Eckhard Bill

Iron-containing proteins and related analogs — complementary Mössbauer, EPR and magnetic susceptibility studies

The three methods covered by this review — Mossbauer spectroscopy, electron paramagnetic resonance and magnetic susceptibility-provide a powerful set of tools for detailed studies of electronic structure and molecular magnetism of iron-containing proteins and related analogs. The interpretation of measured data is based on the spin-Hamiltonian concept which is described in detail. A short introduction to the principles of the three methods as well as a basic description of the spectrometers is given. The major part of the review deals with applications, e.g. to mononuclear iron complexes and to spin-coupled iron complexes. Wherever possible, the complementarity in applying the three methods is described.

Characterization and Reconstitution of a 4Fe-4S Adenylyl Sulfate/ Phosphoadenylyl Sulfate Reductase from Bacillus subtilis*

CysH1 from Bacillus subtilis encodes a 3′-phospho/adenosine-phosphosulfate-sulfonucleotide reductase (SNR) of 27 kDa. Recombinant B. subtilis SNR is a homodimer, which is bispecific and reduces adenylylsulfate (APS) and 3′-phosphoadenylylsulfate (PAPS) alike with thioredoxin 1 or with glutaredoxin 1 as reductants. The enzyme has a higher affinity for PAPS (KmPAPS 6.4 μm Trx-saturating, 10.7 μm Grx-saturating) than for APS (Km APS 28.7 μm Trx-saturating, 105 μm Grx-saturating) at a Vmax ranging from 280 to 780 nmol sulfite mg-1 min-1. The catalytic efficiency with PAPS as substrate is higher by a factor of 10 (Kcat/Km 2.7 × 104-3.6 × 104 liter mol-1 s-1. B. subtilis SNR contains one 4Fe-4S cluster per polypeptide chain. SNR activity and color were lost rapidly upon exposure to air or upon dilution. Mössbauer and absorption spectroscopy revealed that the enzyme contained a 4Fe-4S cluster when isolated, but degradation of the 4Fe-4S cluster produced an inactive intermediate with spectral properties of a 2Fe-2S cluster. Activity and spectral properties of the 4Fe-4S cluster were restored by preincubation of SNR with the iron-sulfur cluster-assembling proteins IscA1 and IscS. Reconstitution of the 4Fe-4S cluster of SNR did not affect the reductive capacity for PAPS or APS. The interconversion of the clusters is thought to serve as oxygen-sensitive switch that suppresses SO3 formation under aerobiosis.

Basic Physical Concepts

Mossbauer spectroscopy is based on recoilless emission and resonant absorption of γ-radiation by atomic nuclei. The aim of this chapter is to familiarize the reader with the concepts of nuclear γ-resonance and the Mossbauer effect, before we describe the experiments and relevant electric and magnetic hyperfine interactions in Chaps. 3 and 4. We prefer doing this by collecting formulae without deriving them; comprehensive and instructive descriptions have already been given at length in a number of introductory books ([7–39] in Chap. 1). Readers who are primarily interested in understanding their Mossbauer spectra without too much physical ballast may skip this chapter at first reading and proceed directly to Chap. 4. However, for the understanding of some aspects of line broadening and the preparation of optimized samples discussed in Chap. 3, the principles described here might be necessary.

Mössbauer-Active Transition Metals Other than Iron

The previous chapters are exclusively devoted to the measurements and interpretation of 57Fe spectra of various iron-containing systems. Iron is, by far, the most extensively explored element in the field of chemistry compared with all other Mossbauer-active elements because the Mossbauer effect of 57Fe is very easy to observe and the spectra are, in general, well resolved and they reflect important information about bonding and structural properties. Besides iron, there are a good number of other transition metals suitable for Mossbauer spectroscopy which is, however, less extensively studied because of technical and/or spectral resolution problems. In recent years, many of these difficulties have been overcome, and we shall see in the following sections a good deal of successful Mossbauer spectroscopy that has been performed on compounds of nickel (61Ni), zinc (67Zn), ruthenium (mainly 99Ru), tantalum (181Ta), tungsten (mainly 182W, 183W), osmium (mainly 189Os), iridium (191Ir, 193Ir), platinum (195Pt), and gold (197Au). The nuclear γ-resonance effect in the case of technetium (99Tc), silver (107Ag), hafnium (176Hf, 177Hf, 178Hf, 180Hf), rhenium (187Re), and mercury (199Hg, 201Hg) has been of relatively little use to the chemists, so far. There are various reasons responsible for this, viz., (1) extraordinary difficulties in measuring the resonance effect because of the long lifetime of the excited Mossbauer level and hence the extremely small transition line width (e.g., in 67Zn), (2) poor resolution of the resonance lines due to either very small nuclear moments or the very short lifetime of the excited Mossbauer level resulting in very broad resonance lines, (3) insufficient resonance effects due to unusually high transition energies between the excited and the ground nuclear levels, which in turn increase the recoil energy and thus reduces the recoilless fraction of emitted and observed γ-rays.

Some Special Applications

We have learned from the preceding chapters that the chemical and physical state of a Mossbauer atom in any kind of solid material can be characterized by way of the hyperfine interactions which manifest themselves in the Mossbauer spectrum by the isomer shift and, where relevant, electric quadrupole and/or magnetic dipole splitting of the resonance lines. On the basis of all the parameters obtainable from a Mossbauer spectrum, it is, in most cases, possible to identify unambiguously one or more chemical species of a given Mossbauer atom occurring in the same material.

Nuclear Resonance Scattering Using Synchrotron Radiation (Mössbauer Spectroscopy in the Time Domain)

Conventional Mossbauer spectroscopy (MS) can be considered as “spectroscopy in the energy domain.” It has been widely used since its discovery in 1958 [1]. Nuclear resonant forward scattering (NFS) of synchrotron radiation has been successfully employed as a time-differential technique since 1991 [2]. Another related technique, nuclear inelastic scattering (NIS) of synchrotron radiation [3], can be regarded as an extension of conventional, energy-resolved MS (in the range 10−9 to 10−7 eV) to energies on the order of molecular vibrations (in the range 10−3 to 10−1 eV). So far only a few “Mossbauer” stations for NFS and NIS measurements have become available in synchrotron laboratories, i.e., in Germany, France, Japan, and the USA.

CCDC 1479124: Experimental Crystal Structure Determination

Related Article: Mei Wang, Thomas Weyhermuller, Eckhard Bill, Shengfa Ye, and Karl Wieghardt|2016|Inorg.Chem.|55|5019|doi:10.1021/acs.inorgchem.6b00609