Imidazoles, coordinated

Reactions of cobalt porphyrin with p-fluoranyl or phenyl-A -butylimidazole proceeds analogously—both fluoranyl and imidazole coordinate with the cobalt contained in the porphyrin complex, but fluoranyl was fixed in its semiquinone, anion-radical form (Okamoto and Fukuzumi 2003). 

In general, there are several chemical ambiguities regarding the imidazole side chain of histidine as it is observed in protein structures. For example, the protonation state of histidine cannot be ascertained from X-ray crystallographic experiments, and this is of particular concern because the near-physiological first pK of the imidazole side chain is about 6.0—6.5. The second pXa of histidine is about 14.0—14.5, and this value may be lowered 2 units by imidazole coordination to a metal ion (Tainer eprotein structures as the protonated imidazolium or as the neutral imidazole, and even sometimes as the deprotonated imidazolate. 

More recent studies of the kinetics of bromination of imidazoles coordinated to cobalt(III) have implied that, in aqueous solution, proton-abstraction from a Wheland intermediate is rate-determining at acidic pH values. In more nearly neutral conditions bromine addition becomes rate-... 

Chelation sometimes does not take place automatically, as illustrated by the coordination of histamine, 4-(2-aminoethyl)imidazole (19 R = H). It appears to tautomerize and coordinates with the non-chelating nitrogen, whereas the NH2 group is protonated, so that a monodentate ligand (in fact a 4-substituted imidazole) coordinates to the metal, in this case Ni with NCS- anions in the compound [Ni(NCS)4(hamH)2] (hamH = histaminium cation).70... 

In the wake of this report, many chiral iron(III)- and Mn(III)-porphyrin complexes have been synthesized and applied to the epoxidation of styrene derivatives [20]. Because these asymmetric epoxidations are discussed in the first edition of this book [21], the discussion on metalloporphyrin-catalyzed epoxidation here is limited to some recent examples. Most chiral metallopor-phyrins bear chiral auxi Maries such as the one derived from a-amino acid or binapthol. Differing from these complexes is complex 6, which has no chiral auxiliary but is endowed with facial chirality by introducing a strap and has been reported by Inoue et al. [20f]. Epoxidation of styrene by using only 6 as the catalyst shows low enantioselectivity, but the selectivity is remarkably enhanced when the reaction is performed in the presence of imidazole (Scheme 6B.11). This result can be explained by assuming that imidazole coordinates to the unhindered face of the complex and the reaction occur on the strapped face [20f. ... 

On the structure of Hb02 two models are presented (Fe(II)-02, Fe(IH)-02 ) (12-14). Iron in Hb02 is in a low spin state. However, when imidazole coordinates as the sixth legand both high spin state and low spin state of Fe(III) are observed. These states relate to the interaction of oxygen with Hb, and analyses using NMR technique have recently been done (15-17). 

One recombinant FetSp mutant is unique among multicopper oxidase species and has been particularly informative about the structure of the type 3 binuclear cluster in these species. This is the T1D/T2D double mutant that contains only this type 3 site (Blackburn et al., 2000). EXAFS analysis of this protein contains contributions from electron ejection and scattering from only the type 3 copper atoms and thus provides direct structural information about this cluster. The K-edge XAS spectrum for this mutant in its oxidized and reduced states is shown in Fig. 21. The oxidized sample has a nearly featureless edge with a midpoint energy of 8990 eV typical of tetragonally distorted type 2 Cu(ll) centers, i.e., those with predominantly histidine imidazole coordination. The reduced type 3 cluster exhibited a pronounced shoulder at 8984 eV just below the... 

As shown in Fig. 10, Fe + CN in the fuUy oxidized state has a C-N stretch band at 2151.5 cm h However, this band splits into bands at 2131.4 and 2091.0 cm on reduction of the metal sites except for the Fe + CN site. The transition between the 2151.5-cm band and the pair of 2131.4 and 2091.0-cm bands is linearly dependent on the electron equivalents added to the system. At intermediate oxidation states between the fully oxidized state and the three-electron-reduced state, approximately one equivalent of cyanide is required to saturate the cyanide-binding site. These results indicate that reduction of Cug+ induces the split and shift of the 2151-cm band. This is the clearest evidence for the interaction between Cub and the ligand bound at Foa. The two bands observed in the partially reduced state are likely to be induced by the two types of ligand environment. For example, one of the histidine imidazoles coordinated to Cug+ could be partly deproto-nated and create a significantly different polar environment in the vicinity of the bound cyanide. In the Cug+ state, no such equilibrium is present (Yoshikawa et al., 1995). X-ray structures of these cyanide derivatives in various oxidation states and pH at high resolution would contribute to an improvement of understanding the mechanism of the cyanide inhibition. 

The type 3 binuclear Cu(II) pair in an MCO is characteristic of this type of copper site in biology. First, both copper atoms are ligated by three protein side chains each all six are His residues. Five of the imidazoles coordinate the Cu(II) atoms via the Ne2 nitrogens while the sixth His does so via N51. This coordination pattern is a conserved feature of the MCO structure.Second, the two Cu(n)... 

Such a bond is realized with polymer ligands containing basic groups with o-dcmor and n-acceptor properties like pyridine, imidazole. Coordination chemistry is nearly equal to low molecular mdal ligands. 

The paramagnetic nature of Cu ion was utilised to characterise these polymeric receptors by studying the interaction between the Cu centres and imidazole groups using ESR spectroscopy [17]. Results from the ESR experiments indicate that Cu -imidazole coordination bonds formed during the assembly of the monomer and template are conserved upon polymerisation. Furthermore, analysis of the ESR data obtained for different polymers suggested a defined arrangement of metal ion sites in the templated materials, which is absent in the non-templated control polymers. 

Fig 3, Representation of copper-imidazole coordination showing the important multiple-scattering pathways. 

Poly(imidazolyl)borates H B(im)4 ra (Scheme 4) can be prepared by analogy with that reported for poly(pyrazolyl)borates.164 HraB(im)4 ra can be used to model the histidine imidazole coordination in metalloproteins. H B(im)4 ra complexes have been widely described,165 but only few metal derivatives are structurally authenticated i.e., the ionic lithium salt of tetrakis(imidazolyl)borate,166... 

The imidazol coordinates were taken from an x-ray diffraction crystallographic study of histamine (24) with a proton being substituted for the ethylamine side chain of histamine. The coordinates of other reactants, products, and intermediate complexes were calculated employing idealized hybridization and average bond distances or are as discussed. 

Recently the synthesis of NHC complexes by a base catalyzed tautomeric rearrangement has been introduced. 7V-Pheny I imidazole coordinates via the unsubstituted nitrogen atom to Mn to give complex 82 (Fig. 26a) [198]. Subsequent attempts to deprotonate the C2 carbon atom in complex 82 led to the intermediate... 

The NMR spectra of native RRB2 show a feature at 24 ppm, which disappears upon D2O exchange (120). The observed shift and solvent exchange behavior is as expected for N-H groups of imidazoles coordinated to (ix-oxo)diiron(III) complexes. Methemerythrin exhibits such a peak at 24 ppm (35), and a variety of model complexes show features associated with imidazole N-H in the 14-20 ppm region (72, 85, 96). When compared to those found for mononuclear high-spin Fe(III)-imidazole complexes at... 

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