A model for the CO-inhibited form of [NiFe] hydrogenase: synthesis of (CO)3Fe(μ-StBu)3Ni{SC6H3-2,6-(mesityl)2} and reversible CO addition at the Ni site

A [NiFe] hydrogenase model compound having a distorted trigonal-pyramidal nickel center, (CO)₃Fe(μ-S^tBu)₃Ni(SDmp), 1 (Dmp = C₆H₃-2,6-(mesityl)₂), was synthesized from the reaction of the tetranuclear Fe-Ni-Ni-Fe complex [(CO)₃Fe(μ-S^tBu)₃Ni]₂(μ-Br)₂, 2 with NaSDmp at -40 °C. The nickel site of complex 1 was found to add CO or CN^tBu at -40 °C to give (CO)₃Fe(S^tBu)(μ-S^tBu)₂Ni(CO)(SDmp), 3, or (CO)₃Fe(S^tBu)(μ-S^tBu)₂Ni(CN^tBu)(SDmp), 4, respectively. One of the CO bands of 3, appearing at 2055 cm^-1 in the infrared spectrum, was assigned as the Ni-CO band, and this frequency is comparable to those observed for the CO-inhibited forms of [NiFe] hydrogenase. Like the CO-inhibited forms of [NiFe] hydrogenase, the coordination of CO at the nickel site of 1 is reversible, while the CN^tBu adduct 4 is more robust.     

Kinetics and mechanistic analysis of an extremely rapid carbon dioxide fixation reaction

Carbon dioxide may react with free or metal-bound hydroxide to afford products containing bicarbonate or carbonate, often captured as ligands bridging two or three metal sites. We report the kinetics and probable mechanism of an extremely rapid fixation reaction mediated by a planar nickel complex [Ni^II(NNN)(OH)]^1- containing a tridentate 2,6-pyridinedicarboxamidate pincer ligand and a terminal hydroxide ligand. The minimal generalized reaction is M-OH + CO₂ → M-OCO₂H; with variant M, previous rate constants are ≲10³ M^-1 s^-1 in aqueous solution. For the present bimolecular reaction, the (extrapolated) rate constant is 9.5 × 10⁵ M^-1 s^-1 in N,N′-dimethylformamide at 298 K, a value within the range of k_cat/K_M≈10⁵–10⁸ M^-1 s^-1 for carbonic anhydrase, the most efficient catalyst of CO₂ fixation reactions. The enthalpy profile of the fixation reaction was calculated by density functional theory. The initial event is the formation of a weak precursor complex between the Ni-OH group and CO₂, followed by insertion of a CO₂ oxygen atom into the Ni-OH bond to generate a four center Ni(η²-OCO₂H) transition state similar to that at the zinc site in carbonic anhydrase. Thereafter, the Ni-OH bond detaches to afford the Ni(η¹-OCO₂H) fragment, after which the molecule passes through a second, lower energy transition state as the bicarbonate ligand rearranges to a conformation very similar to that in the crystalline product. Theoretical values of metric parameters and activation enthalpy are in good agreement with experimental values [ΔH^‡ = 3.2(5) kcal/mol].     

Electron delocalization in the S1 and T1 metal-to-ligand charge transfer states of trans-substituted metal quadruply bonded complexes

The singlet S₁ and triplet T₁ photoexcited states of the compounds containing MM quadruple bonds trans-M₂(TⁱPB)₂(O₂CC₆H₄-4-CN)₂, where TⁱPB = 2,4,6-triisopropylbenzoate and M = Mo (I) or M = W (I^′), and trans-M₂(O₂CMe)₂((N[ⁱ Pr ])₂CC ≡ CC₆H₅)₂, where M = Mo (II) and M = W (II^′), have been investigated by a variety of spectroscopic techniques including femtosecond time-resolved infrared spectroscopy. The singlet states are shown to be delocalized metal-to-ligand charge transfer (MLCT) states for I and I^′ but localized for II and II^′ involving the cyanobenzoate or amidinate ligands, respectively. The triplet states are MoMoδδ* for both I and II but delocalized ³MLCT for I^′ and localized ³MLCT for II^′. These differences arise from consideration of the relative orbital energies of the M₂δ or M₂δ* and the ligand π^∗ as well as the magnitudes of orbital overlap.     

