Photosynthetic water oxidation and O2 formation are catalyzed by a Mn4Ca
complex bound to the proteins of photosystem II (PSII). The catalytic site,
including the inorganic Mn4CaOnHx core and its protein environment, is denoted
as oxygen-evolving complex (OEC). Earlier and recent progress in the endeavor
to elucidate the structure of the OEC is reviewed, with focus on recent
results obtained by (i) X-ray spectroscopy (specifically by EXAFS analyses),
and (ii) X-ray diffraction (XRD, protein crystallography). Very recently, an
impressive resolution of 1.9 Å has been achieved by XRD. Most likely however,
all XRD data on the Mn4CaOnHx core of the OEC are affected by X-ray induced
modifications (radiation damage). Therefore and to address (important) details
of the geometric and electronic structure of the OEC, a combined analysis of
XRD and XAS data has been approached by several research groups. These efforts
are reviewed and extended using an especially comprehensive approach. Taking
into account XRD results on the protein environment of the inorganic core of
the Mn complex, 12 alternative OEC models are considered and evaluated by
quantitative comparison to (i) extended-range EXAFS data, (ii) polarized EXAFS
of partially oriented PSII membrane particles, and (iii) polarized EXAFS of
PSII crystals. We conclude that there is a class of OEC models that is in good
agreement with both the recent crystallographic models and the XAS data. On
these grounds, mechanistic implications for the Osingle bondO bond formation
chemistry are discussed. This article is part of a Special Issue entitled:
Photosystem II