Unique Active Sites Research Articles

The Production of Nitrous Oxide by the Heme/Nonheme Diiron Center of Engineered Myoglobins (FeBMbs) Proceeds through a trans-Iron-Nitrosyl Dimer

Denitrifying NO reductases are transmembrane protein complexes that are evolutionarily related to heme/copper terminal oxidases. They utilize a heme/nonheme diiron center to reduce two NO molecules to N2O. Engineering a nonheme FeB site within the heme distal pocket of sperm whale myoglobin has offered well-defined diiron clusters for the investigation of the mechanism of NO reduction in these unique active sites. In this study, we use FTIR spectroscopy to monitor the production of N2O in solution and to show that the presence of a distal FeBII is not sufficient to produce the expected product. However, the addition of a glutamate side chain peripheral to the diiron site allows for 50% of a productive single-turnover reaction. Unproductive reactions are characterized by resonance Raman spectroscopy as dinitrosyl complexes, where one NO molecule is bound to the heme iron to form a five-coordinate low-spin {FeNO}7 species with ν(FeNO)heme and ν(NO)heme at 522 and 1660 cm–1, and a second NO molecule is bound to the nonheme FeB site with a ν(NO)FeB at 1755 cm–1. Stopped-flow UV–vis absorption coupled with rapid-freeze-quench resonance Raman spectroscopy provide a detailed map of the reaction coordinates leading to the unproductive iron-nitrosyl dimer. Unexpectedly, NO binding to FeB is kinetically favored and occurs prior to the binding of a second NO to the heme iron, leading to a (six-coordinate low-spin heme-nitrosyl/FeB-nitrosyl) transient dinitrosyl complex with characteristic ν(FeNO)heme at 570 ± 2 cm–1 and ν(NO)FeB at 1755 cm–1. Without the addition of a peripheral glutamate, the dinitrosyl complex is converted to a dead-end product after the dissociation of the proximal histidine of the heme iron, but the added peripheral glutamate side chain in FeBMb2 lowers the rate of dissociation of the promixal histidine which in turn allows the (six-coordinate low-spin heme-nitrosyl/FeB-nitrosyl) transient dinitrosyl complex to decay with production of N2O at a rate of 0.7 s–1 at 4 °C. Taken together, our results support the proposed trans mechanism of NO reduction in NORs.

Multilayered Nanoclusters of Platinum and Gold: Insights on Electrodeposition Pathways, Electrocatalysis, Surface and Bulk Compositional Properties

Electrochemical, surface and bulk compositional properties of multilayered nanoclusters of Pt and Au, electrochemically deposited on glassy carbon under conditions involving sequential surface–limited redox–replacement reactions (performed at open–circuit) and voltammetric dealloying of templating adlayers of electrodeposited Cu, have been studied in the direction of electrocatalytic applications. Variations in open–circuit potentials during redox–replacement steps indicated thermodynamically–favored formation of Pt(s) and Au(s). Unique bimetallic interfacial active sites, Pt|Au or Au|Pt, were effectively generated as evidenced by their distinct surface electrochemistry and multicomponent X–ray photoelectron spectral features. The bulk and surface–to–near surface distribution of Pt and Au appeared to be influenced by the stoichiometry of the surface redox–replacement reactions and sequential dealloying processes through which the nanoclusters were synthesized. Interactions between metal centers, carbon and oxygen containing surface functional groups on the glassy carbon appeared to have played a significant role in the overall stabilization and catalytic activity of the nanoclusters. Profound effects were also found on interfacial charge–transfer and adsorptive properties involving carbon monoxide and its subsequent electrooxidation to CO2, as well as on the electrocatalytic activity involving formic acid oxidation reaction, where the Pt–rich (Pt|Au) exhibited the highest activity.

Catalytic Role of O•– Radicals in the Low-Temperature Isotopic Exchange in Dioxygen

In spite of a long investigation history, the low-temperature isotopic exchange in dioxygen taking place without involvement of the catalyst oxygen (R0 exchange) is still an exotic and poorly understood phenomenon in heterogeneous catalysis. Although very strong bonds are to be cleft in O2 molecules (118 kcal/mol), over some metal oxides R0 can be observed even at the temperature of liquid nitrogen. In this work, we studied the R0 exchange over a FeZSM-5 zeolite at 233 K and discovered for the first time a linear dependence of the R0 rate on the concentration of O•– radicals (α-oxygen), which identifies a catalytic role of these species. Upon transition to cryogenic temperatures, O•– species lose the ability to exchange themselves with dioxygen, but start functioning as a principal part of unique active sites capable of catalyzing a very smooth redistribution of the oxygen–oxygen bonds in adsorbed O2 molecules. Running by a highly concerted mechanism, this remarkable process leads to the R0 exchange with ...

