Ligation imidazole

Figure 20. Two-pulse echo decay envelope of Cu(II) complexes of (a) bleomycin, (b) diethylenetriamine and imidazole, and (c) diethylenetriamine and pyrimidine. In traces a and b, one observes a modulation pattern arising from the interaction of Cu(ll) with the remote N of ligated imidazole. The respective magnetic fields and spectrometer frequencies are the following a, 3080 G, 9247 MHz b, 3195 G, 9251 MHz c, 2970 G, 9225 MHz. Lines indicating the periods were added by the authors. From [268], with permission.

Figure 18.4 Structures of heme/Cu oxidases at different levels of detail, (a) Position of the redox-active cofactors relative to the membrane of CcO (left, only two obligatory subunits are shown) and quinol oxidase (right), (b) Electron transfer paths in mammalian CcO. Note that the imidazoles that ligate six-coordinate heme a and the five-coordinate heme are linked by a single amino acid, which can serve as a wire for electron transfer from ferroheme a to ferriheme as. (c) The O2 reduction site of mammalian CcO the numbering of the residues corresponds to that in the crystal structure of bovine heart CcO. The subscript 3 in heme as and heme 03 signifies the heme that binds O2. The structures were generated using coordinates deposited in the Protein Data Bank, lari [Ostermeier et al., 1997] Ifft [Abramson et al., 2000] (a) and locc [Tsukihara et al., 1996] (b, c).

Figure 18.5 Plausible sequence of steps responsible for rapid and selective reduction of O2 to H2O by mixed-valence CcO. The square frames signify the catalytic site (Fig. 18.4c) imidazole ligation of Cub is omitted for clarity in some or aU intermediates, Cub may additionally be ligated by an exogenous ligand, such as H2O (in Cu ) or OH (in Cu ) such ligation is not established, and hence is omitted in all but compound Pm and the putative hydroperoxo intermediate. The dashed frames signify the noncatalytic redox cofactors. Typically used phenomenological names of the spectroscopically observed intermediates (compounds A, E, H, etc.) are also indicated.

Section 18.2). The latest generation of such catalysts (1 in Fig. 18.17) reproduces the key features of the site (i) the proximal imidazole ligation of the heme (ii) the trisi-midazole ligation of distal Cu (iii) the Fe-Cu separation and (iv) the distal phenol covalently attached to one of the imidazoles. As a result, binding of O2 to compound 1 in its reduced (Fe Cu ) state appears to result in rapid reduction of O2 to the level of oxides (—2 oxidation state) without the need for outer-sphere electron transfer steps [Collman et ah, 2007b]. This reactivity is analogous to that of the heme/Cu site of cytochrome c oxidase (see Section 18.2). 

Figure 18.20 A plausible ORR catalytic cycle by biomimetic catalysts 2 (Fig. 18.17). Cu is ligated by three imidazoles (omitted for clarity) and potentially an exogenous ligand, whose nature is not known. All intermediates other than ferric-peroxo and ferric-hydroperoxo were prepared independently.

Fig. 18.17) when adsorbed on a graphite electrode, it manifests catalytic behavior comparable to that of series 2 metalloporphyrins. (b) The simplest structural motif that ensures the axial imidazole ligation of the Fe site [Khvostichenko et al., 2007 Yang et al., 2008]. 

Yang Q, Khvostichenko D, Atkinson J, Boulatov R. 2008. Simple dimer containing dissocia-tively stable mono-imidazole ligated ferrohemes. Chem Commun (8) 963. 

Very recently, the kinetics and thermodynamics of a variety of axial ligation reactions have been investigated with Fe11 and Co11 porphyrins involving the small molecules CO, 02, and NO as ligands 27-30, 40, 92, 93). These experiments lead to the conclusion that the dynamic trans effects observed in these systems cannot alone be explained by the interaction models D and F (Fig. 1). Especially imidazole and its derivatives do not hold the place in various series of trans effects that they should take on the ground of their proton basicities. Therefore, besides their usual a-donor-tr-acceptor function, these unsaturated molecules are ascribed an additional 7r-donor function. 

Nudeobases have pyrimidine and purine (a combination of pyrimidine and imidazole) heterocyclic nuclei which have two ligating nitrogens placed in angular directions. The angular dispositions of pyrimidine, imidazole, and 1,7-purine nitrogens respectively being 120°, 150°, and 90° impart to these bases, heteroatom-induced endocyclic donor ability towards metals which drive self-assembly processes to form macrocyclic systems. The metallacalixarenes formed from nudeobases and various metal salts are discussed with respect to each base. 

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