BIOINORGANIC CHEMISTRY A Short Course Second Edition

orientation of the applied fi eld. In other words, the parallel versus perpendicu-
lar dependence of the M ö ssbauer spectrum becomes less pronounced as g
becomes more anisotropic ( gx ≠ gy ≠ gz ≠ 0) and the spectrum looks more like
Figure 3.27.
The six - line magnetic hyperfi ne splitting spectrum seen for the anisotropic
cases is averaged over all molecular orientations and will have a 3 : 2 : 1 : 1 : 2 : 3
intensity pattern as shown in Figure 3.27. Because g values are determined by
the electron paramagnetic resonance (EPR) technique as discussed above in
Section 3.5 , one can make reasonable predictions about the shape of the
M ö ssbauer spectrum if the results of an EPR study are known. For instance,
a magnetic - fi eld - independent M ö ssbauer spectrum suggests an EPR silent
state; and conversely, a magnetic - fi eld - dependent M ö ssbauer spectrum implies
that an EPR spectrum should be observed.

3.6.4 Descriptive Examples,

M ö ssbauer spectroscopy, along with evidence gathered from EPR and
ENDOR, has identifi ed the behavior of iron atoms in the M center, FeMo
cofactor, of the enzyme nitrogenase from Azobacter vinelandii. Figure 3.28
shows a schematic diagram of nitrogenase ’ s M center. At the time of the EPR
study, it was known that two M center clusters were contained in each nitro-
genase MoFe – protein subunit, that each was of composition Mo Fe 6 – 8 S 9 ± 1 , and
that each cluster contained three unpaired electrons in anS = 3/2 system.^38
The rhombic EPR signal with g factors g 1 (= gx ) = 4.32, g 2 (= gy ) = 3.68 and
g 3 (= gz ) = 2.01 would have the appearance of the rhombic spectrum of
Figure 3.22D in Section 3.5.1. (Also see Figure 16 of reference 39 .)
The electron - nuclear double resonance (ENDOR) technique mentioned
in the previous paragraph is used to study electron – nuclear hyperfi ne

Figure 3.28 The FeMo cofactor, M center, of nitrogenase ’ s MoFe protein.

Fe 1

S S S

Fe 4 Fe 3

cysα 275

Fe 2

S S

Fe 5 Fe^7

S

Fe 6

S S

Mo

S

442 α his

O

O

homocitrate

MÖSSBAUER SPECTROSCOPY 137