Effect of crystal phase composition on the reductive and oxidative abilities of TiO2 nanotubes under UV and visible light

Titania nanotube arrays synthesized by the electrochemical oxidation of titanium foils have generated considerable interest as photocatalysts for their ordered nature and large surface area. Mixed-phase materials combining the anatase and rutile crystal phases of TiO2, however, have been much more widely studied due to their enhanced reactivity in comparison to pure phase materials. In this study, we seek to integrate these two lines of research and investigate the reductive and oxidative reactivity of TiO2 nanotube arrays (anatase phase) supported on TiO2 films of varying crystal phase composition. A series of TiO2 nanotubes 1.2μm in length was synthesized, annealed at varying temperatures to control their crystallinity, and characterized by various physical techniques (e.g. XRD, diffuse reflectance, SEM). Photocatalytic CO2 reduction and acetaldehyde oxidation reactions were performed in the gas phase under UV and visible wavelengths. For CO2 reduction, reaction rates decreased with increasing rutile phase under UV. Rates increased with rutile phase ratio under visible and near visible light. For oxidation, the mixed-phase samples showed enhanced reactivity, with a maximum acetaldehyde destruction rate achieved at a 79:21 ratio of anatase to rutile. The samples were substantially less active under visible light, except for the 620°C composite (77% rutile) which showed a slight rate increase. Nanotubes annealed at 680°C collapsed to a random porous structure, but showed comparable reductive ability to the non-collapsed samples despite the loss of surface area. This is attributed to the creation of additional anatase–rutile crystallite interfacial area leading to the formation of unique active sites. Control of crystal phase composition through anneal temperature is found to be a simple way to tune the reactivity of these materials and enhance their ability to absorb visible light.

Forty Years of Designing Catalytic and Adsorptive Sites in FAU Type Zeolites at K.U. Leuven

An overview of scientific achievements at K.U. Leuven on the design and characterization of active sites in X and Y zeolites and on the development of adsorptive and catalytic processes in which these active sites are operated is provided. The historical development of this research area and the scientific progress are presented. Zeolite Y and ultrastabilized specimen present Bronsted and Lewis sites that can be operated in vapor and liquid phase hydrocarbon conversion processes, bifunctional catalysis, epoxidation and ring opening of epoxides. Examples of molecular shape selective catalysis with Y zeolites are presented. The redox chemistry of transition metal exchanged faujasites is understood in great detail and has been exploited in Wacker chemistry and water splitting in a photochemical-thermal cycle. Selective adsorbents were designed based on knowledge of cation siting in X and Y zeolites. The occlusion of coordination compounds in the faujasite supercages is a means of creating unique active sites. Catalytic oxidative properties of the enzymes can be mimicked by embedding zeozyme in a hydrophobic membrane.

Hydrogenase cluster biosynthesis: organometallic chemistry nature's way

It has been over a decade now since it was revealed that the metal containing active sites of hydrogenases possess carbonyl and cyanide ligands bound to iron. The presence of these ligands in hydrogenases came as a surprise and to-date these ligands have not been observed to be associated with any other enzymatic metallocenter. The elucidation of the structures of these unique metalloenzymes and their associated metal clusters created opportunity for a number of different lines of research. For synthetic chemists, the structures of hydrogenase active sites have provided attractive targets for syntheses that advance our understanding of the electronic structure and reactivity of these unique enzyme active sites. These efforts contribute to the synthesis of first row transition metal catalysts for hydrogen oxidation and hydrogen production that could have significant impacts on alternative and renewable energy solutions. Although effective synthetic approaches have been identified to generate models with a high degree of similarity to these active sites, the details of how these metal clusters are synthesized biochemically have not been resolved. Since hydrogen metabolism is presumed to be an early feature in the energetics of life and hydrogen metabolizing organisms can be traced very early in molecular phylogeny, the metal clusters at hydrogenase active sites are presumed to be among the earliest of known co-factors. Comparison of mineral based precursors and synthetic cluster analog chemistry to what is observed in contemporary biological systems may shed light on how proto-metabolically relevant catalysts first arose prebiotically by the processes of adoption of pre-existing functionality and ligand assisted catalysis.

Safety and Antitumor Efficacy of the Proteasome Inhibitor Carfilzomib (PR-171) Dosed for Five Consecutive Days in Hematologic Malignancies: Phase 1 Results.

