Catechol complexes, simple

Catecholate Siderophores. Simple Catechol Complexes. As noted earlier, the common siderophore for enteric bacteria is the tricatechol, enterobactin (Figure 4). In order to perfect synthetic and... 

The case of the vanadium-catechol complex is also interesting. Under certain conditions, vanadium(V) undergoes reduction by catechol to yield the vanadium(IV)-semiquinone. The vanadium in this complex has a 51V NMR signal and no ESR signal, demonstrating its oxidation state of 5+. One wonders whether the low BVS value reflects a simple weakness in the BVS analysis or the inclination of catechol to reduce vanadium(V). 

At high pH, the tricatecholate ligands derived from 2,3-dihydroxybenzoic acid bind iron via the six phenolic oxygens of the catechol rings just as do simple mono-catechols. However, the dissociation dehavior for tris(mono-catechol) complexes as the pH is lowered is quite different from that observed for the multidentate ligands which incorporate DHB groups. For the mono-catechol complexes, a simple, stepwise dissociation of individual catechol rings is observed and this is associated with the association of two protons per catechol. In contrast, the spectrophotometric behavior... 

Aqueous ferric ion forms a complex with guaiacol with two discrete maxima and a similar extinction (X = 422 nm, s = 2.4 x 10 Lmol emX = 461 nm, s = 2.3 x 10 LmoHcm ) as the catechol complexes [71]. However, 4-substituted phenols are better models than simple guaiacol for the guaiacyl function in lignin. The extinction coefficients for ferric complexes of such compounds range from 4 to 10 LmoHcm", which is 10-100 times smaller than those of catechol complexes [62,71]. 

However, if only Domain III is important in recognition, it would be expected that the simple tris(catecholato)-rhodium(TII) complex would be an equally good inhibitor. In fact, even at concentrations in which the rhodium-catechol complex was in very large excess, no inhibition of iron uptake was observed, suggesting that Domain II is important in the recognition process. 

The visible and circular dichroism spectra of the chromic siderophore complexes are closely related to the corresponding spectra of simple model complexes of hydroxamate or catecholate ligands. This provides a spectroscopic probe for structure in assigning the geometries of the siderophore complexes. 

Chelated Cu complexes are generally more stable than the simple complexes. Examples are the ethylenediamine, oxalato, catechol and jS-diketone complexes bis(acetylacetonato)copper(II) is shown ... 

To conclude, it is necessary to note that the reaction ability of biogenic silica depends on its structure and solubility, determined by various factors. It has been found that soluble forms of silica could be stabilized by S - 7% of glycerin and catechol. It has been determined that the amorphous part of the biogenic silica of siliceous rocks formed the complex with triethylphosphate and actively reacts with polyphenols (the simple one, catehol and complex ones, humic acids), with formation of ethers. The silicon organic derivatives and complexes formed are inert to hydrolysis. 

Most kinetic studies of iron release have focused on pathways involving the use of chelate ligands such as EDTA (217), pyrophosphate 218-220), phosphonates 220,221), catecholates 108,216), hydroxa-mates 120), and nitrilotriacetate (221). In many cases, simple saturation kinetics are observed, and interpreted in terms of the formation of a quaternary complex, ligand-Fe-transferrin-COs (120,122). The failure to observe this complex spectroscopically [in contrast to iron uptake studies (120)] has been explained in terms of a rate-limiting conformational change, giving a basic three-step mechanism, which is essentially the reverse of that given for iron uptake (Section V.A.l). 

As with hydroxamate siderophores, simple tris(catecholato) metallate(lll) complexes have served as models for enterobactin. Unlike hydroxamate, catecholate is a symmetric, bidentate ligand. Consequently, there are no geometrical isomers of simple tris(catecholato) metal complexes, and only A and A optical isomers are possible. However all siderophore catecholates are substituted asymmetrically on the catechol ring, such that geometric isomers may in principle exist. However, in the case of enterobactin molecular models show only the more symmetric cis chelate is possible, as the A or A form. 

