Multifrequency pulsed electron paramagnetic resonance study of the S2 state of the photosystem II manganese cluster

Multifrequency electron spin-echo envelope modulation (ESEEM) spectroscopy is employed to measure the strength of the hyperfine coupling of magnetic nuclei to the paramagnetic (S = 1/2) S2 form of photosystem II (PSII). Previous X-band-frequency ESEEM studies indicated that one or more histidine nitrogens are electronically coupled to the tetranuclear manganese cluster in the S2 state of PSII. However, the spectral resolution was relatively poor at the approximately 9 GHz excitation frequency, precluding any in-depth analysis of the corresponding bonding interaction between the detected histidine and the manganese cluster. Here we report ESEEM experiments using higher X-, P-, and Ka-band microwave frequencies to target PSII membranes isolated from spinach. The X- to P-band ESEEM spectra suffer from the same poor resolution as that observed in previous experiments, while the Ka-band spectra show remarkably well-resolved features that allow for the direct determination of the nuclear quadrupolar couplings for a single I = 1(14)N nucleus. The Ka-band results demonstrate that at an applied field of 1.1 T we are much closer to the exact cancellation limit (alpha iso = 2nu(14)N) that optimizes ESEEM spectra. These results reveal hyperfine (alpha iso = 7.3 +/- 0.20 MHz and alpha dip = 0.50 +/- 0.10 MHz) and nuclear quadrupolar (e(2)qQ = 1.98 +/- 0.05 MHz and eta = 0.84 +/- 0.06) couplings for a single (14)N nucleus magnetically coupled to the manganese cluster in the S 2 state of PSII. These values are compared to the histidine imidazole nitrogen hyperfine and nuclear quadrupolar couplings found in superoxidized manganese catalase as well as (14)N couplings in relevant manganese model complexes.     

Copper(I) complex O(2)-reactivity with a N(3)S thioether ligand: a copper-dioxygen adduct including sulfur ligation, ligand oxygenation, and comparisons with all nitrogen ligand analogues

In order to contribute to an understanding of the effects of thioether sulfur ligation in copper-O(2) reactivity, the tetradentate ligands L(N3S) (2-ethylthio-N,N-bis(pyridin-2-yl)methylethanamine) and L(N3S')(2-ethylthio-N,N-bis(pyridin-2-yl)ethylethanamine) have been synthesized. Corresponding copper(I) complexes, [CuI(L(N3S))]ClO(4) (1-ClO(4)), [CuI(L(N3S))]B(C(6)F(5))(4) (1-B(C(6)F(5))(4)), and [CuI(L(N3S'))]ClO(4) (2), were generated, and their redox properties, CO binding, and O(2)-reactivity were compared to the situation with analogous compounds having all nitrogen donor ligands, [CuI(TMPA)(MeCN)](+) and [Cu(I)(PMAP)](+) (TMPA = tris(2-pyridylmethyl)amine; PMAP = bis[2-(2-pyridyl)ethyl]-(2-pyridyl)methylamine). X-ray structures of 1-B(C(6)F(5))(4), a dimer, and copper(II) complex [Cu(II)(L(N3S))(MeOH)](ClO(4))(2) (3) were obtained; the latter possesses axial thioether coordination. At low temperature in CH(2)Cl(2), acetone, or 2-methyltetrahydrofuran (MeTHF), 1 reacts with O(2) and generates an adduct formulated as an end-on peroxodicopper(II) complex [{Cu(II)(L(N3S))}(2)(mu-1,2-O(2)(2-))](2+) (4)){lambda(max) = 530 (epsilon approximately 9200 M(-1) cm(-1)) and 605 nm (epsilon approximately 11,800 M(-1) cm(-1))}; the number and relative intensity of LMCT UV-vis bands vary from those for [{Cu(II)(TMPA)}(2)(O(2)(2-))](2+) {lambda(max) = 524 nm (epsilon = 11,300 M(-1) cm(-1)) and 615 nm (epsilon = 5800 M(-1) cm(-1))} and are ascribed to electronic structure variation due to coordination geometry changes with the L(N3S) ligand. Resonance Raman spectroscopy confirms the end-on peroxo-formulation {nu(O-O) = 817 cm(-1) (16-18O(2) Delta = 46 cm(-1)) and nu(Cu-O) = 545 cm(-1) (16-18O(2) Delta = 26 cm(-1)); these values are lower in energy than those for [{Cu(II)(TMPA)}(2)(O(2)(2-))](2+) {nu(Cu-O) = 561 cm(-1) and nu(O-O) = 827 cm(-1)} and can be attributed to less electron density donation from the peroxide pi* orbitals to the Cu(II) ion. Complex 4 is the first copper-dioxygen adduct with thioether ligation; direct evidence comes from EXAFS spectroscopy {Cu K-edge; Cu-S = 2.4 Angstrom}. Following a [Cu(I)(L(N3S))](+)/O(2) reaction and warming, the L(N3S) thioether ligand is oxidized to the sulfoxide in a reaction modeling copper monooxygenase activity. By contrast, 2 is unreactive toward dioxygen probably due to its significantly increased Cu(II)/Cu(I) redox potential, an effect of ligand chelate ring size (in comparison to 1). Discussion of the relevance of the chemistry to copper enzyme O(2)-activation, and situations of biological stress involving methionine oxidation, is provided.     

