Catecholate formation constants

Enterobactin (ent), the cycHc triester of 2,3-dihydroxy-A/-benzoyl-l-serine, uses three catecholate dianions to coordinate iron. The iron(III)-enterobactin complex [62280-34-6] has extraordinary thermodynamic stabiUty. For Fe " +ent , the estimated formal stabiUty constant is 10 and the reduction potential is approximately —750 mV at pH 7 (23). Several catecholate-containing synthetic analogues of enterobactin have been investigated and found to have lesser, but still impressively large, formation constants. 

Aromatic polyalcohols act as strong coordinating agents and Table 17 summarizes reported formation constants. The complexes are quite stable this behaviour has been used for the qualitative and quantitative determination of vanadium (e.g. refs. 494 and 495). At pH 3-4, an initial vanadyl catechol complex slowly converts to a tris complex.496 In fact complexes with 1 3 metal-ligand stoichiometry have been isolated (see below), but since in the equilibrium (30) no protons are consumed or liberated, [VO(cat)2]2- and [V(cat)3]2 are not distinguishable by potentiometric studies. 

Tris-catecholate complexes were prepared from symmetrically and unsymmetrically substituted catechols (LH2) 4-chlorocatechol, 4,5-dichlorocatechol, 4-nitrocatechol, 3,4-dinitrocatechol and 4,5-dinitrocatechol202. All of these complexes are prepared in aqueous solution and are water-stable, down to ca pH 4. The 111 NMR signals for the free and complexed catechol moieties are well separated, and enable the determination of formation constants for each of the complexes, according to equation 46, where L is the catechol dianion. 

In contrast to the hydroxamate siderophores, little or nothing is known about the stability constant for the catechol siderophore, entero-bactin. Prior to determining the formation constant of enterobactin (for which hydrolysis of the ligand presents special problems), the reaction of catechol itself with ferric ion has been investigated (31). 

The weak acidity of catechol makes its effective formation constant much less than 1045-9 near physiological pH. However, any chelate effect should tend to make the formation constant for enterobactin in larger than / 3 for catechol. Thus 1045 can be regarded as a lower bound for the reaction Fe3+ + ent6" Fe(ent)3". 

Hydroxamate- or catecholate-containing siderophores are strongly absorbing species with characteristic spectra (see Table 1) which can be utilized for spectrophotometric determination of the complex formation constant. Iron(III) hydroxamates absorb in the visible region, producing a broad absorption band in the 420-440 nm region. Iron(III) catecholates exhibit pH-dependent absorption maxima. Unfortunately, the overall Fe + ion complex formation constants cannot be determined directly at neutral pH, because the extremely high stability of siderophore complexes precludes direct measurements of the equilibrium of interest, which would yield the desired formation constant for a tris-bidentate siderophore complex, /3no (equation (2)). ... 

The three catechol groups of enterobactin are carried on a cyclic serine triester structure. A variety of both cyclic and linear structures are found among other catechol siderophores. " For example, parabactin and agrobactin (Fig. 16-1) contain a backbone of spermidine (Chapter 24). After the Fe -enterobactin complex enters a bacterial cell the ester linkages of a siderophore are cleaved by an esterase. Because of the extremely high formation constant of M for... 

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. 

The reaction of catechol methylation in gas phase at temperatures below 300°C is seen to proceed efficiently over modified y-aluminas with moderate acid sites. At low catechol conversion (X < 0.05), O- and C-methylated products are formed in parallel reaction pathways. The 0/C methylation ratio has been regulated by varying acid/base properties of the catalyst. Modification of Y-AI2O3 by phosphoric acid was observed to increase the selectivity towards guaiacol formation (O-methylation) up to 82=0.89. The catalyst Mg (7.5 at.% )/ Y-AI2O3 showed the maximum selectivity towards 3-methyl catechol formation (C-methylation) to be 3=0.65. It means that a 20-fold change in the O/C methylation ratio was achieved, when the catalyst acidity was modified, keeping constant other reaction conditions. 

Table 4. Formation constants for some actinide (IV) hydroxamates and catecholates...

Figure 1.9 Reaction pathway for phenoi hydroxyiation with H2O2 as the oxidizing agent and TS-1 as the catalyst. The relative rate constants characterizing product (hydroqui-none or catechol) formation and by-product (benzoquinone) formation and secondary reactions depend on the catalyst and are discussed in the text.

The formation constants of the ferric complexes of these synthetic catecholate ligands have been determined spectrophotometri-cally by competition with EDTA, as described above for entero-bactln. The first three (most acidic) ligand protonation constants have been determined by potentlometrlc titration of the free ligand. The second, more basic, set of protonation constants are too large to be determined readily potentlometrically. Thus the proton-dependent stability constant is expressed as... 

Binding of Cu + ions by catechol-terminated SAMs was studied by thin-layer UV-vis spectrophotometry combined with a long-optical-pass cell [43]. Since the pK values are 9.23 and 13.0 for the first and second catechol deprotonation, respectively, experiments were carried out at pH = 7.3 to ensure that the ligand is in its neutral form. Under these conditions, assuming formation of a 1 1 ligand-metal ion complex both in solution and on the surface, formation constants... 

DFT and ab initio calculations have been used to study the mechanism of the gas-phase oxidation of phenol by HO. Addition of HO to the ort/io-position forms P2, which subsequently combines with O2 at the ip o-position to form adduct P2-1-00. A concerted HO2 elimination from P2-1-00 forms 2-hydroxy-3,5-cyclohexadienone (HCH) as the main product and is responsible for the rate constants for the reaction between P2 and O2 to be about two orders of magnitude higher than those between other aromatic-OH adducts and O2. The HCH subsequently isomerizes to catechol, which is thermodynamically more stable than HCH, possibly through a heterogeneous process. Reaction of P2 with NO2 proceeds by addition to form P2-n-N02 ( = 1, 3, 5) followed by HONO elimination from P2-1/3-N02 to form catechol. The barriers for HONO elimination and catechol formation are below the separate reactants P2 and NO2, being consistent with the experimental observation of catechol in the absence of O2, while H2O elimination from P2-I/3-NO2 forms 2-nitrophenol (2NP). The most likely pathway for 2NP is the reaction between phenoxy radical and N02." ... 

Various hydroxyl and amino derivatives of aromatic compounds are oxidized by peroxidases in the presence of hydrogen peroxide, yielding neutral or cation free radicals. Thus the phenacetin metabolites p-phenetidine (4-ethoxyaniline) and acetaminophen (TV-acetyl-p-aminophenol) were oxidized by LPO or HRP into the 4-ethoxyaniline cation radical and neutral V-acetyl-4-aminophenoxyl radical, respectively [198,199]. In both cases free radicals were detected by using fast-flow ESR spectroscopy. Catechols, Dopa methyl ester (dihydrox-yphenylalanine methyl ester), and 6-hydroxy-Dopa (trihydroxyphenylalanine) were oxidized by LPO mainly to o-semiquinone free radicals [200]. Another catechol derivative adrenaline (epinephrine) was oxidized into adrenochrome in the reaction catalyzed by HRP [201], This reaction can proceed in the absence of hydrogen peroxide and accompanied by oxygen consumption. It was proposed that the oxidation of adrenaline was mediated by superoxide. HRP and LPO catalyzed the oxidation of Trolox C (an analog of a-tocopherol) into phenoxyl radical [202]. The formation of phenoxyl radicals was monitored by ESR spectroscopy, and the rate constants for the reaction of Compounds II with Trolox C were determined (Table 22.1). 

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