The manganese complex of oxygenic photosynthesis: conversion of five-coordinated Mn(III) to six-coordinated Mn(IV) in the S2-S3 transition is implied by XANES simulations

Abstract

Photosynthetic water oxidation proceeds at a tetra-manganese complex bound to a cofactor-protein complex denoted as Photosystem II (PSII). To understand this process of fundamental importance for life on Earth, the crucial changes of the active-site occurring upon the individual transitions in the catalytic S-state cycle have been investigated by x-ray absorption spectroscopy at the Mn K-edge. For the S2-S3 transition, the changes in position and shape of the x-ray edge have been interpreted to be either associated with oxidation of Mn(III) to Mn(IV) or to formation of a ligand radical without manganese oxidation. Here we simulate XANES spectra of simple Mn-complexes using modern multiple-scattering code. Features of the XANES spectra are assigned to molecular orbitals by means of comparative semi-empirical calculations. It is concluded that the changes in position and shape of the edge spectra observed for the S2-S3 transition are explainable by the transformation of five-coordinated Mn(III) in a square-pyramidal geometry to six-coordinated Mn(IV).