CHEMICAL MODELS OF ENZYMES - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 184

6.5. Water oxidation

The life giving process of water decomposition in containing manganese photosynthetic
systems still remains one of the most challenging problems to biochemists and chemists.
The evolution dioxygen from water in a cluster of transition metals in the biological
systems at the absorption of light quanta of low energy can occurs by a sequence of
elementary steps: four one-electron steps of oxidation of the manganese complex and,
most probably and by one four-electron step of evolution (Section 3.5.2). In
approaching this problem, a number of artificial manganese clusters and other transition
metal clusters were synthesized and investigated (Shafirovich, 1995; Britt, 1996; Shilov,
1997; Rüttinger and Dismukes, 1997; Rüttinger et al., 2000; Wikaira and Gorun 1999;).
The crystallographic structures of the complex of different
oxidative states (-,0,+) have been determined (Gelasko et al., 1997). These molecules
form dimers with both of the ligands spanning both Mn ions with the alkoxide on the
backbone of the ligand bridging the metals. The following metal-metal distances were
obtained:

Significant structural changes in the polyhedra of X-ray
structures of a series of dimanganese complexes and terpyridine dimanganese oxo
complexes across the range of metal oxidation states, have been observed. The authors
suggested that these changes are reminiscent of the carboxylate shift in metal
carboxylate in the natural complex. It also illustrates how alkoxide ligands can
participate in an analogous alkoxide shift to generate a binding site for an incoming
ligand, such as MeOH, or a substrate, such as H2O2.
A series of dimanganese complexes,
and (terpy

= 2,2 -terpyridine) have been crystallographically characterized (Baffert et al.,

2002). The electrochemical behavior of complex (2) in shows that while this
complex could be oxidized into its stable manganese(IV,IV) species its reduced form
manganese(III,III) is very unstable.
A model water oxidation complex (terpy
is 2,2':6',2"-terpyridine), containing a manganese dimer, was synthesized and
structurally characterized (Limburg et al., 1999). This complex catalyzes the dioxygen
evolution. Oxygen-18 isotope labeling showed that water is the source of the oxygen
atoms in the evolved dioxygen. Another functional model for photosynthetic water
oxidation, the complex,

' :6,2 "

(terpy =

-Terpyridine,) has been synthesized and characterized(Limburg et al., 2000,
2001). This complex catalyzes evolution from either KHSO5 (potassium oxone) or
NaOCl via an intermediate complex The
efficiency of the catalyst was relatively low: Vmax = 2420 mol O2 (mol 1)-1 hr-1 and
mM for oxone and mol O2 (mol 1)-1 hr-1 and
mM for hypochlorite with first-order kinetics observed for both oxidants.
Isotope-labeling studies using and show that evolution proceeds via
an intermediate that can exchange with water. The rate-limiting step of evolution is
proposed to be the formation of a formally MnV:O moiety which could then
competitively react with either oxone or water/hydroxide to produce

2,2 ' :6,2 "

Dioxygen