The Role of Manganese Complexes in Photosynthesis

In reality, it results from the following two thermodynamically disfavoured half-reactions  

The present discussion is only concerned with the structure/redox capacity of the site responsible for the oxidation of water. The starting point is the evidence that the photosynthetic pathway is triggered by photooxidation of the chlorophylls in photosystem II. The need for chlorophylls to recover the electrons lost in photooxidation (in order to regenerate their ability to absorb light) induces water to undergo oxidation, according to  

The mechanism of simultaneously releasing two electrons per water molecule helps Nature to avoid one-electron processes such as  

There is evidence that it is a manganese complex that acts as a mediator in supplying the electrons [through the Mn(II)/Mn(III)/Mn(IV) redox cycle] necessary to return the photo-oxidized chlorophylls back to their reduced state. The manganese centre is able to provide the four electrons produced in the oxidation of water in four successive steps. 

The previously discussed mixed-valent Mn(III)/Mn(IV) complexes [Mn202(phen)4]3 + and [Mn202(bipy)4]3 +, because of their EPR 

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