Third, the generation of electrons is sequen-
tial as each absorption of a photon results in
the transfer of a single electron. Therefore, the
electron equivalents must be stored until the
fourth electron is available to drive the reac-
tion to completion. This function is performed
by the manganese cluster, which consists of four
manganese atoms. The manganese cluster is
oxidized by YZ, with rates ranging from 20 to
300 ns for the different S states. As discussed
above, the molecular arrangement of the four
manganese has not been determined using the
X-ray data due to the limited resolution. How-
ever, two spectroscopic techniques, extended
X-ray absorption spectroscopy (EXAFS) and
EPR, have been very revealing about the mole-
cular arrangement.
As the manganese cluster undergoes the
S-state transitions, the oxidation state of the
cluster changes. The development of EPR
spectroscopy has produced signals from all of
the S states, which show different gvalues and
degrees of resolved hyperfine interactions
(Figure 20.13). The most characteristic EPR
signal is for the S2 state that was found to be
centered at g=2 and contained multiple lines
characteristic of hyperfine coupling between
the unpaired electron and the manganese
nuclei of the cluster. The^55 Mn isotope is
100% naturally abundant, with a nuclear
spin of 1/2, so a single manganese will have
an EPR spectrum with six lines (= 2 I+1/2). The abundance of lines can
be modeled by assuming that the metal cluster must be more complex.
Synthetic Mn(III)–Mn(IV) binuclear clusters have 16 hyperfine lines
that resemble the spectrum of the S2 state. In addition to the hyperfine
lines, the low-temperature EPR spectrum of the S2 state shows a signal
at g=4.1 that has no hyperfine structure. Comparison with model man-
ganese systems led to the conclusion that the g=4.1 signal was due to
the presence of a tetranuclear cluster.
As discussed in Chapter 15, the advent of synchrotrons provided the
opportunity to investigate metal clusters using EXAFS. The measurement
of photosystem II using EXAFS showed the presence of three distinctive
peaks corresponding the distances of 1.8, 2.7, and 3.3 –3.4 Å (Figure 20.14).
The distance of 2.7 Å is consistent with the presence of a di-μ-oxo-bridged
manganese. The peak at 3.3 Å was assigned as representing a distance434 PART 3 UNDERSTANDING BIOLOGICAL SYSTEMS USING PHYSICAL CHEMISTRY
EPR signal
Magnetic field (G)S 0EPR signal
Magnetic field (G)S 2EPR signal
Magnetic field (G)S 1EPR signal
Magnetic field (G)S 2 YZ*Figure 20.13EPR of photosystem II showing
different spectroscopic signatures for each S state.
Modified from Britt et al. (2003).