Mossbauer spectra cluster

The Mossbauer spectrum of L consists of a symmetric doublet which was fitted with parameters 8 = 0.66 0.02 mm/s and AE = 1.51 0.03 mm/s (71). The isomer shift is larger than values (0.45-0.55 mm/s) generally seen for carboxylate-bridged diiron(III) clusters,... 

In the most oxidized state (circa OmV), EPR spectroscopy shows a signal (g = 2.02) characteristic of [3Ee-4S] clusters in the +1 state. In agreement, the low temperature (4.2 K), Mossbauer spectrum shows the correspondent typical magnetic component. However, this component only accounts for approximately 27 per cent of the total iron. The remaining 73 per cent, invisible to EPR spectroscopy, can be assigned to two diamagnetic [4Ee-4S] clusters. 

So far, biological Mossbauer spectroscopy is only sensitive to iron (or to be more precise to the Fe isotope) which rules out the use of this technique to directly probe the Ni site. Unfortunately, the iron site seems to remain always in a low-spin state making its identification by Mossbauer very difficult since it will be probably unresolved from the [3Fe-4S] and [4Fe-4S] cluster components. Also, each iron atom in the protein contributes equally to the Mossbauer spectrum the Fe site of the Ni-Fe centre is only 8.3 per cent of the total absorption. 

Figure 7.12 Analysis of the zero-field Mossbauer spectrum at SOK of H2-reduced A. vinosum [NiFe] hydrogenase. (A) Dotted line experimental spectrum solid line theoretical spectrum for a [3Fe-4S] cluster. (B) Dotted spectrum experimental spectrum in A minus the theoretical spectrum or the reduced 3Fe cluster this was expected to represent the spectrum of the two reduced [4Fe-4S] clusters. Solid line theoretical spectrum for two reduced 4Fe clusters. The additional absorption in the experimental spectrum (arrows) comprised a doublet with a surprisingly small isomer shift (0.05-0.15 mm/s) and accounted for about 8 per cent of the total absorption, i.e. one Fe out of twelve (adapted from Surerus et al. 1994).The present interpretation is that this doublet represents the Fe atom in the Ni-Fe site.

Figure 4.2. Mossbauer spectrum of Hox+i hatched marks) prepared from the spectra of the aerobically purified D. vulgaris hydrogenase recorded at 4.2 K in a magnetic field of 0.05 T applied parallel to the y-rays. Solid line, a least-squares fit to the data, assuming three equal-intensity quadrupole doublets dotted-and-dashed line, doublet corresponding to the [2Fe]n cluster dashed line, superposition of two doublets (doublets 1 and 2) corresponding to the [4Fe-4S]n cluster.

Augite, Mossbauer spectrum of, 6 474 Au(PRj) fragment, addition to homonuclear gold cluster compounds, 39 329-332 Auranofin, 36 21 cytotoxicity, 36 22, 34 Auro-bis(thiosulfate), 36 18-19 Aurosomes, gold in, 36 21-22 Aurothioglucose, 36 18-19 Aurothiomalate, 36 18-21... 

The linewidth (corrected for instrumental effects) may also provide important chemical information of several types. For example, if the chemical environment of a resonant atom is not the same for all of the atoms in the sample, then a broadening of the observed resonance is expected. That is, the observed resonance is a sum of the contributions from each atom, the latter not all having the same Mossbauer parameters. Thus for a small catalyst particle, interesting particle size information might be contained in the linewidth due to the contribution from the surface atoms to the Mossbauer spectrum. The distribution (clustered or uniform) of resonant atoms throughout a multicomponent catalyst particle may also be reflected in the linewidth. 

The Mossbauer spectrum of native ribonucleotide reductase is very similar to that of oxyhaemerythrin, both having a binuclear Fein-Fein cluster with 5 = 0 [127,188]. This suggests that there is negligible coupling of the iron centre to the distant (5A) tyrosine-122 radical in ribonucleotide reductase. This was confirmed by the fact that removing the tyrosine radical with hydroxyurea or hydroxylamine had no effect on the spectrum [188]. However, by rapid-freezing... 

The spectroscopic properties of P clusters are unusual. In the dithi-onite-reduced MoFe protein all the Fe atoms of the P clusters are iron(II), indicating a [4Fe-4S] oxidation state, a level difficult to achieve with model complexes. Oxidation gives rise transiently to an EPR-observable (gav = 1.93) species, which then relaxes to give a very complex Mossbauer spectrum. 

Fig. 4. A 90 K MOssbauer spectrum on an extended velocity scale of apoferritin loaded in air at pH 6,25 with four Fe(II) and frozen at 3 min. The computer fit resolves the spectrum into three doublets b, due to small clusters c, due to oxo-bridged Fe(III) dimers and d, due to Fe(II), Doublet d represents only 5% of the total iron. Reproduced from Rfif 69.

Here, Q is the nuclear quadrupole moment (Fe " "), 3(Fe +) and (Fe +), 3(Fe +) are the mean values of the electric field gradients and isomeric shifts for Fe and Fe ions. The coefficients Uvn and a n are found from the solution of the dynamic vibronic problem for the arbitrary values of ct(T). The expressions for and can be obtained by means of the substitution -o- The total Mossbauer spectrum F(Q) was obtained summing the spectra yielded by different cluster vibronic states in the molecular field, taking into account their equilibrium... 

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