In the photosynthetic evolution of oxygen, water oxidation occurs at a catalytic site that includes four manganese atoms together with the essential cofactors, the calcium and chlorine ions. A structural model and a determination of the manganese oxidation states based on x-ray absorption spectroscopy are presented. The salient features, in both higher plants and cyanobacteria, are a pair of di-mu-oxo bridged manganese binuclear clusters linked by a mono-mu-oxo bridge, one proximal calcium atom, and one halide. In dark-adapted samples, manganese occurs in oxidation states (III) and (IV). Data from oriented membranes display distinct dichroism, precluding highly symmetrical structures for the manganese complex.
The photosynthetic oxygen-evolving complex contains a cluster of four manganese atoms and requires both Ca and Cl for activity. The question of Ca proximity to the Mn cluster has been investigated by performing Mn X-ray absorption experiments on native samples of photosystem II (PS II) and on samples depleted of Ca and reconstituted by either Ca or Sr. Analysis of X-ray K-edge spectra demonstrates no significant differences in oxidation state or symmetry between Ca- and Sr-reactivated preparations. Differences are observed in the extended X-ray absorption fine structure (EXAFS). The amplitude of a Fourier transform peak due to scatters at distances greater than 3 A is larger for samples reactivated with strontium than for calcium-reactivated samples. Taking into account the stoichiometry of Mn and Ca atoms in PS II, and considering physically reasonable structures, curve-fitting analyses of the EXAFS data using FEFF5-calculated parameters favor a model where both manganese and calcium (or strontium) scatterers contribute to the Fourier peak at approximately 3 A. Other models for the approximately 3 A peak with multiple Mn-Mn interactions or multiple Mn-Ca(Sr) interactions can also be fit to the data, but are considered less likely. This result provides confirmation for the structural proximity of Ca to the Mn cluster suggested previously [Yachandra, V. K., et al. (1993) Science 260, 675-679]. Possible structural arrangements for a calcium-binding site are discussed.
This paper describes a method, discovered and refined by parallel screening, for the epoxidation of alkenes. It uses hydrogen peroxide as the terminal oxidant, is promoted by catalytic amounts (1.0-0.1 mol %) of manganese(2+) salts, and must be performed using at least catalytic amounts of bicarbonate buffer. Peroxymonocarbonate, HCO(4)(-), forms in the reaction, but without manganese, minimal epoxidation activity is observed in the solvents used for this research, that is, DMF and (t)BuOH. More than 30 d-block and f-block transition metal salts were screened for epoxidation activity under similar conditions, but the best catalyst found was MnSO(4). EPR studies show that Mn(2+) is initially consumed in the catalytic reaction but is regenerated toward the end of the process when presumably the hydrogen peroxide is spent. A variety of aryl-substituted, cyclic, and trialkyl-substituted alkenes were epoxidized under these conditions using 10 equiv of hydrogen peroxide, but monoalkyl-alkenes were not. To improve the substrate scope, and to increase the efficiency of hydrogen peroxide consumption, 68 diverse compounds were screened to find additives that would enhance the rate of the epoxidation reaction relative to a competing disproportionation of hydrogen peroxide. Successful additives were 6 mol % sodium acetate in the (t)BuOH system and 4 mol % salicylic acid in the DMF system. These additives enhanced the rate of the desired epoxidation reaction by 2-3 times. Reactions performed in the presence of these additives require less hydrogen peroxide and shorter reaction times, and they enhance the yields obtained from less reactive alkene substrates. Possible mechanisms for the reaction are discussed.
We report an extensive advanced paramagnetic resonance
characterization of the mixed-valence dinuclear
Fe center of methane monooxygenase hydroxylase (MMOHmv)
from Methylococcus capsulatus (Mc) (Bath) and
of
binding to it by the exogenous ligand DMSO. We employ continuous
wave and pulsed electron nuclear double
resonance (ENDOR) spectroscopy, both at Q-band microwave frequencies,
to examine 14,15N, 1,2H, 13C, and
57Fe
nuclei. Preliminary 1H ENDOR results were communicated
previously (DeRose, V. J.; Liu, K. E.; Hoffman,
B.
M.; Lippard, S. J. J. Am. Chem. Soc.
1993,
115, 6440−6441). ENDOR-derived 14,15N
hyperfine tensors are interpreted
in terms of the spin distribution on histidyl ligands bound to the
dinuclear center. Determination of the 57Fe
hyperfine
tensors gives a complete picture of the spin-coupled Fe2+
and Fe3+ ions. The 1,2H ENDOR results
disclose the
presence of a bridging hydroxide and an aqua ligand in both native and
DMSO-treated enzyme. A novel procedure
for describing the 1H hyperfine tensor of the bridge gives
the orientation of the g-tensor relative to the cluster
framework
in both enzyme forms, information that is normally obtained only from
full single-crystal EPR studies. DMSO is
found to cause small perturbations of both histidyl ligands, and little
change in the 57Fe hyperfine tensors.
However,
Q-band pulsed 2H and 13C ENDOR measurements of
labeled DMSO show that this exogenous ligand binds in a
distinct site with a well-ordered structure, and further indicate that
it is O-bound to the Fe3+ ion of the mixed-valence cluster. The analysis, coupled with 2H X-band
electron spin−echo envelope modulation data, places
limitations
on the possible orientation of the bound DMSO. These geometric
restrictions have been used to guide molecular
modeling of DMSO bound to the MMOHmv diiron active site.
The results reported here provide a basis with which
to study other dinuclear Fe−carboxylate proteins.
X-ray absorption spectroscopy has been performed on oriented photosystem II membrane particles isolated from spinach. Structural features of the tetranuclear Mn cluster and the orientation of the cluster with respect to the lipid bilayer were determined in both the S1 and S2 states of the Kok cycle. Variation of the sample orientation with respect to the X-ray e-vector yields highly dichroic K-edge and extended X-ray absorption fine structure spectra (EXAFS), indicative of an asymmetric tetranuclear cluster. Mn-Mn vectors at 2.72 and 3.38 A can be resolved from these measurements using quantitative analysis. The 2.72-A vector, consisting of at least two component vectors, is oriented at an average angle of 60 degrees +/- 7 degrees to the membrane normal, with an average of 1.1 +/- 0.1 interactions per Mn atom. The 3.38-A vector, most probably an average of two vectors, makes an angle of 43 degrees +/- 10 degrees with respect to the membrane normal, with an average of 0.45 +/- 0.07 backscatterer per Mn atom. Upon advance to the S2 state, the orientation of these vectors and the average numbers of backscatterers are approximately invariant. Analysis of more subtle features of the EXAFS reveals changes accompanying this S-state advance that are consistent with the oxidation of Mn during this transition. However, the dominant structural features of the oxygen-evolving complex remain constant in the S1 and S2 states. The structure of the Mn complex and the orientation of the complex in the membrane within the context of dichroism of the X-ray absorption data are discussed.
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