Oxidation of exogenous substrates by the O 2-evolving center of photosystem II and related catalytic air oxidation of secondary alcohols via a tetranuclear manganese(IV) complex.

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 OO 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.