Synthesis, crystal structure, EPR, and DFT studies of an unusually distorted vanadium(II) complex.

We report the synthesis and structure of the most highly distorted four-coordinate d ion known to date that also serves as the second known example of a bis(biphenolato) transition metal complex. We demonstrate the application of density functional theory to calculate the magnetic parameters derived from the experimental and simulated EPR spectra.

Binding of the substrate analog methanol in the oxygen-evolving complex of photosystem II in the D1-N87A genetic variant of cyanobacteria.

The solar water-splitting protein complex, photosystem II (PSII), catalyzes one of the most energetically demanding reactions in nature by using light energy to drive a catalyst capable of oxidizing water. The water oxidation reaction is catalyzed at the MnCa-oxo cluster in the oxygen-evolving complex (OEC), which cycles through five light-driven S-state intermediates (S-S). A detailed mechanism of the reaction remains elusive as it requires knowledge of the delivery and binding of substrate water in the higher S-state intermediates.

Shedding Light on Primary Donors in Photosynthetic Reaction Centers.

Chlorophylls (Chl)s exist in a variety of flavors and are ubiquitous in both the energy and electron transfer processes of photosynthesis. The functions they perform often occur on the ultrafast (fs-ns) time scale and until recently, these have been difficult to measure in real time. Further, the complexity of the binding pockets and the resulting protein-matrix effects that alter the respective electronic properties have rendered theoretical modeling of these states difficult.

A dimeric chlorophyll electron acceptor differentiates type I from type II photosynthetic reaction centers.

This research addresses one of the most compelling issues in the field of photosynthesis, namely, the role of the accessory chlorophyll molecules in primary charge separation. Using a combination of empirical and computational methods, we demonstrate that the primary acceptor of photosystem (PS) I is a dimer of accessory and secondary chlorophyll molecules, Chl and Chl, with an asymmetric electron charge density distribution. The incorporation of highly coupled donors and acceptors in PS I allows for extensive delocalization that prolongs the lifetime of the charge-separated state, providing for high quantum efficiency.

Determining the Electronic Structure of Paramagnetic Intermediates in membrane proteins: A high-resolution 2D H hyperfine sublevel correlation study of the redox-active tyrosines of photosystem II.

The photosynthetic reaction center, photosystem II (PSII), catalyzes one of the most energetically demanding reactions in nature by using light energy to drive water oxidation. The four-electron water oxidation reaction occurs at the tetranuclear manganese‑calcium-oxo (MnCa-oxo) cluster that is present in the oxygen-evolving complex (OEC) of PSII. The water oxidation reaction is facilitated by proton-coupled electron transfer (PCET) at the redox-active tyrosine residue, Y, in the OEC which is one of the two symmetric tyrosine residues, Y and Y, in PSII.

HYSCORE and DFT Studies of Proton-Coupled Electron Transfer in a Bioinspired Artificial Photosynthetic Reaction Center.

The photosynthetic water-oxidation reaction is catalyzed by the oxygen-evolving complex in photosystem II (PSII) that comprises the MnCaO cluster, with participation of the redox-active tyrosine residue (Y) and a hydrogen-bonded network of amino acids and water molecules. It has been proposed that the strong hydrogen bond between Y and D1-His190 likely renders Y kinetically and thermodynamically competent leading to highly efficient water oxidation. However, a detailed understanding of the proton-coupled electron transfer (PCET) at Y remains elusive owing to the transient nature of its intermediate states involving Y⋅.

Elucidating the Structural Chemistry of a Hysteretic Iron(II) Spin-Crossover Compound From its Copper(II) and Zinc(II) Congeners.

Annealing [FeL ][BF ] ⋅2 H O (L=2,6-bis-[5-methyl-1H-pyrazol-3-yl]pyridine) affords an anhydrous material, which undergoes a spin transition at T =205 K with a 65 K thermal hysteresis loop. This occurs through a sequence of phase changes, which were monitored by powder diffraction in an earlier study. [CuL ][BF ] ⋅2 H O and [ZnL ][BF ] ⋅2 H O are not perfectly isostructural but, unlike the iron compound, they undergo single-crystal-to-single-crystal dehydration upon annealing.