Introduction: Carfilzomib (CFZ) is a structurally- and mechanistically-novel proteasome inhibitor (PI) of the peptide epoxyketone class that exhibits a high level of selectivity for the unique N-terminal threonine active sites within the proteasome. CFZ is similar to bortezomib (BTZ; VELCADE®), in that it is a potent inhibitor of the chymotrypsin-like activity, but unlike BTZ, CFZ has shown minimal cross-reactivity with the other catalytic sites within the proteasome or across other protease classes. Preclinical human tumor xenograft models indicate that consecutive day (D1, D2) dosing resulting in 48 hours of proteasome suppression was superior to split dosing (D1, D4).Methods: This phase 1 sequential dose escalation trial tested an intensive proteasome inhibition schedule with CFZ administered as an intravenous push (IVP) daily (QD) for 5 consecutive days on a 14 day cycle. The doses administered ranged from 1.2 mg/m2 to 20 mg/m2. Multiple myeloma (MM), non-Hodgkin's lymphoma (NHL) that included mantle cell lymphoma (MCL), Hodgkin's disease (HD), or Waldenström's macroglobulinemia (WM) were eligible if they had relapsed after 2 or more prior therapies.Results: A total of 29 subjects have been treated. The minimal effective dose (MED) was 11 mg/m2. A maximum of 80% proteasome inhibition was achieved in peripheral blood and mononuclear cells at ≥15 mg/m2. Five responses have occurred in 14 patients (6 MM, 3 MCL, 1 WM, 4 NHL) enrolled at ≥ the MED: 1 MM partial response (PR), 2 MM minimal responses (MR), 1 WM MR and one macroscopic CR in a patient with gastrointestinal MCL. Responses occurred in patients whose disease had failed therapy with BTZ, immunomodulatory agents, and stem cell transplant. Four NHL patients have had stable disease and remained on treatment for up to 7 months. Dose limiting toxicity reported at 20 mg/m2 included febrile neutropenia (reversible) and grade 4 thrombocytopenia. Grade 3 (25%) or grade 4 events (8%) were primarily hematologic events. Most withdrawals occurred due to the inconvenience of frequent visits, or progressive disease.Conclusions: This study demonstrates that intensive CFZ dosing leading to sustained proteasome inhibition is well tolerated. The selectivity of the epoxyketone class of PI may promote a favorable safety profile. Furthermore, antitumor responses in heavily pretreated patients suggest non-cross resistant activity. Phase 2 trials in MM and solid tumors are proceeding with an alternate IV schedule. An oral agent suitable for frequent dosing schedules is under development.

Phase 1 Single Agent Antitumor Activity of Twice Weekly Consecutive Day Dosing of the Proteasome Inhibitor Carfilzomib (PR-171) in Hematologic Malignancies.

Carfilzomib (CFZ) is a structurally- and mechanistically-novel proteasome inhibitor of peptide epoxyketone class that exhibits a high level of selectivity for the unique N-terminal threonine active sites within the proteasome. CFZ is similar to bortezomib (BTZ,Velcade®), in that it is a potent inhibitor of the proteasome chymotrypsin-like activity, but unlike BTZ, CFZ has shown minimal cross-reactivity with the other catalytic sites within the proteasome or across other protease classes. Preclinical human tumor xenograft models indicated that consecutive day (D1, D2) dosing resulting in 48 hours of proteasome suppression was superior to split dosing (D1, D4). This phase 1 sequential dose escalation trial tested twice weekly consecutive day dosing (48 hour proteasome inhibition) with carfilzomib. Carfilzomib was administered as an IVP on a 28 day cycle (D1, 2, 8, 9, 15 and 16 with 12 days rest) at doses from 1.2 mg/m2 to 27 mg/m2. Patients with Multiple myeloma (MM), non-Hodgkin's Lymphoma (NHL) that included mantle cell lymphoma (MCL), Hodgkin's disease (HD), or Waldenström's macroglobulinemia (WM) were eligible if they had relapsed after at least 2 prior therapies. A total of 37 subjects have been treated. The minimal effective dose (MED) was 15 mg/m2, and maximum (80%) proteasome inhibition was achieved in peripheral blood and mononuclear cells at this dose. Five objective responses have occurred in 16 patients (13 MM, 3 MCL) enrolled at ≥ the MED: 4 MM partial response (PR), 1 MM minimal responses (MR). The responses have been durable (134 to 392 days) and occurred in patients who had failed bortezomib, immunomodulatory agents and stem cell transplant. Six additional patients have had stable disease (SD): 2 MM, 1 T NHL, 2 MCL, 1 B NHL. Dose limiting toxicity reported at 27 mg/m2 included a hypoxic event and Grade 2 thrombocytopenia increasing to Grade 4. A reversible Grade 2 creatinine D2 was reported in 3/5 MM patients treated at 27 mg/m2 and was associated with a rapid decline in M-protein without evidence of tumor lysis syndrome and the event did not occur on rechallenge. Cyclic thrombocytopenia was rapidly reversible and painful peripheral neuropathy was not reported. This study demonstrates that consecutive CFZ dosing leading to sustained proteasome inhibition is well tolerated on a weekly basis. The selectivity of epoxyketone class of proteasome inhibitors may provide a wide therapeutic index. Patients have remained on therapy for over a year, and furthermore, antitumor responses in heavily pretreated MM patients suggest non-cross resistant activity. Phase 2 trials in relapse and refractory MM and relapse solid tumors are proceeding with this schedule.

