Mossbauer spectroscopy intensities

The spectroscopic techniques that have been most frequently used to investigate biomolecular dynamics are those that are commonly available in laboratories, such as nuclear magnetic resonance (NMR), fluorescence, and Mossbauer spectroscopy. In a later chapter the use of NMR, a powerful probe of local motions in macromolecules, is described. Here we examine scattering of X-ray and neutron radiation. Neutrons and X-rays share the property of being found in expensive sources not commonly available in the laboratory. Neutrons are produced by a nuclear reactor or spallation source. X-ray experiments are routinely performed using intense synclirotron radiation, although in favorable cases laboratory sources may also be used. 

Mossbauer spectroscopy has been used to characterize the iron clusters in fuscoredoxin isolated from D. desulfuricans (133). The authors explained why the iron nuclearity was incorrectly determined, and studied the protein in three different oxidation states fully oxidized, one-electron reduced, and two-electron reduced. The error made in determining the iron cluster nuclearity was caused by the assumption that in the as-purified fuscoredoxin, cluster 2 is in a pure S = state. This assumption was proven to be false and unnecessary. In fact, the observation of four resolved, equal intensity (8% of total Fe absorption) spectral components associated with the S = i species in the as-purified protein is consistent with cluster 2 being a tetranuclear Fe cluster. The 4x8 = 32% Fe absorption for the four components indicates that only 64% of clusters 2 are in the S = state (the total Fe absorption for cluster 2 is 50% of the total Fe absorption). The remaining clusters 2 are in a different oxidation state, the spectrum of which is unresolved from that of cluster 1. 

Ga( Zn), Sn, Te( I) Mossbauer spectroscopy, no modifications of the local symmetry of lattice sites, electronic structure of atoms and intensity of electron-phonon interaction are revealed for Pbi Sn Te solid solutions in the gapless state at 80 and 295 K... 

Atoms are not rigidly bound to the lattice, but vibrate around their equilibrium positions. If we were able to look at the crystal with a very short observation time, we would see a slightly disordered lattice. Incident electrons see these deviations, and this, for example, is the reason that in LEED the spot intensities of diffracted beams depend on temperature at high temperatures the atoms deviate more from their equilibrium position than at low temperatures, and a considerable number of atoms are not at the equilibrium position necessary for diffraction. Thus, spot intensities are low and the diffuse background high. Similar considerations apply in other scattering techniques, as well as in EXAFS and in Mossbauer spectroscopy. 

As described above, the combination of EPR and Mossbauer spectroscopies, when applied to carefully prepared parallel samples, enables a detailed characterization of all the redox states of the clusters present in the enzyme. Once the characteristic spectroscopic properties of each cluster are identified, the determination of their midpoint redox potentials is an easy task. Plots of relative amounts of each species (or some characteristic intensive property) as a function of the potential can be fitted to Nernst equations. In the case of the D. gigas hydrogenase it was determined that those midpoint redox potentials (at pFi 7.0) were —70 mV for the [3Fe-4S] [3Fe-4S]° and —290 and —340mV for each of the [4Fe-4S]> [4Fe-4S] transitions. 

Typical X-ray diffraction patterns of the Fe oxides are shown in Figure 7.16. They provide the three parameters, namely line (angle) position, width and intensity from which the nature of the oxide, its quantity (in a mixture), its unit cell parameters and its crystallinity (crystal size and order) can be deduced. The crystal structure of unknown compounds may also be determined. XRD is still the most reliable way to identify a particular oxide because it is based on the long range order of the atoms, whereas most other methods (e.g. Mossbauer spectroscopy, EXAFS) characterize the atoms and their immediate (short range) environment. 

Mossbauer spectroscopy also provides structural information.97-100 In particular it is possible to deduce from peak asymmetry the separate appearance of surface atoms as size is decreased they cause an increase in intensity of quadrupole splitting, and two quadropole components indicate two kinds of surface atoms.97 Au1 and Au111 species are also sometimes recognised,99 and sometimes not.98... 

Mossbauer spectroscopy provides phase identification, determination of oxidation states, and incidentally structure information and particle size. A little used application is to follow in real time the kinetics of phase transitions (carburization, reduction) in catalysts by monitoring the intensities of a few selected peaks in a single velocity experiment. Examples of applications on catalysts have recently been reviewed [43]. 

Vibrational (IR and Raman), UV-visible, photoelectron, NMR, and Mossbauer spectroscopy have all been reported for bis(bistrimethylsilyl-methyl)tin(II) and analogous tin(II) amides. Since an unusual tin-tin double bond has been proposed for the solid state of [Sn[CH(SiMe3)2]2]2 the Raman spectrum of this compound was of interest. Unfortunately, the compound decomposed in the laser beam however, an intense band at 300 cm-1 has been assigned as the Ge—Ge stretching frequency for the analogous germanium compound (68). 

---