Effects of melatonin and dexpanthenol on antioxidant parameters when combined with estrogen treatment in ovariectomized rats

The purpose of the study was to assess whether it is possible to reduce the oxidative damage using antioxidant agents combined with hormone replacement therapy after menopause. In this prospective experimental study, 50 mature female Wistar albino rats weighing 270–310 g were used. Rats were divided into the following six groups: (1) Ovx group (n = 7): the animals underwent bilateral ovariectomy. No drug was administered following bilateral ovariectomy. (2) Ovx + E₂group (n = 7): bilateral ovariectomy + 17β-estradiol (100 μg/kg/day); (3) Ovx + E₂ + MT5 group (n = 7): bilateral ovariectomy + 17β-estradiol (100 μg/kg/day) + melatonin (5 mg/kg/day); (4) Ovx + E₂ + MT20 group (n = 7): bilateral ovariectomy + 17β-estradiol (100 μg/kg/day) + melatonin (20 mg/kg/day); (5) Ovx + E₂ + Dxp250 group (n = 7): bilateral ovariectomy + 17β-estradiol (100 μg/kg/day) + dexpanthenol (250 mg/kg/day); (6) Ovx + E₂ + Dxp500 group (n = 7): bilateral ovariectomy + 17β-estradiol (100 μg/kg/day) + dexpanthenol (500 mg/kg/day), and the activity of these antioxidative enzymes and oxidative stress products were measured. Enzymatic activity levels of catalase (CAT), superoxide dismutase (SOD), glutathione peroxidase(GSH-Px), and glutathione reductase and levels of free radicals (malondialdehyde (MDA) and nitric oxide) were both analyzed. We observed an increase in the level of GSH activity, but no significant differences in levels of CAT, SOD, and GSH-Px enzymatic activity and in levels of free radical MDA following 17β-estradiol or additional antioxidant treatment (melatonin or dexpanthenol). Despite the present study indicating that the addition of melatonin and dexpanthenol into the hormone replacement therapy regimen may contribute to the antioxidant effect of estrogen, the existence of limited data in this field indicates that further studies are warranted.     

Splitting CO2 into CO and O2 by a single catalyst

The metal complex [(tpy)(Mebim-py)Ru^II(S)]²⁺ (tpy = 2,2^′ : 6^′,2^′′-terpyridine; Mebim-py = 3-methyl-1-pyridylbenzimidazol-2-ylidene; S = solvent) is a robust, reactive electrocatalyst toward both water oxidation to oxygen and carbon dioxide reduction to carbon monoxide. Here we describe its use as a single electrocatalyst for CO₂ splitting, CO₂ → CO + 1/2 O₂, in a two-compartment electrochemical cell.     

Mimicking the electron transfer chain in photosystem II with a molecular triad thermodynamically capable of water oxidation

In the photosynthetic photosystem II, electrons are transferred from the manganese-containing oxygen evolving complex (OEC) to the oxidized primary electron-donor chlorophyll P680^•+ by a proton-coupled electron transfer process involving a tyrosine-histidine pair. Proton transfer from the tyrosine phenolic group to a histidine nitrogen positions the redox potential of the tyrosine between those of P680^•+ and the OEC. We report the synthesis and time-resolved spectroscopic study of a molecular triad that models this electron transfer. The triad consists of a high-potential porphyrin bearing two pentafluorophenyl groups (PF₁₀), a tetracyanoporphyrin electron acceptor (TCNP), and a benzimidazole-phenol secondary electron-donor (Bi-PhOH). Excitation of PF₁₀ in benzonitrile is followed by singlet energy transfer to TCNP (τ = 41 ps), whose excited state decays by photoinduced electron transfer (τ = 830 ps) to yield . A second electron transfer reaction follows (τ < 12 ps), giving a final state postulated as BiH⁺-PhO^•-PF₁₀-TCNP^•-, in which the phenolic proton now resides on benzimidazole. This final state decays with a time constant of 3.8 μs. The triad thus functionally mimics the electron transfers involving the tyrosine-histidine pair in PSII. The final charge-separated state is thermodynamically capable of water oxidation, and its long lifetime suggests the possibility of coupling systems such as this system to water oxidation catalysts for use in artificial photosynthetic fuel production.     

