4 From Small Molecules to Complex Systems: A Survey of Chemical …
207
and the isomer shift δ can—as a first input for the simulation be taken from measurements in zero field—this holds for the S = 2 case—or from measurements taken
at higher T for those paramagnetic centers where magnetic splitting is observed at
4.2 K. For δ one can use e.g. measurements at T = 77 K since the second order
Doppler shift between 77 and 4.2 K is often negligible, nevertheless a temperature
dependence of ΔE Q can occur due to population of excited orbital states.
Quite a lot of Fe–S proteins have 2Fe–2S iron centers. These centers occur in
the [2Fe–2S]
2+ state and can be reduced to the [2Fe–2S]
1+ state. Thus, also 2Fe–
2S centers support electron transfer in proteins. A typical example are ferredoxins.
These proteins have 2Fe–2S centers which are coordinated to the protein via 4 cystein
residues and which are connected via a disulfide bridge. There are also 2Fe–2S
proteins which have one iron site coordinated to two histidine ligands, the most wellknown of them being the Rieske-cluster of the bc 1 complex [86]. Very recently a
bishistidinyl-coordinated non-Rieske [2Fe–2S] protein was identified also by means
of Mössbauer spectroscopy [87]. Figure 4.22 shows the Mössbauer spectrum of the
wild-type protein APD showing two doublets. One doublet with δ 1 = 0.24 mms
−1 is
typical for a ferric FeS 4 center whereas the other doublet has a slightly higher isomer
shift of δ 2 = 0.35 mms
−1 . This points to a coordination with two sulfurs and two
nitrogen ligands from histidin like in the Rieske potein (FeS 2 N 2 ). The fact that APD
has also a diferric center can be interfered by the analysis of a Mössbauer spectrum
obtained in a high external field (Fig. 4.22b). The so observed Mössbauer pattern can
be reproduced by a simulation which assumes that the total spin of the [2Fe–2S]
2+
center is zero. This can be explained by an antiparallel spin coupling of two ferric
high spin sites as depicted in Fig. 4.22.
After reduction of the [2Fe–2S]
2+ center to a [2Fe–2S]
1+ center by one electron the
Mössbauer spectrum at low field changes completely (Fig. 4.22c). Now already at low
fields a magnetic splitting can be observed. From EPR spectroscopy one deduces that
the total spin of the [2Fe–2S]
1+ center is S = 1/2. This spin state together with the gtensor components obtained from EPR can be used as input for the spin Hamiltonian
simulations which are drawn as solid curves in Fig. 4.22c, d. The resulting total spin,
which often also is called system spin S = 1/2 originates from an antiparallel spin
coupling of the ferric high spin S 1 = 5/2 and the ferrous high spin S 2 = 2. For a
more in deep discussion of the spin coupling between mixed valent iron sulfur sites
we refer to reviews [34, 35] and the references given therein.
Figure 4.23 displays the Mössbauer spectra of an iron sulfur protein having a
[4Fe–4S]
2+ center. Also these clusters are quite common in iron sulfur proteins. In
the majority of cases known to date these also function as electron transfer centers,
but in some 4Fe–4S proteins the cluster itself is also catalytically active as shown in
Sect. 5.2. The protein the Mössbauer spectrum of which is displayed in Fig. 4.23 is
called PhrB [88] and it belongs to the cryptochrome/photolyase family (CPF). The
function of PhrB has been shown to be related to DNA repair.
Despite there are four iron sites in the [4Fe–4S]
2+ cluster there is only one doublet
in the spectrum of the
57 Fe-enriched PhrB both at T = 77 K (Fig. 4.23a) and T = 4.2 K
(Fig. 4.23b) with δ = 0.42 mms
−1 and ΔE Q = 1.26 mms
−1 (Fig. 4.23a). This shows
that all four iron sites are equivalent and have the same coordinated ligands. The
207
and the isomer shift δ can—as a first input for the simulation be taken from measurements in zero field—this holds for the S = 2 case—or from measurements taken
at higher T for those paramagnetic centers where magnetic splitting is observed at
4.2 K. For δ one can use e.g. measurements at T = 77 K since the second order
Doppler shift between 77 and 4.2 K is often negligible, nevertheless a temperature
dependence of ΔE Q can occur due to population of excited orbital states.
Quite a lot of Fe–S proteins have 2Fe–2S iron centers. These centers occur in
the [2Fe–2S]
2+ state and can be reduced to the [2Fe–2S]
1+ state. Thus, also 2Fe–
2S centers support electron transfer in proteins. A typical example are ferredoxins.
These proteins have 2Fe–2S centers which are coordinated to the protein via 4 cystein
residues and which are connected via a disulfide bridge. There are also 2Fe–2S
proteins which have one iron site coordinated to two histidine ligands, the most wellknown of them being the Rieske-cluster of the bc 1 complex [86]. Very recently a
bishistidinyl-coordinated non-Rieske [2Fe–2S] protein was identified also by means
of Mössbauer spectroscopy [87]. Figure 4.22 shows the Mössbauer spectrum of the
wild-type protein APD showing two doublets. One doublet with δ 1 = 0.24 mms
−1 is
typical for a ferric FeS 4 center whereas the other doublet has a slightly higher isomer
shift of δ 2 = 0.35 mms
−1 . This points to a coordination with two sulfurs and two
nitrogen ligands from histidin like in the Rieske potein (FeS 2 N 2 ). The fact that APD
has also a diferric center can be interfered by the analysis of a Mössbauer spectrum
obtained in a high external field (Fig. 4.22b). The so observed Mössbauer pattern can
be reproduced by a simulation which assumes that the total spin of the [2Fe–2S]
2+
center is zero. This can be explained by an antiparallel spin coupling of two ferric
high spin sites as depicted in Fig. 4.22.
After reduction of the [2Fe–2S]
2+ center to a [2Fe–2S]
1+ center by one electron the
Mössbauer spectrum at low field changes completely (Fig. 4.22c). Now already at low
fields a magnetic splitting can be observed. From EPR spectroscopy one deduces that
the total spin of the [2Fe–2S]
1+ center is S = 1/2. This spin state together with the gtensor components obtained from EPR can be used as input for the spin Hamiltonian
simulations which are drawn as solid curves in Fig. 4.22c, d. The resulting total spin,
which often also is called system spin S = 1/2 originates from an antiparallel spin
coupling of the ferric high spin S 1 = 5/2 and the ferrous high spin S 2 = 2. For a
more in deep discussion of the spin coupling between mixed valent iron sulfur sites
we refer to reviews [34, 35] and the references given therein.
Figure 4.23 displays the Mössbauer spectra of an iron sulfur protein having a
[4Fe–4S]
2+ center. Also these clusters are quite common in iron sulfur proteins. In
the majority of cases known to date these also function as electron transfer centers,
but in some 4Fe–4S proteins the cluster itself is also catalytically active as shown in
Sect. 5.2. The protein the Mössbauer spectrum of which is displayed in Fig. 4.23 is
called PhrB [88] and it belongs to the cryptochrome/photolyase family (CPF). The
function of PhrB has been shown to be related to DNA repair.
Despite there are four iron sites in the [4Fe–4S]
2+ cluster there is only one doublet
in the spectrum of the
57 Fe-enriched PhrB both at T = 77 K (Fig. 4.23a) and T = 4.2 K
(Fig. 4.23b) with δ = 0.42 mms
−1 and ΔE Q = 1.26 mms
−1 (Fig. 4.23a). This shows
that all four iron sites are equivalent and have the same coordinated ligands. The