Iron imidazole

Heme complexes and heme proteins have also been the subject of NIS studies. Of specific interest have been three features the in-plane vibrations of iron, which have not been reported by Resonance Raman studies [108], the iron-imidazole stretch, which has not been identified in six-coordinated porphyrins before, and the heme-doming mode, which was assumed to be a soft mode. 

Figure 1. Structures of some of the synthetic compounds studied. The iron-imidazole bond in. CO is unstrained while it is strained i-ET-CO, 1-ST-CO and 1-AD-CO.

Point charges and the electric field located both at the distal and proximal sites strongly affect the Fe-C and C-0 bond indices as well as the C-0 vibration frequency. This was revealed in a detailed study by Kushkuley and Stavrov [80,81] using INDO calculations to follow the effect of the distortion of the porphyrin ring, the iron...imidazole distances, the iron displacement out of the... 

Chang and Traylor argued that in heme-peptide models the side chains containing the imidazole had either too few or too many atoms to achieve a strain-free iron-imidazole bond On the other hand it was argued that condensation of l-(3-aminopropyl) imidazole 21 with the acid chloride of pyrroporphyrin XV 22 followed by insertion of iron would give a strain-free five-coordinate-system 23 (Scheme 11). This chelated heme was capable of binding dioxygen in a reversible manner in the solid state or when... 

In the centrosymmetric imidazole complex (39a) the Cu-N distances of 2.045(7) and 1.913(7) A differ considerably. It is believed that one pair of mutually irons imidazole ligands co-ordinates to the metal as shown in (39b) whereas the other pair are deprotonated and co-ordinate as in (39c). The bond lengths in the ligand rings support this suggestion, as do the short contacts with oxygen atoms of 2.81 and 2.84 A made by the nitrogen atom not bonded to the metal in (39c). ... 

Electrons from cytochrome c are transferred to Cu sites and then passed to the heme iron of cytochrome a. Cu is liganded by two cysteines and two histidines (Figure 21.18). The heme of cytochrome a is liganded by imidazole rings of histidine residues (Figure 21.18). The Cu and the Fe of cytochrome a are within 1.5 nm of each other. 

FIGURE 21.18 (a) The Cn site of cytochrome oxidase. Copper ligands inclnde two histidine imidazole groups and two cysteine side chains from the protein, (b) The coordination of histidine imidazole ligands to the iron atom in the heme a center of cytochrome oxidase. 

The structure of the diamagnetic, cherry-red vitamin B12 is shown in Fig. 26.6 and it can be seen that the coordination sphere of the cobalt has many similarities with that of iron in haem (see Fig. 25.7). In both cases the metal is coordinated to 4 nitrogen atoms of an unsaturated macrocycle (in this case part of a corrin ring which is less symmetrical and not so unsaturated as the porphyrin in haem) with an imidazole nitrogen in the fifth position. A major... 

It is noteworthy that carbene function may be manifested in the derivatives of tris(imidazol-l-yl)borate 44 (96AGE310). Its reaction first with n-butyllithium, then with iron(II) chloride, and finally with sodium tetraphenylborate gives the iron(III) carbene derivative 45. 

Flooding. An iron-phthaloc/anine complex reacts with carbon monoxide to liberate methyl imidazole 26... 

More recently 233) it has been reported that cross-linked polystyrene containing imidazole ligands did not provide a support rigid enough to prevent dimerization, and that the p-oxo dimer was benzene extracted from oxygenated tetraphenyl porphyrin iron(ll), Fe(TPP), which had been attached to the modified polystyrene. A discussion of model synthetic porphyrins, from which definitive structural and other physical data are obtained, is given in section V.C. 

A silica gel containing 3-imidazole propyl groups bonded to surface atoms of silicon has been prepared (253), and was treated 254) with a solution of Fe(TPP)L2, where L = py or pip, where upon the iron(II) porphyrin became attached to the imidazole groups on the surface of the gel,... 

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