Dispersion‐Force‐Assisted Disproportionation: A Stable Two‐Coordinate Copper(II) Complex

The synthesis of the first linear coordinated Cu^II complex Cu{N(SiMe₃)Dipp}₂ ( 1 Dipp=C₆H₅‐2,6Prⁱ₂) and its Cu^I counterpart [Cu{N(SiMe₃)Dipp}₂]^? ( 2 ) is described. The formation of 1 proceeds through a dispersion force‐driven disproportionation, and is the reaction product of a Cu^I halide and LiN(SiMe₃)Dipp in a non‐donor solvent. The synthesis of 2 is accomplished by preventing the disproportionation into 1 by using the complexing agent 15‐crown‐5. EPR spectroscopy of 1 provides the first detailed study of a two‐coordinate transition‐metal complex indicating strong covalency in the Cu?N bonds.     

Spectroscopic and density functional studies of the red copper site in nitrosocyanin: role of the protein in determining active site geometric and electronic structure

The electronic structure of the red copper site in nitrosocyanin is defined relative to that of the well understood blue copper site of plastocyanin by using low-temperature absorption, circular dichroism, magnetic circular dichroism, resonance Raman, EPR and X-ray absorption spectroscopies, combined with DFT calculations. These studies indicate that the principal electronic structure change in the red copper site is the sigma rather than the pi donor interaction of the cysteine sulfur with the Cu 3d(x2-y2) redox active molecular orbital (RAMO). Further, MCD data show that there is an increase in ligand field strength due to an increase in coordination number, whereas resonance Raman spectra indicate a weaker Cu-S bond. The latter is supported by the S K-edge data, which demonstrate a less covalent thiolate interaction with the RAMO of nitrosocyanin at 20% relative to plastocyanin at 38%. EXAFS results give a longer Cu-S(Cys) bond distance in nitrosocyanin (2.28 A) compared to plastocyanin (2.08 A) and also show a large change in structure with reduction of the red copper site. The red copper site is the only presently known blue copper-related site with an exogenous water coordinated to the copper. Density functional calculations reproduce the experimental properties and are used to determine the specific protein structure contributions to exogenous ligand binding in red copper. The relative orientation of the CuNNS and the CuSC(beta) planes (determined by the protein sequence) is found to be key in generating an exchangeable coordination position at the red copper active site. The exogenous water ligation at the red copper active site greatly increases the reorganization energy (by approximately 1.0 eV) relative to that of the blue copper protein site, making the red site unfavorable for fast outer-sphere electron transfer, while providing an exchangeable coordination position for inner-sphere electron transfer.     