? Divergent coordination behavior of early-transition metals towards MOF-5.

Treatment of MOF-5 with NbCl(THF) in acetonitrile leads to incorporation of Nb(iv) centers in a fashion that diverges from the established cation metathesis reactivity of this iconic material. A combination of X-ray absorption spectroscopy analysis and reactivity studies altogether supported by density functional theory computational studies document an unprecedented binding mode for the ZnO(OC-) secondary building units (SBUs), which in Nb(iv)-MOF-5 function as -chelating ligands for NbCl moieties, with no exchange of Zn observed. This unusual reactivity expands the portfolio of post-synthetic modification techniques available for MOFs, exemplified here by MOF-5, and underscores the diverse coordination environments offered by this and potentially other MOFs towards heterometal species.

Rigidification of a macrocyclic tris-catecholate scaffold leads to electronic localisation of its mixed valent redox product.

The catecholate groups in [{Pt(L)}3(μ3-tctq)] (H6tctq = 2,3,6,7,10,11-hexahydroxy-4b,8b,12b,12d-tetramethyltribenzotriquinacene; L = a diphosphine chelate) undergo sequential oxidation to their semiquinonate forms by voltammetry, with ΔE1/2 = 160-170 mV. The monoradical [{Pt(dppb)}3(μ3-tctq˙)]+ is valence-localised, with no evidence for intervalence charge transfer in its near-IR spectrum. This contrasts with previously reported [{Pt(dppb)}3(μ3-ctc˙)]+ (H6ctc = cyclotricatechylene), based on the same macrocyclic tris-dioxolene scaffold, which exhibits partly delocalised (class II) mixed valency.

Cages on a plane: a structural matrix for molecular 'sheets'.

A family of heterometallic Anderson-type 'wheels' of general formula [MM(hmp)] (M = Cr or Al and M = Ni or Zn, Hhmp = 2-pyridinemethanol) has been extended to include M = Cr or Al and M = Co, Fe, Mn or Cu, affording five new species of formulae [CrCo(hmp)](ClO) (1), [CrFe(hmp)](ClO) (2), [CrMn(hmp)](ClO) (3), [CrCu(hmp)](ClO)(NO) (4) and [AlCo(hmp)](ClO) (5). As per previous family members, the metallic skeleton common to the cations of 1-5 describes a centred hexagon with the two M sites disordered around the outer wheel, with the exception of compound 4 where the Cu sites are localised. A structurally related, but enlarged planar disc possessing a [MM] hexagon capped on each edge by a Cu ion can be formed, but only when M = Al and M = Cu.

Modular [FeM] (M = Pd, Co, Ni, Cu) Coordination Cages.

The reaction of the simple metalloligand [FeL] [HL = 1-(4-pyridyl)butane-1,3-dione] with a variety of different M salts results in the formation of a family of heterometallic cages of formulae [FePdL]Cl (1), [FeCuL(HO)Br]Br (2), [FeCuL(HO)](NO) (3), [FeNiL(SCN)Cl] (4), and [FeCoL(SCN)(HO)]Cl (5). The metallic skeleton of each cage describes a cube in which the Fe ions occupy the eight vertices and the M ions lie at the center of the six faces. Direct-current magnetic susceptibility and magnetization measurements on 3-5 reveal the presence of weak antiferromagnetic exchange between the metal ions in all three cases.

Unpaired Electron Spin Density Distribution across Reduced [2Fe-2S] Cluster Ligands by C-Cysteine Labeling.

Iron-sulfur clusters are one of the most versatile and ancient classes of redox mediators in biology. The roles that these metal centers take on are predominantly determined by the number and types of coordinating ligands (typically cysteine and histidine) that modify the electronic structure of the cluster. Here we map the spin density distribution onto the cysteine ligands for the three major classes of the protein-bound, reduced [2Fe-2S](His)(Cys) (n = 0, 1, 2) cluster by selective cysteine-C isotope labeling.

