Mononuclear EXAFS spectroscopy

Since the first purifications of Fe hydrogenases in the early 1970s a range of different models for the H-cluster active site have been proposed including mononuclear iron and clusters of 2, 3, 4, and 6Fe [56,57,72-77], At least in part this changing stoichiometry reflects improvements in purification, elemental analysis, and spectroscopy. The more recent models propose the H cluster to contain approximately 6 Fe on the basis of elemental analysis [56,57] and a putative 3.3 A Fe-Fe distance indicated by EXAFS spectroscopy [58], The data are indicative at best, because counting Fe in proteins has an uncertainty typically of the order of 10% (i.e., l-2Fe), and because no EXAFS on 6Fe models has been published. 

Table 19.1 Summary of isolated supported mononuclear metal complexes with structure determined by EXAFS spectroscopy... 

In an effort to understand adsorption mechanisms, Waite et al. (1994) (see also Chisholm-Brause and Morris 1992) examined the character of U(V1) adsorption sites on the HFO surface with uranium EXAFS spectroscopy. They concluded that a single inner-sphere, mononuclear, bidentate complex, (sFe02)U02, could explain their low pH-adsorption results and that U(VI) desorption at alkaline pH s could be modeled assuming a (sFe02)U02C03 surface species. Waite et al. (1994) used the DL model in their study and assumed the existence of both weak and strong adsorption sites (see Fig. 13.14 and Chap. 10). 

Costas et al. have reported spectroscopic evidence for an Fe Fe complex that can be considered a structural model for the putative Fe Fe (/x-0)2 core of methane monooxygenase intermediate The synthetic complex was prepared at —80 °C in CH2CI2 by decay of a mononuclear low-spin Fe peroxo precursor. The Mossbauer spectra showed that all iron in the sample is intermediate spin (5 = 1) Fe, but the data were compatible with either a mononuclear site or a weakly coupled ( J <5cm ) symmetric dimer. Combination of the Mossbauer technique with resonance Raman and EXAFS spectroscopies provided evidence for a bis-/x-oxo bridged diiron(IV) complex. The complex of Costas et al however, is not an electronic model for intermediate Q, as the latter contains high-spin Fe sites. 

The method has been applied to NaY encaged [Ir4(CO)i2] (Ikble 4-5). The results confirm the structure of the zeolite-entrapped spedes that had been inferred from the infrared spectra (Fig. 4-10). However, the EXAFS data indicate that [Ir4(CO)i2] is not the only iridium spedes present in the zeolite. The data suggest that some (unidentified) mononuclear iridium spedes (perhaps iridium carbonyls) are also present and comprise about 20 % of the iridium. [78] EXAFS spectroscopy is perhaps the best suited technique for the detection of such minority spedes because infrared spectroscopy alone is often not suffident to detect them. 

X-ray absorption spectroscopy has been performed on the isolated Rieske protein from bovine heart mitochondrial bc complex 69) as well as on the Rieske-type cluster in Burkholderia cepacia phthalate dioxygenase (PDO) (72). The analysis performed by Powers et al. 69) was significantly hampered by the fact that the presence of two histidine ligands was not fully recognized therefore, only the results obtained with the dioxygenase where the mononuclear iron has been depleted will be considered here. Table VII gives a comparison of the distances obtained from the fit of the EXAFS spectra assuming an idealized Rieske model and of the distances in the crystal structures... 

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