Oxidation of exogenous substrates by the O₂-evolving center of photosystem II and related catalytic air oxidation of secondary alcohols via a tetranuclear manganese(IV) complex.

The O₂-evolving center of Photosystem II (PSII) contains atetranuclear Mn complex that acts as the catalyst for photosynthetic H₂O oxidation. The catalytic cycle for H₂O oxidation is thought to involve the light-driven generation of a very strongly oxidizing state of the Mn complex prior to the formation of the OO bond. However, the Mn complex also exhibits oxidation chemistry in its lower oxidation states. Exogenous ligands, such as primary amines and hydroxylamines, have been shown previously to interact with the O₂-evolving center in the dark S₁ oxidation state by coordinating to a Cl^--binding site near the Mn complex. In this paper, we present evidence that primary and secondary amines irreversibly inactivate the Mn complex in the S₁ state by a reductive mechanism, leading to the liberation of Mn(II) ions and a concomitant loss of O₂-evolution activity. Hence, the Mn complex in the O₂-evolving center acts as a strong oxidant even in the dark-stable S₁ state. The synthetic tetranuclear Mn(IV) complex [(TACN)₄Mn₄O₆]Br₄, which has been suggested as a model for the Mn complex in the O₂-evolving center, is a strong oxidant as well. We find that it catalyzes the air oxidation of secondary alcohols to ketones and triphenylphosphine to triphenylphosphine oxide.