Recent advances in serial femtosecond crystallography
(SFX) of
photosystem II (PSII), enabled by X-ray free electron lasers (XFEL),
provided the first geometric models of distinct intermediates in the
catalytic S-state cycle of the oxygen-evolving complex (OEC). These
models are obtained by flash-advancing the OEC from the dark-stable
state (S1) to more oxidized intermediates (S2and S3), eventually cycling back to the most reduced
S0. However, the interpretation of these models is controversial
because geometric parameters within the Mn4CaO5cluster of the OEC do not exactly match those expected from coordination
chemistry for the spectroscopically verified manganese oxidation states
of the distinct S-state intermediates. Here we focus on the first
catalytic transition, S1→ S2, which
represents a one-electron oxidation of the OEC. Combining geometric
and electronic structure criteria, including a novel effective oxidation
state approach, we analyze existing 1-flash (1F) SFX-XFEL crystallographic
models that should depict the S2state of the OEC. We show
that the 1F/S2equivalence is not obvious, because the
Mn oxidation states and total unpaired electron counts encoded in
these models are not fully consistent with those of a pure S2state and with the nature of the S1→ S2transition. Furthermore, the oxidation state definition in two-flashed
(2F) structural models is practically impossible to elucidate. Our
results advise caution in the extraction of electronic structure information
solely from the literal interpretation of crystallographic models
and call for re-evaluation of structural and mechanistic interpretations
that presume exact correspondence of such models to specific catalytic
intermediates of the OEC.