X-ray absorption spectroscopic study of the reduced hydroxylases of methane monooxygenase and toluene/o-xylene monooxygenase: differences in active site structure and effects of the coupling proteins MMOB and ToMOD

The diiron active sites of the reduced hydroxylases from methane monooxygenase (MMOH(red)) and toluene/o-xylene monooxygenase (ToMOH(red)) have been investigated by X-ray absorption spectroscopy (XAS). Results of Fe K-edge and extended X-ray absorption fine structure analysis reveal subtle differences between the hydroxylases that may be correlated to access of the active site. XAS data were also recorded for each hydroxylase in the presence of its respective coupling protein. MMOB affects the outer-shell scattering contributions in the diiron site of MMOH(red), whereas ToMOD exerts its main effect on the first-shell ligation of ToMOH(red); it also causes a slight decrease in the Fe-Fe separation. These results provide an initial step toward delineating the differences in structure and reactivity in bacterial multicomponent monooxygenase proteins.     

Biosynthesis of the catalytic H-cluster of [FeFe] hydrogenase: the roles of the Fe–S maturase proteins HydE,HydF, and HydG

[FeFe] hydrogenases carry out the redox interconversion of protons and molecular hydrogen (2H⁺ + 2e⁻ ⇌ H₂) at a complex Fe–S active site known as the H-cluster. The H-cluster consists of a [4Fe–4S] subcluster, denoted here as [4Fe]_H, linked via a cysteine sulfur to an interesting organometallic [2Fe]_H subcluster thought to be the subsite where the catalysis occurs. This [2Fe]_H subcluster consists of two Fe atoms, linked with a bridging CO and a bridging SCH₂NHCH₂S azadithiolate (adt), with additional terminal CO and CN ligands bound to each Fe. Synthesizing such a complex organometallic unit is a fascinating problem in biochemistry, complicated by the toxic nature of both the CO and CN⁻ species and the relative fragility of the azadithiolate bridge. It has been known for a number of years that this complex biosynthesis is carried out by a set of three essential Fe–S proteins, HydE, HydF, and HydG. HydF is a GTPase, while HydE and HydG are both members of the large family of radical S-adenosylmethionine (rSAM) enzymes. In this perspective we describe the history of research and discovery concerning these three Fe–S “maturase” proteins and describe recent evidence for a sequential biosynthetic pathway beginning with the synthesis of a mononuclear organometallic [Fe(ii)(CO)₂CN(cysteine)] complex by the rSAM enzyme HydG and its subsequent activation by the second rSAM enzyme HydE to form a highly reactive Fe(i)(CO)₂(CN)S species. In our model a pair of these Fe(i)(CO)₂(CN)S units condense to form the [Fe(CO)₂(CN)S]₂ diamond core of the [2Fe]_H cluster, requiring only the installation of the central CH₂NHCH₂ portion of the azadithiolate bridge, whose atoms are all sourced from the amino acid serine. This final step likely occurs with an interplay of HydE and HydF, the details of which yet remain to be elucidated.

Fe–S cluster enzymes HydG, HydE, and HydF provide sequential assembly of the catalytic H-cluster of [FeFe] hydrogenase.     

Revisiting the polyoxometalate-based late-transition-metal-oxo complexes: the "oxo wall" stands

Terminal oxo complexes of the late transition metals Pt, Pd, and Au have been reported by us in Science and Journal of the American Chemical Society. Despite thoroughness in characterizing these complexes (multiple independent structural methods and up to 17 analytical methods in one case), we have continued to study these structures. Initial work on these systems was motivated by structural data from X-ray crystallography and neutron diffraction and (17)O and (31)P NMR signatures which all indicated differences from all previously published compounds. With significant new data, we now revisit these studies. New X-ray crystal structures of previously reported complexes K(14)[P(2)W(19)O(69)(OH(2))] and "K(10)Na(3)[Pd(IV)(O)(OH)WO(OH(2))(PW(9)O(34))(2)]" and a closer examination of these structures are provided. Also presented are the (17)O NMR spectrum of an (17)O-enriched sample of [PW(11)O(39)](7-) and a careful combined (31)P NMR-titration study of the previously reported "K(7)H(2)[Au(O)(OH(2))P(2)W(20)O(70)(OH(2))(2)]." These and considerable other data collectively indicate that previously assigned terminal Pt-oxo and Au-oxo complexes are in fact cocrystals of the all-tungsten structural analogues with noble metal cations, while the Pd-oxo complex is a disordered Pd(II)-substituted polyoxometalate. The neutron diffraction data have been re-analyzed, and new refinements are fully consistent with the all-tungsten formulations of the Pt-oxo and Au-oxo polyoxometalate species.     