Metal-organic charge transfer can produce biradical states and is mediated by conical intersections

The present paper illustrates key features of charge transfer between calcium atoms and prototype conjugated hydrocarbons (ethylene, benzene, and coronene) as elucidated by electronic structure calculations. One- and two-electron charge transfer is controlled by two sequential conical intersections. The two lowest electronic states that undergo a conical intersection have closed-shell and open-shell dominant configurations correlating with the 4s² and 4s¹3d¹ states of Ca, respectively. Unlike the neutral-ionic state crossing in, for example, hydrogen halides or alkali halides, the path from separated reactants to the conical intersection region is uphill and the charge-transferred state is a biradical. The lowest-energy adiabatic singlet state shows at least two minima along a single approach path of Ca to the π system: (i) a van der Waals complex with a doubly occupied highest molecular orbital, denoted , and a small negative charge on Ca and (ii) an open-shell singlet (biradical) at intermediate approach (Ca⋯C distance ≈2.5–2.7 Å) with molecular orbital structure ϕ₁ϕ₂, where ϕ₂ is an orbital showing significant charge transfer form Ca to the π-system, leading to a one-electron multicentered bond. A third minimum (iii) at shorter distances along the same path corresponding to a closed-shell state with molecular orbital structure has also been found; however, it does not necessarily represent the ground state at a given Ca⋯C distance in all three systems. The topography of the lowest adiabatic singlet potential energy surface is due to the one- and two-electron bonding patterns in Ca-π complexes.     

Younger Dryas cooling and the Greenland climate response to CO2

Greenland ice-core δ¹⁸O-temperature reconstructions suggest a dramatic cooling during the Younger Dryas (YD; 12.9–11.7 ka), with temperatures being as cold as the earlier Oldest Dryas (OD; 18.0–14.6 ka) despite an approximately 50 ppm rise in atmospheric CO₂. Such YD cooling implies a muted Greenland climate response to atmospheric CO₂, contrary to physical predictions of an enhanced high-latitude response to future increases in CO₂. Here we show that North Atlantic sea surface temperature reconstructions as well as transient climate model simulations suggest that the YD over Greenland should be substantially warmer than the OD by approximately 5 °C in response to increased atmospheric CO₂. Additional experiments with an isotope-enabled model suggest that the apparent YD temperature reconstruction derived from the ice-core δ¹⁸O record is likely an artifact of an altered temperature-δ¹⁸O relationship due to changing deglacial atmospheric circulation. Our results thus suggest that Greenland climate was warmer during the YD relative to the OD in response to rising atmospheric CO₂, consistent with sea surface temperature reconstructions and physical predictions, and has a sensitivity approximately twice that found in climate models for current climate due to an enhanced albedo feedback during the last deglaciation.     

Titanium dioxide induced inflammation in the small intestine

AIM: To investigate the effects of titanium dioxide (TiO₂) nanoparticles (NPTiO₂) and microparticles (MPTiO₂) on the inflammatory response in the small intestine of mice.METHODS: Bl 57/6 male mice received distilled water suspensions containing TiO₂ (100 mg/kg body weight) as NPTiO₂ (66 nm), or MPTiO₂ (260 nm) by gavage for 10 d, once a day; the control group received only distilled water. At the end of the treatment the duodenum, jejunum and ileum were extracted for assessment of cytokines, inflammatory cells and titanium content. The cytokines interleukin (IL)-1b, IL-4, IL-6, IL-8, IL-10, IL-12, IL-13, IL-17, IL-23, tumor necrosis factor-α (TNF-α), intracellular interferon-γ (IFN-γ) and transforming growth factor-β (TGF-β) were evaluated by enzyme-linked immunosorbent assay in segments of jejunum and ileum (mucosa and underlying muscular tissue). CD4⁺ and CD8⁺ T cells, natural killer cells, and dendritic cells were evaluated in duodenum, jejunum and ileum samples fixed in 10% formalin by immunohistochemistry. The titanium content was determined by inductively coupled plasma atomic emission spectrometry.RESULTS: We found increased levels of T CD4⁺ cells (cells/mm²) in duodenum: NP 1240 ± 139.4, MP 1070 ± 154.7 vs 458 ± 50.39 (P < 0.01); jejunum: NP 908.4 ± 130.3, MP 813.8 ± 103.8 vs 526.6 ± 61.43 (P < 0.05); and ileum: NP 818.60 ± 123.0, MP 640.1 ± 32.75 vs 466.9 ± 22.4 (P < 0.05). In comparison to the control group, the groups receiving TiO₂ showed a statistically significant increase in the levels of the inflammatory cytokines IL-12, IL-4, IL-23, TNF-α, IFN-γ and TGF-β. The cytokine production was more pronounced in the ileum (mean ± SE): IL-12: NP 33.98 ± 11.76, MP 74.11 ± 25.65 vs 19.06 ± 3.92 (P < 0.05); IL-4: NP 17.36 ± 9.96, MP 22.94 ± 7.47 vs 2.19 ± 0.65 (P < 0.05); IL-23: NP 157.20 ± 75.80, MP 134.50 ± 38.31 vs 22.34 ± 5.81 (P < 0.05); TNFα: NP 3.71 ± 1.33, MP 5.44 ± 1.67 vs 0.99 ± 019 (P < 0.05); IFNγ: NP 15.85 ± 9.99, MP 34.08 ± 11.44 vs 2.81 ± 0.69 (P < 0.05); and TGF-β: NP 780.70 ± 318.50, MP 1409.00 ± 502.20 vs 205.50 ± 63.93 (P < 0.05).CONCLUSION: Our findings indicate that TiO₂ particles induce a Th1-mediated inflammatory response in the small bowel in mice.     

