4 research outputs found
FTIR Study of Manganese Dimers with Carboxylate Donors As Model Complexes for the Water Oxidation Complex in Photosystem II
The carboxylate stretching frequencies of two high-valent,
di-μ-oxido
bridged, manganese dimers has been studied with IR spectroscopy in
three different oxidation states. Both complexes contain one monodentate
carboxylate donor to each Mn ion, in one complex, the carboxylate
is coordinated perpendicular to the Mn-(μ-O)<sub>2</sub>-Mn
plane, and in the other complex, the carboxylate is coordinated in
the Mn-(μ-O)<sub>2</sub>-Mn plane. For both complexes, the difference
between the asymmetric and the symmetric carboxylate stretching frequencies
decrease for both the Mn<sub>2</sub><sup>IV,IV</sup> to Mn<sub>2</sub><sup>III,IV</sup> transition and the Mn<sub>2</sub><sup>III,IV</sup> to Mn<sub>2</sub><sup>III,III</sup> transition, with only minor
differences observed between the two arrangements of the carboxylate
ligand versus the Mn-(μ-O)<sub>2</sub>-Mn plane. The IR spectra
also show that both carboxylate ligands are affected for each one
electron reduction, i.e., the stretching frequency of the carboxylate
coordinated to the Mn ion that is not reduced also shifts. These results
are discussed in relation to FTIR studies of changes in carboxylate
stretching frequencies in a one electron oxidation step of the water
oxidation complex in Photosystem II
Room-Temperature Energy-Sampling Kβ X‑ray Emission Spectroscopy of the Mn<sub>4</sub>Ca Complex of Photosynthesis Reveals Three Manganese-Centered Oxidation Steps and Suggests a Coordination Change Prior to O<sub>2</sub> Formation
In
oxygenic photosynthesis, water is oxidized and dioxygen is produced
at a Mn<sub>4</sub>Ca complex bound to the proteins of photosystem
II (PSII). Valence and coordination changes in its catalytic S-state
cycle are of great interest. In room-temperature (in situ) experiments,
time-resolved energy-sampling X-ray emission spectroscopy of the Mn
Kβ<sub>1,3</sub> line after laser-flash excitation of PSII membrane
particles was applied to characterize the redox transitions in the
S-state cycle. The Kβ<sub>1,3</sub> line energies suggest a
high-valence configuration of the Mn<sub>4</sub>Ca complex with MnÂ(III)<sub>3</sub>MnÂ(IV) in S<sub>0</sub>, MnÂ(III)<sub>2</sub>MnÂ(IV)<sub>2</sub> in S<sub>1</sub>, MnÂ(III)ÂMnÂ(IV)<sub>3</sub> in S<sub>2</sub>, and
MnÂ(IV)<sub>4</sub> in S<sub>3</sub> and, thus, manganese oxidation
in each of the three accessible oxidizing transitions of the water-oxidizing
complex. There are no indications of formation of a ligand radical,
thus rendering partial water oxidation before reaching the S<sub>4</sub> state unlikely. The difference spectra of both manganese Kβ<sub>1,3</sub> emission and K-edge X-ray absorption display different
shapes for MnÂ(III) oxidation in the S<sub>2</sub> → S<sub>3</sub> transition when compared to MnÂ(III) oxidation in the S<sub>1</sub> → S<sub>2</sub> transition. Comparison to spectra of manganese
compounds with known structures and oxidation states and varying metal
coordination environments suggests a change in the manganese ligand
environment in the S<sub>2</sub> → S<sub>3</sub> transition,
which could be oxidation of five-coordinated MnÂ(III) to six-coordinated
MnÂ(IV). Conceivable options for the rearrangement of (substrate) water
species and metal–ligand bonding patterns at the Mn<sub>4</sub>Ca complex in the S<sub>2</sub> → S<sub>3</sub> transition
are discussed
Kα X‑ray Emission Spectroscopy on the Photosynthetic Oxygen-Evolving Complex Supports Manganese Oxidation and Water Binding in the S<sub>3</sub> State
The
unique manganese–calcium catalyst in photosystem II (PSII)
is the natural paragon for efficient light-driven water oxidation
to yield O<sub>2</sub>. The oxygen-evolving complex (OEC) in the dark-stable
state (S<sub>1</sub>) comprises a Mn<sub>4</sub>CaO<sub>4</sub> core
with five metal-bound water species. Binding and modification of the
water molecules that are substrates of the water-oxidation reaction
is mechanistically crucial but controversially debated. Two recent
crystal structures of the OEC in its highest oxidation state (S<sub>3</sub>) show either a vacant Mn coordination site or a bound peroxide
species. For purified PSII at room temperature, we collected Mn Kα
X-ray emission spectra of the S<sub>0</sub>, S<sub>1</sub>, S<sub>2</sub>, and S<sub>3</sub> intermediates in the OEC cycle, which
were analyzed by comparison to synthetic Mn compounds, spectral simulations,
and OEC models from density functional theory. Our results contrast
both crystallographic structures. They indicate Mn oxidation in three
S-transitions and suggest additional water binding at a previously
open Mn coordination site. These findings exclude Mn reduction and
render peroxide formation in S<sub>3</sub> unlikely
Cobaloxime-Based Artificial Hydrogenases
Cobaloximes are popular H<sub>2</sub> evolution molecular catalysts but have so far mainly been studied
in nonaqueous conditions. We show here that they are also valuable
for the design of artificial hydrogenases for application in neutral
aqueous solutions and report on the preparation of two well-defined
biohybrid species via the binding of two cobaloxime moieties, {CoÂ(dmgH)<sub>2</sub>} and {CoÂ(dmgBF<sub>2</sub>)<sub>2</sub>} (dmgH<sub>2</sub> = dimethylglyoxime), to apo <i>Sperm-whale</i> myoglobin
(<i>Sw</i>Mb). All spectroscopic data confirm that the cobaloxime
moieties are inserted within the binding pocket of the <i>Sw</i>Mb protein and are coordinated to a histidine residue in the axial
position of the cobalt complex, resulting in thermodynamically stable
complexes. Quantum chemical/molecular mechanical docking calculations
indicated a coordination preference for His93 over the other histidine
residue (His64) present in the vicinity. Interestingly, the redox
activity of the cobalt centers is retained in both biohybrids, which
provides them with the catalytic activity for H<sub>2</sub> evolution
in near-neutral aqueous conditions