Geometric structure determination of N694C lipoxygenase: a comparative near-edge X-ray absorption spectroscopy and extended X-ray absorption fine structure study

The mononuclear nonheme iron active site of N694C soybean lipoxygenase (sLO1) has been investigated in the resting ferrous form using a combination of Fe-K-pre-edge, near-edge (using the minuit X-ray absorption near-edge full multiple-scattering approach), and extended X-ray absorption fine structure (EXAFS) methods. The results indicate that the active site is six-coordinate (6C) with a large perturbation in the first-shell bond distances in comparison to the more ordered octahedral site in wild-type sLO1. Upon mutation of the asparagine to cysteine, the short Fe-O interaction with asparagine is replaced by a weak Fe-(H(2)O), which leads to a distorted 6C site with an effective 5C ligand field. In addition, it is shown that near-edge multiple scattering analysis can give important three-dimensional structural information, which usually cannot be accessed using EXAFS analysis. It is further shown that, relative to EXAFS, near-edge analysis is more sensitive to partial coordination numbers and can be potentially used as a tool for structure determination in a mixture of chemical species.     

Electrochemical water oxidation with cobalt-based electrocatalysts from pH 0-14: the thermodynamic basis for catalyst structure, stability, and activity

Building upon recent study of cobalt-oxide electrocatalysts in fluoride-buffered electrolyte at pH 3.4, we have undertaken a mechanistic investigation of cobalt-catalyzed water oxidation in aqueous buffering electrolytes from pH 0-14. This work includes electrokinetic studies, cyclic voltammetric analysis, and electron paramagnetic resonance (EPR) spectroscopic studies. The results illuminate a set of interrelated mechanisms for electrochemical water oxidation in alkaline, neutral, and acidic media with electrodeposited Co-oxide catalyst films (CoO(x)(cf)s) as well as for a homogeneous Co-catalyzed electrochemical water oxidation reaction. Analysis of the pH dependence of quasi-reversible features in cyclic voltammograms of the CoO(x)(cf)s provides the basis for a Pourbaix diagram that closely resembles a Pourbaix diagram derived from thermodynamic free energies of formation for a family of Co-based layered materials. Below pH 3, a shift from heterogeneous catalysis producing O(2) to homogeneous catalysis yielding H(2)O(2) is observed. Collectively, the results reported here provide a foundation for understanding the structure, stability, and catalytic activity of aqueous cobalt electrocatalysts for water oxidation.     

Ligand K-edge X-ray absorption spectroscopy of [Fe4S4]1+,2+,3+ clusters: changes in bonding and electronic relaxation upon redox

Sulfur K-edge X-ray absorption spectroscopy (XAS) is reported for [Fe(4)S(4)](1+,2+,3+) clusters. The results are quantitatively and qualitatively compared with DFT calculations. The change in covalency upon redox in both the [Fe(4)S(4)](1+/2+) (ferredoxin) and the [Fe(4)S(4)](2+/3+) (HiPIP) couple are much larger than that expected from just the change in number of 3d holes. Moreover, the change in the HiPIP couple is higher than that of the ferredoxin couple. These changes in electronic structure are analyzed using DFT calculations in terms of contributions from the nature of the redox active molecular orbital (RAMO) and electronic relaxation. The results indicate that the RAMO of HiPIP has 50% ligand character, and hence, the HiPIP redox couple involves limited electronic relaxation. Alternatively, the RAMO of the ferredoxin couple is metal-based, and the ferredoxin redox couple involves extensive electronic relaxation. The contributions of these RAMO differences to ET processes in the different proteins are discussed.