Intersystem crossing and dynamics in O(3P) + C2H4 multichannel reaction: experiment validates theory

The O(³P) + C₂H₄ reaction, of importance in combustion and atmospheric chemistry, stands out as a paradigm reaction involving triplet- and singlet-state potential energy surfaces (PESs) interconnected by intersystem crossing (ISC). This reaction poses challenges for theory and experiments owing to the ruggedness and high dimensionality of these potentials, as well as the long lifetimes of the collision complexes. Primary products from five competing channels (H + CH₂CHO, H + CH₃CO, H₂ + CH₂CO, CH₃ + HCO, CH₂ + CH₂O) and branching ratios (BRs) are determined in crossed molecular beam experiments with soft electron-ionization mass-spectrometric detection at a collision energy of 8.4 kcal/mol. As some of the observed products can only be formed via ISC from triplet to singlet PESs, from the product BRs the extent of ISC is inferred. A new full-dimensional PES for the triplet state as well as spin-orbit coupling to the singlet PES are reported, and roughly half a million surface hopping trajectories are run on the coupled singlet-triplet PESs to compare with the experimental BRs and differential cross-sections. Both theory and experiment find almost equal contributions from the two PESs to the reaction, posing the question of how important is it to consider the ISC as one of the nonadiabatic effects for this and similar systems involved in combustion chemistry. Detailed comparisons at the level of angular and translational energy distributions between theory and experiment are presented for the two primary channel products, CH₃ + HCO and H + CH₂CHO. The agreement between experimental and theoretical functions is excellent, implying that theory has reached the capability of describing complex multichannel nonadiabatic reactions.     

Aberrant TGF-β1 signaling contributes to the development of primary biliary cirrhosis in murine model

AIM:To investigate whether transforming growth factor-β1(TGF-β1)signaling pathway is involved in the pathogenesis of primary biliary cirrhosis(PBC).METHODS:A murine model of PBC was developed by injection of polyinosinic polycytidylic acids(polyⅠ:C)in C57BL/6 mice,and the liver expressions of TGFβ1,TGF-βreceptorⅠ(TβRⅠ),TGF-βreceptorⅡ(TβRⅡ),p-Smad2/3,monoclonalα-smooth muscle actin antibody(α-SMA)andα1(Ⅰ)collagen in the mouse model and control mice were evaluated by immunohistochemistry,immunoblotting and real-time polymerase chain reaction(RT-PCR).Lymphocyte subsets in liver were analyzed using flow cytometry.RESULTS:The mouse model had several key phenotypic features of human PBC,including elevated levels of alkaline phosphatase,antimitochondrial antibodies,portal bile ducts inflammation,and progressive collagen deposition.Compared with control mice,protein and mRNA levels of TGFβ1,TβRⅠ,TβRⅡ,p-Smad2/3,α-SMA andα1(Ⅰ)collagen in liver(1.7±0.4 vs 8.9±1.8,0.8±0.2 vs 5.1±1.5,0.6±0.01 vs5.1±0.1,0.6±0.3 vs 2.0±0.3,0.9±0.4 vs 3.4±0.6,0.8±0.4 vs 1.7±0.3,1.1±1.2 vs 11.8±0.6,P<0.05),and the total number and percentage of CD4+CD25+FOXP3+and CD8+lymphocytes(0.01±0.001vs 0.004±0.00,0.12±0.04 vs 0.52±0.23,P<0.01)were higher in the mouse model.CONCLUSION:TGFβ1 might play a dual role in the development of PBC:it suppresses inflammatory response but operates to enhance fibrogenesis.The aberrant activity of TGF-β1 signaling contributes to the development of PBC.     