The simple model complex, tris(cateeholato)chromate(III) has been prepared, and complete resolution of the optical isomers was achieved at pH 13. The known crystal structure of [Cr(cat)3]3- and arguments similar to those for the hydroxamate chromium complexes lead to the assignments of absolute configuration of the CD spectra. It was found that the CD spectra of A-[Cr(cat)3f and [Cr(ent)3f " are essentially identical, and the mirror image of chromic desferriferrichrome (Fig. 28), which shows that enterobactin has a predominant A-cis absolute configuration 147). Unfortunately the usual oxidation sensitivity of the catechol dianion is substantially increased in the chromium complexes, which precludes their use as biological probes l47). 

OH group may be assumed to form the anion/molecule complex 139 which, however, is non-reactive with respect to further tautomerization. Rather, this complex loses the entire carboxylate residue as the fragment ion m/z 255), leaving the phenolic unit as a quinoid neutral fragment (Scheme 37) °. In a further work, 3,4-dihydroxybenzyl carboxylates derived from stearic acid (cf. 140), dihydrocinnamic acid and phenylacetic acid were studied under NC1(NH3)-MS/MS conditions. In these cases, deprotonation was found to take place exclusively at the phenolic sites, owing to the increased acidity of hydroxyl groups in a catechol nucleus, in contrast to simple phenols. Heterolysis of the benzylic C—O bond, e.g. in ions [140 — gives rise to reactive anion/molecule complexes,... 

Flavonoids are widely distributed in fruits and vegetables and are very common nutritional supplements as antioxidants. The results on antioxidant activities of simple catechols provide a useful basis for evaluating results for the many, more complex natural compounds containing the catechol structure, such as the flavonoids, steroidal catechols and hormonal catecholamines. There are several reviews on the antioxidant properties of flavonoids and several reports on experimental " and theoretical evidence linking their antioxidant properties to the catechol moiety usually found in their structure. The basic flavonoid structure (29) is shown in Chart 1, with a few selected examples (30-36) from different groups to illustrate some of the relationships between their detailed structures and related antioxidant properties. Efforts to elucidate these relationships are hampered by their very low solubility in non-polar solvents, and the tendency of some researchers to employ metal ions as initiators of oxidation in aqueous media so that one cannot distinguish between their action as chain-breaking... 

Excellent reviews on dioxirane-mediated oxidations have appeared. One of the most eharacteristic points is that dioxiranes can be applied to the epoxidation of labile olefins such as enol ethers, enol acrylates, allenes and others. Dioxiranes have also been utilized for phenolic oxidation, but in relatively rare cases. Oxidation of simple phenols and anisoles with dimethyldioxirane (544) provided only a complex mixture, so that hindered phenols are more favorable. On treatment with dimethyldioxirane (4 equiv.) in acetone, 2,4-di(terf-butyl)phenol (216) was oxidized to afford in 79% yield the corresponding o-benzoquinone 220, which reacted with 544 and aq. NaHS03 to give catechol 545. Dimethyldioxirane-promoted oxidation of 545 provided again a quantitative yield of 220. Further oxidation of 220 produced a 52% yield of two epoxides 546 and 547 in a ratio of 1 20. Oxidation of thymol (548) was effected with dimethyldioxirane in acetone to afford fhe four oxidation producfs 549-552 in 10, 20, 10 and 10% yields, respectively (Scheme 102). ... 

A polymer (P-DHB) (XI) based on catechol, the active functional group of enterobactin, was recently synthesised by the reaction of polyvinyl amine with the ethyl ester of 2,3-dihy-droxybenzoic acid (DHB). Only about one third of the amine groups was found to be substituted with DHB units. The formation constant of the iron(III) complex (log K = 40) is the same as that reported for the simple dimethyl amide of DHB and so there does not appear to be any appreciable chelate effect. 

As with the hydroxamate siderophores, our initial approach has been to study simple tris(catecholato)metallate(III) complexes as models for the tricatecholate siderophore enterobactin. Unlike hydroxamates, catecholate is a symmetric, bidentate ligand. 

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