Ferric catecholates

Figure 13. Distribution curve, as a function of pH, of the various species formed in the ferric-catechol titration experiment.

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). 

The use of a heterogeneous system under nitrogen consisting of a stirred suspension of silica gel with adsorbed ferric-catechol complex in benzene treated with 35% hydrogen peroxide has been reported to result after 2.5 hours in a 60% yield of phenol (ref. 14). The formation of phenol in 56% yield resulted from a mixture of benzene and vanadium(V) catalyst in acetonitrile under nitrogen when reacted for 2 hours at ambient temperature (ref. 15). More recent studies have involved the conversion of benzene in trifluoromethanesulphonic acid to phenol by the electroreduction of dioxygen (ref. 16) and from generation... 

The asymmetric bonding motif of the catecholate to the ferric ion in these complexes duplicates the binding motif in the enzyme-substrate structures that have been characterized. While the original belief that this asymmetry in Fe—O bond lengths was critical to the reactivity of the systems has since proven incorrect,the complexes remain as functional models for the catechol dioxygenases due to both their structural and reactivity features. All complexes have been characterized as high-spin ferric catecholate complexes by UV-visible, EPR and NMR spectroscopy, and an examination of C—O bond length in these complexes shows all of the complexes to bind in the catecholate form with no semiquinonate character. ... 

Dissolve a few crystals of phenol in water and add ferric chloride solution a violet coloration is produced. Repeat, using i 2 drops of m-cresol shaken up with about i ml. of water a violet coloration is again produced. Catechol (in dilute solution) gives a green coloration. 

METHOD 1 [112l-5g phenol in dH20 is stirred 5 hours at 20 C with some ferric sulfate (Fe2(S04)3, an additional 7mLs dH20, 13mLs 6% H2O2 and a pinch of aluminum oxide (AI2O3). Yield of catechol is 2.5g (50%). 

Fe7-ric chloride reaclion.—Dissolve a drop of the free phenol in water and add a drop of ueutral ferric chloride. A green (catechol), blue (orcinol, pyiogallol) or purple (phenol, resorcinol) colouration is produced, which is often destroyed by acid or alkali. Quinol is o.xidised to quinone, and turns biown (p. 193). The naphthols give precipitates of dinaphthol (p. 220). 

As strong metal ion chelators due to their catechol structure, tea flavonoids are able to bind and thus decrease the level of free cellular ferric and ferrous ions, which are required for the generation of reactive oxygen radicals via the Fenton reaction (Yang and Wang, 1993). 

Carboxy-3,5-dichlorophenyl diazo-nium chloride, 31, 97 Catalyst, ammonium acetate, 31, 25, 27 copper chromite, 31, 32 ferric nitrate, hydrated, 31, 53 piperidine, 31, 35 piperidine acetate, 31, 57 Catechol, 33, 74... 

As mentioned above, transport of siderophores across the cytoplasmic membrane is less specific than the translocation through the outer membrane. In E. coli three different outer membrane proteins (among them FepA the receptor for enterobactin produced by most E. coli strains) recognise siderophores of the catechol type (enterobactin and structurally related compounds), while only one ABC system is needed for the passage into the cytosol. Likewise, OM receptors FhuA, FhuE, and Iut are needed to transport a number of different ferric hydroxamates, whereas the FhuBCD proteins accept a variety of hydroxamate type ligands such as albomycin, ferrichrome, coprogen, aerobactin, shizokinen, rhodotorulic acid, and ferrioxamine B [165,171], For the vast majority of systems, the substrate specificity has not been elucidated, but it can be assumed that many siderophore ABC permeases might be able to transport several different but structurally related substrates. 

---