Clofarabine 5'-di and -triphosphates inhibit human ribonucleotide reductase by altering the quaternary structure of its large subunit

Human ribonucleotide reductases (hRNRs) catalyze the conversion of nucleotides to deoxynucleotides and are composed of α- and β-subunits that form active α_nβ_m (n, m = 2 or 6) complexes. α binds NDP substrates (CDP, UDP, ADP, and GDP, C site) as well as ATP and dNTPs (dATP, dGTP, TTP) allosteric effectors that control enzyme activity (A site) and substrate specificity (S site). Clofarabine (ClF), an adenosine analog, is used in the treatment of refractory leukemias. Its mode of cytotoxicity is thought to be associated in part with the triphosphate functioning as an allosteric inhibitor of hRNR. Studies on the mechanism of inhibition of hRNR by ClF di- and triphosphates (ClFDP and ClFTP) are presented. ClFTP is a reversible inhibitor (K_i = 40 nM) that rapidly inactivates hRNR. However, with time, 50% of the activity is recovered. D57N-α, a mutant with an altered A site, prevents inhibition by ClFTP, suggesting its A site binding. ClFDP is a slow-binding, reversible inhibitor (; t_1/2 = 23 min). CDP protects α from its inhibition. The altered off-rate of ClFDP from E•ClFDP^∗ by ClFTP (A site) or dGTP (S site) and its inhibition of D57N-α together implicate its C site binding. Size exclusion chromatography of hRNR or α alone with ClFDP or ClFTP, ± ATP or dGTP, reveals in each case that α forms a kinetically stable hexameric state. This is the first example of hexamerization of α induced by an NDP analog that reversibly binds at the active site.     

Populations of the three major backbone conformations in 19 amino acid dipeptides

The amide III region of the peptide infrared and Raman spectra has been used to determine the relative populations of the three major backbone conformations (P_II, β, and α_R) in 19 amino acid dipeptides. The results provide a benchmark for force field or other methods of predicting backbone conformations in flexible peptides. There are three resolvable backbone bands in the amide III region. The major population is either P_II or β for all dipeptides except Gly, whereas the α_R population is measurable but always minor (≤ 10%) for 18 dipeptides. (The Gly φ,ψ map is complex and so is the interpretation of the amide III bands of Gly.) There are substantial differences in the relative β and P_II populations among the 19 dipeptides. The band frequencies have been assigned as P_II, 1,317–1,306 cm^-1; α_R, 1,304–1,294 cm^-1; and β, 1,294–1,270 cm^-1. The three bands were measured by both attenuated total reflection spectroscopy and by Raman spectroscopy. Consistent results, both for band frequency and relative population, were obtained by both spectroscopic methods. The β and P_II bands were assigned from the dependence of the ³J(H_N,H_α) coupling constant (known for all 19 dipeptides) on the relative β population. The P_II band assignment agrees with one made earlier from Raman optical activity data. The temperature dependences of the relative β and P_II populations fit the standard model with Boltzmann-weighted energies for alanine and leucine between 30 and 60 °C.     