3d/4f Coordination Clusters as Cooperative Catalysts for Highly Diastereoselective Michael Addition Reactions.

Michael addition (MA) is one of the most well studied chemical transformation in synthetic chemistry. Here, we report the synthesis and crystal structures of a library of 3d/4f coordination clusters (CCs) formulated as [ZnYL(solv)(Z)] and study their catalytic properties toward the MA of nitrostyrenes with barbituric acid derivatives. Each CC presents two borderline hard/soft Lewis acidic Zn centers and two hard Lewis acidic Y centers in a defect dicubane topology that brings the two different metals into a proximity of ∼3.

Structural and electronic elucidation of a N-heterocyclic silylene vanadocene adduct.

The solid and solution state structure of the vanadium(ii) N-heterocyclic silylene (NHSi) complex, [(IPr)V(Cp)] (1) is reported (IPr: 1,3-bis(2,6-diisopropylphenyl)-1,3-diaza-2-silacyclopent-4-en-2-ylidene). The electronic structure of 1 is probed using a combination of magnetic measurements, EPR spectroscopy and computational studies. The V-Si bond strength and complex forming mechanism between vanadocene and an NHSi ligand is elucidated using computational methods.

In Situ Spectroelectrochemical Investigations of the Redox-Active Tris[4-(pyridin-4-yl)phenyl]amine Ligand and a Zn(2+) Coordination Framework.

An investigation of the redox-active tris[4-(pyridin-4-yl)phenyl]amine (NPy3) ligand in the solution state and upon its incorporation into the solid-state metal-organic framework (MOF) [Zn(NPy3)(NO2)2·xMeOH·xDMF]n (MeOH = methanol and DMF = N,N-dimethylformamide) was conducted using in situ UV/vis/near-IR, electron paramagentic resonance (EPR), and fluorescence spectroelectrochemical experiments. Through this multifaceted approach, the properties of the ligand and framework were elucidated and quantified as a function of the redox state of the triarylamine core, which can undergo a one-electron oxidation to its radical cation. The use of pulsed EPR experiments revealed that the radical generated was highly delocalized throughout the entire ligand backbone.

The molecular basis of polysaccharide cleavage by lytic polysaccharide monooxygenases.

Lytic polysaccharide monooxygenases (LPMOs) are copper-containing enzymes that oxidatively break down recalcitrant polysaccharides such as cellulose and chitin. Since their discovery, LPMOs have become integral factors in the industrial utilization of biomass, especially in the sustainable generation of cellulosic bioethanol. We report here a structural determination of an LPMO-oligosaccharide complex, yielding detailed insights into the mechanism of action of these enzymes.

Platinum(ii) complexes of mixed-valent radicals derived from cyclotricatechylene, a macrocyclic tris-dioxolene.

Three complexes of cyclotricatechylene (Hctc), [{PtL}(μ-ctc)], have been synthesised: (L = 1,2-bis(diphenylphosphino)benzene {dppb}, ; L = 1,2-bis(diphenylphosphino)ethane {dppe}, ; L = 4,4'-bis(-butyl)-2,2'-bipyridyl { Bubipy}, ). The complexes show three low-potential, chemically reversible voltammetric oxidations separated by 180 mV, corresponding to stepwise oxidation of the [ctc] catecholato rings to the semiquinonate level. The redox series and have been characterised by UV/vis/NIR spectroelectrochemistry.

Catalytic Amine Oxidation under Ambient Aerobic Conditions: Mimicry of Monoamine Oxidase B.

The flavoenzyme monoamine oxidase (MAO) regulates mammalian behavioral patterns by modulating neurotransmitters such as adrenaline and serotonin. The mechanistic basis which underpins this enzyme is far from agreed upon. Reported herein is that the combination of a synthetic flavin and alloxan generates a catalyst system which facilitates biomimetic amine oxidation.

