Catechol coordination

Figure 9 Possible protonation schemes of tris(catecholate) metal complexes. In path 1, the metal complex undergoes a series of two overlapping one-proton steps to generate a mixed salicylate-catecholate coordination. Further protonation results in the precipitation of a tris(salicylate) complex (e.g. enterobactin, MECAM). This differs from path 2, in which a single two-proton step dissociates one arm of the ligand to form a bis(catecholate) chelate. Path 3 incorporates features of paths 1 and 2. In this model, the metal again imdergoes a series of two overlapping one-proton reactions. However, unlike the case of path 1, the second proton displaces a catecholate arm, which results in a bis(catecholate) metal complex...

Studies of the oxidation of ferric catecholate coordination complexes have been useful in exploring mechanistic possibilities for these enzymes. A series of ferric complexes of 3,5-di-r-butyl-catechol with different ligands L have been found to react with O2 to give oxidation of the catechol ligand (Reaction 5.58). 

Fig. 3 (a)The coordinate vector for catechol coordination to a metal center, (b) The chelate plane defined by a metal ion and the coordinate vectors in a tris-catechol complex, (c). The approach angle for each chelate of the tris-catechol complex, (d) A M4L6 tetrahedral cluster (adaptedfrom Ref. [24]). (View this art in color at www.dekker.com.)... 

As Scheme 4 involves unidentate catechol coordination to iron in the various intermediates, mixed ligand complexes Fe(salen)(CATH), Fe(saloph) (CATH) and Fe(salen) (NCATH) have been prepared and characterized [91] (CATH = catecholato monoanion NCATH = 4-nitrocatecholato monoanion). The X-ray structures of [Fe(salophen)catH] and [Fe(salen)l CHQ) show axially coordinated... 

Fe-N bond in [Fe(NTA) (2,3-DTBC) ]. This bond cleavage leads to unidentate catecholate coordination, which allows for reaction with... 

There are two types of discussions on the structure of ES. One concerns whether catechol coordinates to the ferric center as a monodentate (32, 33) or a bidentate (34) ligand shown in Fig. 9. The other concerns the character of the catecholate ligand, i.e., how the catecholate ligand is activated for oxygenation whether the catecholate ligand has a radical character as in (35) or an anionic character as in (33). 

In contrast, the monodentate coordination of catechols to the ferric center in the form of 31 has been shown in the case of 1,2-CTD. Resonance Raman spectroscopy does not readily distinguish between the mono- and bidentate coordination of mode of the catecholate ligand. However, H and H NMR of the model complexes and 1,2-CTD after coordination of catechol and phenols have supported strongly that catechols coordinate to 1,2-CTD in the monodentate manner (see Chapter 2) [88, 89, 91]. The observed data clearly indicate that catechols are coordinated to the ferric center solely through the 0(1) oxygen. 

In the enzymatic system, it was initially proposed that first oxygen and then catechol coordinate to Fe " " [95, 120]. The coordination of catechol to Fe prior to the coordination of O2 subsequently has been proposed based on enzymatic studies [103,104, 121-123]. Coordination of catechol to Fe is thought to occur via an axial site and one equatorial position, and formation of a square-pyramidal ternary species has been proposed based on the spectral change indicating the binding of azide [104]. 

The iron ligands are not known, but it is suggested based on Mossbauer and EXAFS that there is no tyrosine or sulfur coordination [138, 139]. Histidine ligation is probable since six histidines are conserved in the sequences [130]. An environment of histidines and carboxylate(s) for iron has been proposed based on the resonance Raman studies of the catecholate complexes of TH [139-141]. Coordination of catecholamine is also suggested since the enzyme as isolated from mammalian tissue contains a nearly stoichiometric amount of tightly bound catecholamines and the resonance Raman vibrations strongly indicate the presence of a bidentate catecholamine-Fe(III) complex in the enzyme [140, 141]. Thus, the feedback inhibiting effect of catecholamines in the catecholamine biosynthesis is explained by coordination of catecholamines to the TH active-site iron center [140, 142]. Comparison of the resonance Raman bands of Relabeled dopamine and phenolate complexes at the 3- and/or 4-positions clearly indicated that the catechols coordinate to iron in the chelate mode [139]. 

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