Anti-Arrhenius cleavage of covalent bonds in bottlebrush macromolecules on substrate

Spontaneous degradation of bottlebrush macromolecules on aqueous substrates was monitored by atomic force microscopy. Scission of C─C covalent bonds in the brush backbone occurred due to steric repulsion between the adsorbed side chains, which generated bond tension on the order of several nano-Newtons. Unlike conventional chemical reactions, the rate of bond scission was shown to decrease with temperature. This apparent anti-Arrhenius behavior was caused by a decrease in the surface energy of the underlying substrate upon heating, which results in a corresponding decrease of bond tension in the adsorbed macromolecules. Even though the tension dropped minimally from 2.16 to 1.89 nN, this was sufficient to overpower the increase in the thermal energy (k_BT) in the Arrhenius equation. The rate constant of the bond-scission reaction was measured as a function of temperature and surface energy. Fitting the experimental data by a perturbed Morse potential V = V₀(1 - e^-βx)² - fx, we determined the depth and width of the potential to be V₀ = 141 ± 19 kJ/mol and β^-1 = 0.18 ± 0.03 Å, respectively. Whereas the V₀ value is in reasonable agreement with the activation energy E_a = 80–220 kJ/mol of mechanical and thermal degradation of organic polymers, it is significantly lower than the dissociation energy of a C─C bond D_e = 350 kJ/mol. Moreover, the force constant K_x = 2β²V₀ = 1.45 ± 0.36 kN/m of a strained bottlebrush along its backbone is markedly larger than the force constant of a C─C bond K_l = 0.44 kN/m, which is attributed to additional stiffness due to deformation of the side chains.     

Energetics of Al₁₃ Keggin cluster compounds

The ϵ-Al₁₃ Keggin aluminum hydroxide clusters are essential models in establishing molecular pathways for geochemical reactions. Enthalpies of formation are reported for two salts of aluminum centered ϵ-Keggin clusters, Al₁₃ selenate, (Na(AlO₄)Al₁₂(OH)₂₄(SeO₄)₄•12H₂O) and Al₁₃ sulfate, (NaAlO₄Al₁₂(OH)₂₄(SO₄)₄•12H₂O). The measured enthalpies of solution, ΔH_sol, at 28 °C in 5 N HCl for the ε-Al₁₃ selenate and sulfate are −924.57 (± 3.83) and −944.30 ( ± 5.66) kJ·mol^-1, respectively. The enthalpies of formation from the elements, ΔH_f,el, for Al₁₃ selenate and sulfate are −19,656.35 ( ± 67.30) kJ·mol^-1, and −20,892.39 ( ± 70.01) kJ·mol^-1, respectively. In addition, ΔH_f,el for sodium selenate decahydrate was calculated using data from high temperature oxide melt solution calorimetry measurements: −4,006.39 ( ± 11.91) kJ·mol^-1. The formation of both ε-Al₁₃ Keggin cluster compounds is exothermic from oxide-based components but energetically unfavorable with respect to a gibbsite-based assemblage. To understand the relative affinity of the ϵ-Keggin clusters for selenate and sulfate, the enthalpy associated with two S-Se exchange reactions was calculated. In the solid state, selenium is favored in the Al₁₃ compound relative to the binary chalcogenate, while in 5 N HCl, sulfur is energetically favored in the cluster compound compared to the aqueous solution. This contribution represents the first thermodynamic study of ε-Al₁₃ cluster compounds and establishes a method for other such molecules, including the substituted versions that have been created for kinetic studies. Underscoring the importance of ε-Al₁₃ clusters in natural and anthropogenic systems, these data provide conclusive thermodynamic evidence that the Al₁₃ Keggin cluster is a crucial intermediate species in the formation pathway from aqueous aluminum monomers to aluminum hydroxide precipitates.     

Photo-ribonucleotide reductase β2 by selective cysteine labeling with a radical phototrigger

Photochemical radical initiation is a powerful tool for studying radical initiation and transport in biology. Ribonucleotide reductases (RNRs), which catalyze the conversion of nucleotides to deoxynucleotides in all organisms, are an exemplar of radical mediated transformations in biology. Class Ia RNRs are composed of two subunits: α2 and β2. As a method to initiate radical formation photochemically within β2, a single surface-exposed cysteine of the β2 subunit of Escherichia coli Class Ia RNR has been labeled (98%) with a photooxidant ([Re ] = tricarbonyl(1,10-phenanthroline)(methylpyridyl)rhenium(I)). The labeling was achieved by incubation of S355C-β2 with the 4-(bromomethyl)pyridyl derivative of [Re] to yield the labeled species, [Re]-S355C-β2. Steady-state and time-resolved emission experiments reveal that the metal-to-ligand charge transfer (MLCT) excited-state ³[Re ]^∗ is not significantly perturbed after bioconjugation and is available as a phototrigger of tyrosine radical at position 356 in the β2 subunit; transient absorption spectroscopy reveals that the radical lives for microseconds. The work described herein provides a platform for photochemical radical initiation and study of proton-coupled electron transfer (PCET) in the β2 subunit of RNR, from which radical initiation and transport for this enzyme originates.     

