36
M. Stein
troscopic observables (such as chemical shift, g-values, spin–spin interactions and
zero-field splitting) [1, 2]. Here, the concept of the descriptive Spin Hamiltonian,
which is used in the analysis of spectra from magnetic resonance spectroscopy, is
linked to computationally accessible spectroscopic parameters from quantum chemistry.
The focus in this chapter is on Electron Paramagnetic Resonance (EPR) spectroscopy but its fundamentals are also valid and can be formulated in an analogous
way for Nuclear Magnetic Resonance (NMR). The interpretation of magnetic resonance data in the context of structural and chemical biology of paramagnetic metallic
cofactors does require knowledge not only of the magnitude and strength but also
the directionality and orientation of these magnetic interactions with respect to the
compound or protein structural frame. Here, systems containing transition metal
ions are being discussed since they are challenging to study experimentally but also
computationally demanding in terms of model size and accuracy of results.
Transition metals are in particular interesting and provide the following challenges
and difficulties:
(i) A multitude of oxidation states
(ii) Different electronic ground states (low-spin, intermediate spin, high spin) for
a metal in a particular oxidation state
(iii) Redox activity of transition metal during catalytic cycle
(iv) Changes in spin state during a reaction (‘spin state catalysis’).
Thus, spectra of transition metal-containing complexes or proteins are particularly
cumbersome to analyze and interpret in terms of molecular structures. The EPR gvalues for transition metal complexes vary over a wide spectral range due to spin–orbit
coupling and zero-field splitting, depending on the geometry of the complex and
this variability in nature and number of ligands makes it impossible to establish an
empirical approach to predict g-values of most transition metal systems.
As a subset, selected examples from nickel, molybdenum and manganese containing transition metal complexes and enzymatic systems are discussed.
Nickel occurs in coordination complexes and metalloenzymes in different oxidation states which may be para- or diamagnetic depending on the ligand environment. Nickel enzymes catalyze a wide range of chemical reactions using mononuclear, homo- or heterodinuclear active sites and are present among others in urease,
superoxide dismutase, carbon monoxide dehydrogenase, acetyl-coenzyme A synthase/decarbonylase and methyl-coenzyme M reductase [3, 4]. [NiFe]-hydrogenase
enzymes possess a heterobimetallic active site and the nickel ion occurs in the +I, +II,
and +III oxidation states during the heterolytic splitting of molecular hydrogen (H 2 ).
It is covered here as one example of the application of EPR spectroscopy in solution
and in protein single crystals (for reviews see [5, 6]) hand in hand with computational means to identify the intermediates by calculating spectroscopic observables
and elucidate the enzymatic reaction mechanism [7, 8].
Molybdoenzymes are widespread in eukaryotic and prokaryotic organisms where
they have crucial functions in detoxification reactions in the metabolism of humans
and bacteria [9]. In human, molybdenum xanthine oxidoreductase (XO), aldehyde
M. Stein
troscopic observables (such as chemical shift, g-values, spin–spin interactions and
zero-field splitting) [1, 2]. Here, the concept of the descriptive Spin Hamiltonian,
which is used in the analysis of spectra from magnetic resonance spectroscopy, is
linked to computationally accessible spectroscopic parameters from quantum chemistry.
The focus in this chapter is on Electron Paramagnetic Resonance (EPR) spectroscopy but its fundamentals are also valid and can be formulated in an analogous
way for Nuclear Magnetic Resonance (NMR). The interpretation of magnetic resonance data in the context of structural and chemical biology of paramagnetic metallic
cofactors does require knowledge not only of the magnitude and strength but also
the directionality and orientation of these magnetic interactions with respect to the
compound or protein structural frame. Here, systems containing transition metal
ions are being discussed since they are challenging to study experimentally but also
computationally demanding in terms of model size and accuracy of results.
Transition metals are in particular interesting and provide the following challenges
and difficulties:
(i) A multitude of oxidation states
(ii) Different electronic ground states (low-spin, intermediate spin, high spin) for
a metal in a particular oxidation state
(iii) Redox activity of transition metal during catalytic cycle
(iv) Changes in spin state during a reaction (‘spin state catalysis’).
Thus, spectra of transition metal-containing complexes or proteins are particularly
cumbersome to analyze and interpret in terms of molecular structures. The EPR gvalues for transition metal complexes vary over a wide spectral range due to spin–orbit
coupling and zero-field splitting, depending on the geometry of the complex and
this variability in nature and number of ligands makes it impossible to establish an
empirical approach to predict g-values of most transition metal systems.
As a subset, selected examples from nickel, molybdenum and manganese containing transition metal complexes and enzymatic systems are discussed.
Nickel occurs in coordination complexes and metalloenzymes in different oxidation states which may be para- or diamagnetic depending on the ligand environment. Nickel enzymes catalyze a wide range of chemical reactions using mononuclear, homo- or heterodinuclear active sites and are present among others in urease,
superoxide dismutase, carbon monoxide dehydrogenase, acetyl-coenzyme A synthase/decarbonylase and methyl-coenzyme M reductase [3, 4]. [NiFe]-hydrogenase
enzymes possess a heterobimetallic active site and the nickel ion occurs in the +I, +II,
and +III oxidation states during the heterolytic splitting of molecular hydrogen (H 2 ).
It is covered here as one example of the application of EPR spectroscopy in solution
and in protein single crystals (for reviews see [5, 6]) hand in hand with computational means to identify the intermediates by calculating spectroscopic observables
and elucidate the enzymatic reaction mechanism [7, 8].
Molybdoenzymes are widespread in eukaryotic and prokaryotic organisms where
they have crucial functions in detoxification reactions in the metabolism of humans
and bacteria [9]. In human, molybdenum xanthine oxidoreductase (XO), aldehyde