Electron paramagnetic resonance cobalt complexes

Electron paramagnetic resonance (continued) cobalt-thermolysin complex, 28 334, 335 exchange reactions, 31 106-107 glutamine synthetase, 28 358-364 invisible oxygen species, 31 94-95 metalloenzymes, 28 324, 326 metal particle size distribution, 36 99-100, 104... 

The reaction of cobalt(II) porphyrins with CO has been studied by Wayland et al. (153-156) in some detail by electron paramagnetic resonance (EPR) in frozen solutions. They conclude that Co(II)TPP forms an axially symmetric weakly bonded 1 1 adduct with CO (153-156). To the best of our knowledge, there are no other studies on CoP-CO complexes, neither experimental nor theoretical. 

Electron paramagnetic resonance spectroscopy has proved a valuable tool in the study of AdoCbl-dependent enzymes. AdoCbl itself is EPR-silent, but upon homolysis to form Cbl(II), two spins are formed, one on the cobalt (which now has low-spin d configuration) and one on the organic radical. Typically, the two unpaired electrons remain close enough in the enzymeis active site that they interact with one another to give complex, but informative, EPR spectra. 

We have mentioned earlier the dissimilarities between the spectral properties of chromophoric metal ions at the active sites of metalloen-zymes and the properties of simple bidentate model complexes of the same metals. Cobalt phosphatase has served well to illustrate such a dissimilarity and, in Figure 9, the data for phosphatase, representative of a cobalt enzyme, are shown again along with those for plastocyanin, a copper enzyme, and ferredoxin, an iron enzyme. Each enzyme spectrum is unusual compared with the simple model complexes shown at the bottom of the figure. More detailed spectral data as well as comparison of other physical properties of metalloenzymes—e.g., electron paramagnetic resonance spectra—with those of model complexes have been summarized previously (10). 

The kinetics of outer-sphere ion-pair formation between the sulphate ion and the cobalt(iii) species [CoCNHaje] and [Coens] + have been measured in aqueous solution using the ultrasonic technique. As expected, rates typical of a diffusion-controlled process are found. The electron paramagnetic resonance (e.p.r.) line-broadening technique was used to obtain the thermodynamic parameters for the formation of outer- and inner-sphere complexes between Mn" and dithionite ions. For the formation of the ion-pair, values of AG° = — 1-57 kcalmol S =... 

USE OF ELECTRON PARAMAGNETIC RESONANCE SPECTROSCOPY TO STUDY THE INTERACTION BETWEEN COBALT SCHIFF BASE COMPLEXES AND PHOSPHINES OR PHOSPHITES IN SOLUTION+... 

The reduction of cyanocobalamin gives three possible oxidation states for the cobalt atom (Fig. 2). Electron spin resonance studies with Bi2-r reveals that this molecule is the only paramagnetic species giving a spectrum expected for a tetragonal low spin Co(II) complex. Controlled potential reduction of cyanocobalamin to Bi2-r proves that this reduction involves one electron, and further reduction of Bi2-r to B12-S requires a second single electron (16—19). At one time B12-S was considered to be a hydride of Co(III), but controlled potential coulometry experiments provided evidence against a stable hydride species (16). However, these experimental data do not exclude the possibility of a stable Co(III) hydride as the functional species in enzyme catalyzed oxidation reduction reactions. 

The conclusion that the cobalt and iron complexes 2.182 and 2.183 are formally TT-radical species is supported by a wealth of spectroscopic evidence. For instance, the H NMR spectrum of the cobalt complex 2.182 indicated the presence of a paramagnetic system with resonances that are consistent with the proposed cobalt(III) formulation (as opposed to a low-spin, paramagnetic cobalt(IV) corrole). Further, the UV-vis absorption spectrum recorded for complex 2.182 was found to be remarkably similar to those of porphyrin 7r-radicals. In the case of the iron complex 2.183, Mdssbauer spectroscopy was used to confirm the assignment of the complex as having a formally tetravalent metal and a vr-radical carbon skeleton. Here, measurements at 120 K revealed that the formal removal of one electron from the neutral species 2.177 had very little effect on the Mdssbauer spectrum. This was interpreted as an indication that oxidation had occurred at the corrole ligand, and not at the metal center. Had metal oxidation occurred, more dramatic differences in the Mdssbauer spectrum would have been observed. 

The third principal application of the electron spin resonance technique is to the study of paramagnetic transition metal ions in biochemical systems. Most examples are complexes of copper, iron, manganese, chromium, cobalt and molybdenum. Other metals such as titanium, vanadium and nickel are sometimes employed as structural probes. Only four of these ions, Cu ", Mn, Gd " and VO ", are seen in ESR spectroscopy at room temperature under virtually all conditions. Therefore, they are of special importance. 

Electron spin resonance spectra provide direct information about paramagnetic metal complexes. Much experimental work has been done on cobalt(II) Schiff-base complexes and their 0 - adducts. Some related... 

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