One-electron oxidation of an oxoiron(IV) complex to form an [O═FeV═NR]+ center

Oxoiron(V) species are postulated to be involved in the mechanisms of the arene cis-dihydroxylating Rieske dioxygenases and of bioinspired nonheme iron catalysts for alkane hydroxylation, olefin cis-dihydroxylation, and water oxidation. In an effort to obtain a synthetic oxoiron(V) complex, we report herein the one-electron oxidation of the S = 1 complex [Fe^IV(O)(TMC)(NCCH₃)]²⁺ (1, where TMC is tetramethylcyclam) by treatment with tert -butyl hydroperoxide and strong base in acetonitrile to generate a metastable complex 2 at -44 °C, which has been characterized by UV-visible, resonance Raman, Mössbauer, and EPR methods. The defining spectroscopic characteristic of 2 is the unusual x/y anisotropy observed for the ⁵⁷Fe and ¹⁷O A tensors associated with the high-valent Fe═O unit and for the ¹⁴N A tensor of a ligand derived from acetonitrile. As shown by detailed density functional theory (DFT) calculations, the unusual x/y anisotropy observed can only arise from an iron center with substantially different spin populations in the d_xz and d_yz orbitals, which cannot correspond to an Fe^IV═O unit but is fully consistent with an Fe^V center, like that found for [Fe^V(O)(TAML)]^- (where TAML is tetraamido macrocyclic ligand), the only well-characterized oxoiron(V) complex reported. Mass spectral analysis shows that the generation of 2 entails the addition of an oxygen atom to 1 and the loss of one positive charge. Taken together, the spectroscopic data and DFT calculations support the formulation of 2 as an iron(V) complex having axial oxo and acetylimido ligands, namely [Fe^V(O)(TMC)(NC(O)CH₃)]⁺.     

Highly efficient and robust molecular ruthenium catalysts for water oxidation

Water oxidation catalysts are essential components of light-driven water splitting systems, which could convert water to H₂ driven by solar radiation (H₂O + hν → 1/2O₂ + H₂). The oxidation of water (H₂O → 1/2O₂ + 2H⁺ + 2e^-) provides protons and electrons for the production of dihydrogen (2H⁺ + 2e^- → H₂), a clean-burning and high-capacity energy carrier. One of the obstacles now is the lack of effective and robust water oxidation catalysts. Aiming at developing robust molecular Ru-bda (H₂bda = 2,2^′-bipyridine-6,6′-dicarboxylic acid) water oxidation catalysts, we carried out density functional theory studies, correlated the robustness of catalysts against hydration with the highest occupied molecular orbital levels of a set of ligands, and successfully directed the synthesis of robust Ru-bda water oxidation catalysts. A series of mononuclear ruthenium complexes [Ru(bda)L₂] (L = pyridazine, pyrimidine, and phthalazine) were subsequently synthesized and shown to effectively catalyze Ce^IV-driven [Ce^IV = Ce(NH₄)₂(NO₃)₆] water oxidation with high oxygen production rates up to 286 s^-1 and high turnover numbers up to 55,400.     

Resolvent positive linear operators exhibit the reduction phenomenon

The spectral bound, s(αA + βV), of a combination of a resolvent positive linear operator A and an operator of multiplication V, was shown by Kato to be convex in . Kato''s result is shown here to imply, through an elementary “dual convexity” lemma, that s(αA + βV) is also convex in α > 0, and notably, ∂s(αA + βV)/∂α ≤ s(A). Diffusions typically have s(A) ≤ 0, so that for diffusions with spatially heterogeneous growth or decay rates, greater mixing reduces growth. Models of the evolution of dispersal in particular have found this result when A is a Laplacian or second-order elliptic operator, or a nonlocal diffusion operator, implying selection for reduced dispersal. These cases are shown here to be part of a single, broadly general, “reduction” phenomenon.