132 INSTRUMENTAL METHODS
attached to the C ε atom of histidine), provided spin density and structural
information — how the histidine ligands are arranged in relation to the Cu 2 S 2
nearly planar core. The researchers found another weak coupling that was
assigned to an amide proton in the main chain of one of the cysteine ligands.
Their conclusions included the following: (1) The ENDOR data were consis-
tent with the electron - spin density being similar for the two Cu ions ( ρCu ) and
two sulfur atoms ( ρS ) in the Cu 2 S 2 core, although the COX fragment M160T9
appeared to have a somewhat largerρS than the other Cu A sites; (2) the more
weakly coupled histidine H ε 1 protons indicated their positions outside of the
nearly planar Cu 2 S 2 core; and (3) the most weakly coupled amide proton of
Cu ligand cys200 (in the bovine heart cytochrome c oxidase X - ray crystallo-
graphic structure, PDB: 1OCC discussed in Section 7.8) might suggest its
signifi cance for electron transfer from COX ’ s redox partner, cytochrome c.
3.6 M Ö SSBAUER SPECTROSCOPY
3.6.1 Theoretical Aspects,
The M ö ssbauer effect as a spectroscopic method probes transitions within an
atom ’ s nucleus and therefore requires a nucleus with low - lying excited states.
The effect has been observed for 43 elements. For applications in bioinorganic
chemistry, the^57 Fe nucleus has the greatest relevance and the focus will be
exclusively on this nucleus here. M ö ssbauer spectroscopy requires (a) the
emission ofγ rays from the source element in an excited nuclear state and (b)
absorption of these by the same element in the sample under study. The M ö ss-
bauer phenomenon requires that the emission and absorption of theγ radia-
tion take place in a recoil - free manner; this is accomplished by placing the
nucleus in a solid or frozen solution matrix. Three main types of interaction
of the nuclei of interest with the chemical environment surrounding it cause
detectable changes in the energy required for absorption: (1) resonance line
shifts from changes in electron environment — see discussion of the isomer
shift, δ , below; (2) quadrupole interactions — see discussion of ΔEQ below; and
(3) magnetic interactions. The last type, magnetic interactions, is especially
important in studying bioinorganic systems such as iron – sulfur clusters found
in aconitase (Section 7.9.2.1 ), cytochrome bc 1 (Section 7.6), and cytochrome
b(6)f (Section 7.5 ).
The source for the 14.41 - keV γ radiation used in M ö ssbauer experiments is
indicated by the boldface arrow in Figure 3.24.^3 Origin of the isomer shift and
quadrupole splitting phenomena are indicated at the right - hand side of the
diagram.
The electronic environment about the sample ’ s nucleus infl uences the
energy of theγ ray necessary to cause the nuclear transition from the ground
to the excited state. The energies of the γ rays from the source can be varied
by moving the source relative to the sample. In order to obtain the M ö ssbauer
spectrum, the source is moved relative to the fi xed sample, and the source