Abstract
The O 2-evolving center of Photosystem II (PSII) contains atetranuclear Mn complex that acts as the catalyst for photosynthetic H 2O oxidation. The catalytic cycle for H 2O 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 O 2-evolving center in the dark S 1 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 S 1 state by a reductive mechanism, leading to the liberation of Mn(II) ions and a concomitant loss of O 2-evolution activity. Hence, the Mn complex in the O 2-evolving center acts as a strong oxidant even in the dark-stable S 1 state. The synthetic tetranuclear Mn(IV) complex [(TACN) 4Mn 4O 6]Br 4, which has been suggested as a model for the Mn complex in the O 2-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.
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