Spectroscopic studies Mossbauer spectroscopy

Until then, the purification of the Fepr protein had been a laborous job as a 240-L batch yielded only as little as 5 mg of protein. With the overexpression clones of the Fepr proteins, the range of proteinconsuming studies such as Mossbauer spectroscopy, EXAFS, and, last but not least, crystallization experiments was greatly extended. Thus, several groups set off to systematically investigate the spectroscopic properties of both Fepr proteins, poised at all four (proposed) redox states. 

Russo, U., Long, G.J. Mossbauer spectroscopic studies of the high oxidation states of iron. In Long, G.J., Grandjean, F. (eds.) Mossbauer Spectroscopy Applied to Inorganic Chemistry, pp. 289-329. Plenum, New York (1989)... 

The study of metals in biological systems requires techniques, some of them highly specific, some limited to certain aspects of the metal ion in question, some of more general applicability. Thus, Mossbauer spectroscopy in biological systems is restricted to iron-containing systems because the only element available with a Mossbauer nucleus is 57Fe. The EPR spectroscopic techniques will be of application only if the metal centre has an unpaired electron. In contrast, provided that crystals can be obtained, X-ray diffraction allows the determination of the 3-D structure of metalloproteins and their metal centres. 

Spectroscopic techniques may provide the least ambiguous methods for verification of actual sorption mechanisms. Zeltner et al. (Chapter 8) have applied FTIR (Fourier Transform Infrared) spectroscopy and microcalorimetric titrations in a study of the adsorption of salicylic acid by goethite these techniques provide new information on the structure of organic acid complexes formed at the goethite-water interface. Ambe et al. (Chapter 19) present the results of an emission Mossbauer spectroscopic study of sorbed Co(II) and Sb(V). Although Mossbauer spectroscopy can only be used for a few chemical elements, the technique provides detailed information about the molecular bonding of sorbed species and may be used to differentiate between adsorption and surface precipitation. 

Ti ossbauer spectroscopy is the term now used to describe a new ana-lytical technique which has developed using y-ray nuclear resonance fluorescence or the Mossbauer effect. For most of the time since Rudolf Mossbauer s discovery in 1958 it was the physicist who utilized this new tool. Starting approximately in 1962 some chemists realized the potential of this new technique. Since then they have applied Mossbauer spectroscopy to the study of chemical bonding, crystal structure, electron density, ionic states, and magnetic properties as well as other properties. It is now considered a complimentary tool to other accepted spectroscopic techniques such as NMR, NQR, and ESR. 

As will be explained in Chapter 7, spectroscopic methods are a powerful way to probe the active sites of the hydrogenases. Often spectroscopic methods are greatly enhanced by judicious enrichment of the active sites with a stable isotope. For example, Mossbauer spectroscopy detects only the isotope Fe, which is present at only 2.2 per cent abundance in natural iron. Hydrogen atoms, which cannot be seen by X-ray diffraction for example, can be studied by EPR and ENDOR spectroscopy, which exploit the hyperfine interactions between the unpaired electron spin and nuclear spins. More detailed information has been derived from hyperfine interactions with nuclei such as Ni and Se, in the active sites. In FTTR spec-... 

Spectroscopic studies on the Fe-Mo protein by EPR and Mossbauer spectroscopy have shown six iron atoms each in a distinctive magnetic environment coupled to an overall S=3/2 spin system (6,7,8) and electron nuclear double resonance (ENDOR) studies suggest one molybdenum per spin system (8). The 5 Fe signals (five or six doublets) observed in the ENDOR spectra (8) indicate a rather asymmetric structure for the Fe/Mo/S aggregate in which the iron atoms roughly can be grouped into two sets of trios, each set having very similar hyperfme parameters. 

