MECHANISMS OF CHOSEN ENZYME REACTIONS - New Trends in Enzyme Catalysis and Biomimetic Chemical Reactions - page 127

Water splitting Mn-containing system
Manganese has long been assumed to play a role in the photosynthetic decomposition of
water by plants. In the classical work of Koch and subsequent investigations (Koch et
al., 1970; Widrzynski and Sauer, 1980; Covindjee et al., 1985; Shutilova, 2000; Yagi
and Kaneko, 2001; Dismukes, 2001; Dismukes et al., 2000; Carrell et al., 2002) the key
role of Mn has been confirmed. It was shown that for to be evolved four light flashes
are required. In the currently adopted terminology, the redox state of the evolution
system is designated as in accordance with the number of electrons
transferred from the complex. After prolonged dark adaptation the complex is found in
the state. The evolution of is realized in the presence inorganic ligands and

Photosystem II containes four Mn atoms in the complex attached to a protein. The
cluster nature of the complex has been convincingly demonstrated by a whole arsenal of
physical and biochemical methods. As an example, in the work of Kulikov et al., (1983),
manganese was first removed from a preparation of FS II; then Mn was gradually
introduced. The magnetic state of the Mn complex was traced by the extent of saturation
of the ESR signal of the anion radical of pheophytin. The incorporation of one Mn atom
into the system sharply charges the saturation curve parameters of the ESR signal; the
introduction of the second Mn atom removes this effect. A third Mn atom again affects
on the saturated curve, while the incorporation of further Mn atoms into the medium has
no effect the ESR spectrum. Multiple evidence for strong magnetic interaction within
four-nuclear Mn cluster were reported in 1970's and 1980's by the ESR method (Evans
et a., 2000, 1977; Yocum et al., 1981; De Paula et al. 1986; Hoff, 1986; Dismukes, 1986;
Govingee et al., 1985) and was confirmed in recent investigations (Matsukava et al,
1999; Hanley et al., 2000;

I oannidis and Petrouleas, 2000; Yagi and Kaneko, 2001; and

references therein).