilized with a fcidentate nitrogen ligand, but no (catalytic) oxidation behavior towards catechol was noted (a related trinu-clear copper species converted 2,4-di-ferf-butylphenol stoichiometrically towards the biphenol derivative) [224], Stack et al. have described a similar ] -peroxo-(Cu )2 species (28, vide supra) that could be considered a structural and functional model for tyrosinase-activity, as it efficiently reacted with catechol, benzyl alcohol and benzylamine to yield quinone (95%), benzaldehyde (80%) and benzonitrile (70%) [172,173]. This dinuclear per-0X0 species is generated by association of two monomeric copper centers, in contrast to the systems based on dinucleating Ugand scaffolds described above. 

Models for human and pig COMT are easy to build using the experimental structure of the rat COMT, due to the high degree of homology between the rat, human, and pig COMT enzymes (Figure 4). The active sites are especially well conserved—the few differences in the active-site residues are collected in Table 2. The kinetic data show that the Km values of common substrates for rat and human COMT are very similar. Pig COMT shows, however, a considerably higher Km value for catechol [46]. The same difference is apparent for inhibitors represented by the K values in Table 1. 

Humic acido from ooilo and ligniteo have been examined by ERR spectrometry. All samples showed a stable free organic radical content of about 1018 spins per gram. When these samples were converted to their sodium salts, a marked increase in radical content occurred. This was interpreted to indicate that a quinhydrone moiety exists in the humic acid macromolecule. Synthetic humic acid, prepared by oxidizing catechol in the presence of amino acids, also showed similar ERR spectra, as did selected quinhydrone model compounds. The radical moiety appeared to be stable to severe oxidation and hydrolytic conditions. Reduction in basic media caused an initial decrease in radical species continued reduction generated new radical species. A proposed model for humic acid based on a hydroxyquinone structure is proposed. 

Figure 14 Structure of the cluster model of catechol adsorbed on Ti02 (anatase) particle (left). Calculated absorption threshold of catechol sensitized Ti02 as a function of wavelength in nm (right). The solid line refers to a catechol sensitized Ti02 cluster, while the dashed line refers to a bare TiC>2 cluster.

Figure 2.3 Molecular structure of [Fe(Me3TACN)(DBC)CI], a model complex for a catechol dioxygenase coordinated to its substrate molecule [28]. Hydrogen atoms have been omitted for clarity. The l,4,7-trimethyl-l,4,7-triazacyclononane (Me3TACN) ligand coordinates facially to the iron center. The remaining three coordination sites are occupied by 3,5-di-tert-butylcatecholate (DBC) and a chlorido ligand.

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