The association of sulfur and iron into simple to more complex molecular assemblies allows a great flexibility of electron transfer relays and catalysis in metalloproteins. Indeed, the array of different structures, the interactions with amino-acid residues and solvent and their effect on redox potential and spectroscopic signatures is both inspiring for chemists and electrochemists, and of paramount importance for the study of these centers in native conditions. Most of the simpler natural clusters have been synthesized and studied in the laboratory. Particularly, the multiple redox and spin states can be studied on pure synthetic samples with electrochemical and spectroscopic techniques such as EPR or Fe Mossbauer spectroscopy. More complex assembhes still resist structural... 

Spectroscopic studies, including vibrational spectroscopy, UV-visible absorption and MCD spectroscopy and 197Au Mossbauer spectroscopy, suggest that isocyanides act largely as a donors to gold(I) with very little d -p back-bonding,69,404,408 It is this polarization R—... 

These criteria for an ideal spectroscopic technique have been met, to a large extent, by Mossbauer spectroscopy. UHV conditions are not required since one utilizes y rays with energies in the keV range, and the technique lends itself easily to in situ studies. Furthermore, the technique is ideally suited for studies of small particle systems, and in several instances information about particle size can also be obtained. The unique feature of Mossbauer spectroscopy is the extremely high energy sensitivity of the technique. This allows detailed chemical, structural, and magnetic information to be obtained about atoms on the surface or in the bulk phase. 

However, spectroscopic studies of activated BLM indicate that it is not an Fev=0 species. It exhibits an S - 1/2 EPR spectrum with g values at 2.26, 2.17, and 1.94 [15], which is typical of a low-spin Fe111 center. This low-spin Fem designation is corroborated by Mossbauer and x-ray absorption spectroscopy [16,19], Furthermore, EXAFS studies on activated BLM show no evidence for a short Fe—0 distance, which would be expected for an iron-oxo moiety [19], These spectroscopic results suggest that activated BLM is a low-spin iron(III) peroxide complex, so the two oxidizing equivalents needed for the oxidation chemistry would be localized on the dioxygen moiety, instead of on the metal center. This Fe(III)BLM—OOH formulation has been recently confirmed by electrospray ionization mass spectrometry [20] and is supported by the characterization of related synthetic low-spin iron(III) peroxide species, e.g., [Fe(pma)02]+ [21] and [Fe(N4py)OOH]2+ [22], The question then arises whether the peroxide intermediate is itself the oxidant in these reactions or the precursor to a short-lived iron-oxo species that effects the cytochrome P-450-like transformations. This remains an open question and the subject of continuing interest. 

Of the various techniques routinely available, IR and 13C NMR spectroscopy usually provide the most valuable information in terms of the determination of the most appropriate valence description (A-D, Chart 1) of the carbon fragment. Mossbauer spectroscopy has also been used with good effect with iron-containing poly-carbon complexes.89 This solution-based work is complemented by a significant number of solid-state structural studies, which are described in greater detail below. Electronic spectroscopic methods, including luminescence methods, have been used to probe the electronic structures of a small number of poly-yndiyl complexes and polymers.288 315 340 342 377 380 Selected IR, 13C NMR, and UV-vis data have been given in Tables I-VIII, above. 

This paper reviews literature data on model fluid cracking catalysts (FCC) and reports results of spectroscopic studies performed in our laboratories for catalysts in which vanadium has been passivated by tin. Mossbauer spectroscopy, luminescence, x-ray powder diffraction and electron paramagnetic methods have been used to monitor vanadium-tin interactions in all the catalysts studied. 

The [Fe(terpy)2] cation is low-spin, as demonstrated by Mossbauer, electronic, H NMR, and resonance Raman spectroscopy and magnetic measurements (20,184,187, 228, 266). Similarly, spectroscopic studies of the iron(III) cation have indicated a low-spin ( B) ground term (382). There have been numerous electrochemical studies of the bis complexes (177, 200, 256, 298, 332, 344, 373, 378, 379, 397, 398). Ligand-centered reductions to formal oxidation states of iron(I), iron(O), and iron( — 1) and oxidations to iron(III) are observed. The complex [Fe(terpy)L][C104]2 [L = tris(2 -pyridyl)l,3,5-triazine (Fig. 15)] has been prepared (399, 442). 

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