Ligand properties spinning analysis

Extensive analysis of the EPR and redox behavior of this unusual copper protein led to the hypothesis that the protein might contain a Cu(A) site similar to that in cytochrome oxidase (Riester et ai, 1989) and that the unusual seven-line EPR is due to the Cu(A)-type site. An alternative interpretation of this EPR is based on electron spin-echo spectroscopy as well, and that is that the seven-line EPR is due to a half-met Cu—Cu pair and to unusual type I sites (Jin et ai, 1989). Three sets of spin-echo peaks can be attributed to nitrogens on imidazole ligands to a CuA-type site and to another imidazole on the half-met site. The electron spin-echo spectra of cytochrome oxidase are similar, although there is not enough copper in cytochrome oxidase for a half-met site. Conceivably, the property of delocalization of the paramagnetic electron could be effected by the proposed bridging between Cub and heme as (nomenclature summarized by Capaldi, 1990), which are proposed to be 3-4 A apart. 

An obvious approach to the problem of conformational analysis in these complexes is the use of NMR spectroscopy. However, several general problems immediately arise, namely, that most metal ions have nuclear spin, many are labile to substitution of the ligands, and they are frequently paramagnetic. In addition the ligands often contain N14. All these properties potentially hinder the observation of fine structure and thus the analysis of the NMR spectra. However, in some complexes the problems are either eliminated or reduced. For example Co(III) compounds are usually diamagnetic and inert to substitution, and in some complexes the ligand-proton fine structure is observed to be relatively uncomplicated by Co or N quad-rupole relaxation effects, for example, (CoEDTA)", Fig. 18. 

These materials are characterized by common techniques such as H NMR, IR, elemental analysis, DSC, and TGA.54 58 59 60 64 68 Other techniques such as electron spin resonance (ESR),54 magnetic susceptibilities54 and Mossbauer spectroscopy (when they apply),56,58 60 64 71 uV-visible spectroscopy,58,59 60 61 64-67 and solid-state electric conductivity measurements5456 59-63 66 67 71 were also employed. These materials were carefully compared to model bis(mono-isocyanide) adducts for better understanding of the physical properties. Important solubility problems are often observed when no alkyl side chain is used. So, these more soluble substituents are incorporated either on the macrocycles or the bridging ligands for better characterization. 

A model complex with properties almost identical to the high spin complex was obtained in a crystalline form from hemin-dimethylester and 4-nitrobenzene thiolate By X-ray analysis it proved to be a five-coordinated complex with an Fe-S distance of 2.32 A low affinity sixth ligand would thus allow a low-spin/high-spin equilibrium with an equilibrium constant close to unity. Both substrates and an increase in temperature would shift this equilibrium to the high-spin side. The significance of this mechanism for enzymatic catalysis will be discussed in Chap. 6.1. 

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