Catecholate complexes, model

For simpler catechols employed in plant chemical defense we have synthesized and studied more complex analogs into which biological-receptor-site-directing functionality has been integrated to examine the potential of these redox-sensitive systems for inactivating targeted receptors. These modified catechols are models for selective receptor inactivation. 

Catecholate complexes of the type Zrans-[0s(0)2L2]2 (H2L = dopa, dopamine, adrenaline, noradrenaline, or isoproterenol) are prepared from the reactions between Zrans-[0s(0)2(0H)4]2- and H2L. The complexes have been characterized using Raman, IR, and NMR (JH and 13C) spectroscopies, which indicate that the ligands are bound via the catecholate oxygens. These types of complexes are believed to be models for the staining of catecholamine rich sites in biological tissues... 

It can be seen from molecular models that two diastereoisomers are possible for the ferric enterobactin complex, A-cis and A-cis. These are not mirror images because of the optical activity of the ligand. The similarity of the roles played by the ferrichromes and enterobactin lent additional speculative interest to the preferred absolute configuration of the iron complex (20). The structural studies of the tris catechol complexes (vide infra) and the spectroscopic properties of the chromic... 

Karaliota A., Kamariotaki M., Hadjipanagioti D. and Aletras V., Molybdenum catecholates as models for Mo in biological systems. 1. Synthesis and spectroscopic study on Mo complexes with 3,4-dihydroxybenzoic and 3,4-dihydroxyphenylacetic acid. J. Inorg. Biochem. 69 (1998) pp. 79-90. 

Very little work on structural analogues has been reported. Que and Heistand38) have prepared Fe(salen)catH and Fe(salen)ncatH as models for the proposed mono-coordinated catecholate complex. [Abbreviations used salen, ethylenebis(salicylal-dimine) catH, catechol monoanion ncatH, 4-nitrocatechol monoanion saloph, o-phe-nylenebis(salicylaldimme).] This mode of binding is suggested by the presence of an 0—H stretch at 3380 cm-1 which shifts to 2520 cm-1 when the complex is synthesized with catechol deuterated at the hydroxyl functions. A crystal structure of the corresponding Fe(saloph)catH complex has been obtained and confirms the structural assignment (Fig. 6). 

Fig. 5.21. Top Self-assembly drives the formation of helical, homochiral dimeric titanium tris-catecholate complexes. Dimerization is only mediated by LC, while Na and K" " do not lead to comparable products. Bottom Crystal structures of the dimers formed from the aldehyde (left, R = H), the ethyl ketone (centre, R = C2H5), and the methyl ester (right, R = OCH3). Shown is a side view in space-filling representation and a ball-and-stick model with a view along the Ti—Ti...

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]. 

In addition to preparing the model catecholate complexes of rhodium and chromium, the analogous enterobactin complexes were also prepared and their CD spectra recorded (62). From examination of molecular models it is apparent that either the A-cis or A-cis diastereomers of a metal enterobactin complex are structurally possible. In theory, these diastereomers should be separable by chromatographic techniques analogous to those used for the hydroxamates (vide supra) however, under a variety of conditions only one chromatographic fraction is obtained. We conclude that one isomer predominates to the exclusion of the other. 

The structural characterization of a series of copper-catecholate complexes, including model structures for catechol oxidase, which have not been known before, and the investigation of a catechol oxidase mechanism (192, 213). 

Figure 37.3. The smallest measurable particle sizes for biogenic silica in the branches of Equisetum arvense [black bars], silica produced in a model system from a silicon catecholate complex [striped bars], and silica produced in a model system in the presence of a biosilica protein containing extract from Equisetum arvense [hatched bars].

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. ... 

Step behind their counterparts, the intradiol-cleaving dioxygenases. In particular, no iron(II) catecholate complexes have been reported to carry out the oxygenation chemistry of the enzymes. The development of model systems that proceed via the same pathway as the enzymes presents itself as the next challenge in the effective development of functional model systems. 

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... 

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