Structural evidence of a productive active site architecture for an evolved quorum-quenching GKL lactonase.

The in vitro evolution and engineering of quorum-quenching lactonases with enhanced reactivities was achieved using a thermostable GKL enzyme as a template, yielding the E101G/R230C GKL mutant with increased catalytic activity and a broadened substrate range [Chow, J. Y., Xue, B.

Dissection of hydrogen bond interaction network around an iron-sulfur cluster by site-specific isotope labeling of hyperthermophilic archaeal Rieske-type ferredoxin.

The electronic structure and geometry of redox-active metal cofactors in proteins are tuned by the pattern of hydrogen bonding with the backbone peptide matrix. In this study we developed a method for selective amino acid labeling of a hyperthermophilic archaeal metalloprotein with engineered Escherichia coli auxotroph strains, and we applied this to resolve the hydrogen bond interactions with the reduced Rieske-type [2Fe-2S] cluster by two-dimensional pulsed electron spin resonance technique. Because deep electron spin-echo envelope modulation of two histidine (14)N(δ) ligands of the cluster decreased non-coordinating (15)N signal intensities via the cross-suppression effect, an inverse labeling strategy was employed in which (14)N amino acid-labeled archaeal Rieske-type ferredoxin samples were examined in an (15)N-protein background.

Hydrogen bonding between the Q(B) site ubisemiquinone and Ser-L223 in the bacterial reaction center: a combined spectroscopic and computational perspective.

In the Q(B) site of the Rhodobacter sphaeroides photosynthetic reaction center, the donation of a hydrogen bond from the hydroxyl group of Ser-L223 to the ubisemiquinone formed after the first flash is debatable. In this study, we use a combination of spectroscopy and quantum mechanics/molecular mechanics (QM/MM) calculations to comprehensively explore this topic. We show that ENDOR, ESEEM, and HYSCORE spectroscopic differences between mutant L223SA and the wild-type sample (WT) are negligible, indicating only minor perturbations in the ubisemiquinone spin density for the mutant sample.

Interactions of intermediate semiquinone with surrounding protein residues at the Q(H) site of wild-type and D75H mutant cytochrome bo3 from Escherichia coli.

Selective (15)N isotope labeling of the cytochrome bo(3) ubiquinol oxidase from Escherichia coli with auxotrophs was used to characterize the hyperfine couplings with the side-chain nitrogens from residues R71, H98, and Q101 and peptide nitrogens from residues R71 and H98 around the semiquinone (SQ) at the high-affinity Q(H) site. The two-dimensional ESEEM (HYSCORE) data have directly identified N(ε) of R71 as an H-bond donor carrying the largest amount of unpaired spin density. In addition, weaker hyperfine couplings with the side-chain nitrogens from all residues around the SQ were determined.

Exploring by pulsed EPR the electronic structure of ubisemiquinone bound at the QH site of cytochrome bo3 from Escherichia coli with in vivo 13C-labeled methyl and methoxy substituents.

The cytochrome bo(3) ubiquinol oxidase from Escherichia coli resides in the bacterial cytoplasmic membrane and catalyzes the two-electron oxidation of ubiquinol-8 and four-electron reduction of O(2) to water. The one-electron reduced semiquinone forms transiently during the reaction, and the enzyme has been demonstrated to stabilize the semiquinone. The semiquinone is also formed in the D75E mutant, where the mutation has little influence on the catalytic activity, and in the D75H mutant, which is virtually inactive.

Nitrogen oxide reaction with six-atom silver clusters supported on LTA zeolite.

Silver containing catalysts were prepared by aqueous ion exchange of Ag(+) against Na(+) in an LTA zeolite. A well-defined paramagnetic cluster consisting of six equivalent silver nuclei was obtained after oxidation and hydrogen reduction. Continuous wave EPR demonstrates that the reduced Ag(6)(+) clusters are isolated and all silver atoms are close to equivalent.