A “Green Route” to Propene through Selective Hydrogen Oxidation

The pros and cons of oxidative dehydrogenation of propane are outlined and a new catalytic system based on metal-doped cerianite catalysts is introduced. These novel materials catalyze the selective combustion of hydrogen from a mixture of hydrogen, propane, and propene at 550 degrees C. This gives three key advantages: energy is supplied directly where needed, product separation is made easier, and the dehydrogenation equilibrium is shifted to the desired products. A set of eighteen doped cerianites was synthesized in parallel, characterized, and screened for activity, selectivity, and stability in a cyclic redox system. The best results were obtained with Ce(0.89)Cr(0.02)Fe(0.09)O(2), Ce(0.98)Sn(0.02)O(2), and Ce(0.96)Cu(0.02)Zn(0.02)O(2), which gave 98 %, 91 %, and 98 % selectivity, respectively. Ce(0.89)Cr(0.02)Fe(0.09)O(2) also shows excellent stability in over 120 cycles (66 h on stream at 550 degrees C). Importantly, these doped cerias are monophasic crystalline materials. The dopants are incorporated as solid solutions throughout the fluorite lattice. This means that these catalysts are very stable (they do not sinter during reduction) as opposed to traditional supported metal oxides. The results show that both activity and selectivity towards hydrogen combustion can be tuned (increased or decreased) by selecting the appropriate dopant. Furthermore, the trends in selectivity differ from those measured on supported oxides of the same elements, which indicates that these novel materials indeed contain unique active sites. The factors governing selectivity towards hydrogen oxidation and the nature of the active site are discussed.

Ammonia decomposition over Ru and Ni catalysts supported on fumed SiO 2, MCM-41, and SBA-15

The effects of using fumed SiO 2, MCM-41, and SBA-15 as supports for Ru and Ni catalysts on ammonia decomposition were investigated. It was found that the supported catalysts on these siliceous materials are more active than those supported on ordinary silica. In general, the supported Ru catalysts are more active than the Ni catalysts, and MCM-41 is the best support material for Ru and Ni. Significant enhancement in activity was observed when the supported Ru catalysts were modified by KOH, but such effect was minimal in the cases of supported Ni catalysts. The results of N 2 adsorption and transmission electron microscopy studies revealed that the Ni particles of Ni/MCM-41(TIE) prepared by template-ion exchange (TIE) method are largely located inside the pores of MCM-41. Compared with the Ni/MCM-41(IMP) catalyst prepared by impregnation method, the Ni/MCM-41(TIE) catalyst exhibited appropriate Ni dispersion and weaker Ni/support interaction, and consequently higher catalytic activity. In terms of the remarkable changes in turnover frequency (TOF) with Ni dispersion on different catalysts, it was demonstrated that NH 3 decomposition over Ni is significantly structure sensitive. Some unique metal clusters similar to B5 active sites should be responsible for the catalytic activities. Preparation methodology has a substantial influence on metal dispersion. For the supported Ni catalysts prepared by the TIE method, the generated metal particles are very small and the constitution of unique active sites becomes unfavorable, which accounts for the apparently lower TOF values.

Selective catalytic reduction of NO 2 with urea in nanocrystalline NaY zeolite

In this study, the selective catalytic reduction (SCR) of NO 2 with urea in nanocrystalline NaY zeolite was investigated with in situ transmission Fourier transform infrared (FTIR) spectroscopy and solid-state nuclear magnetic resonance spectroscopy. At T = 473 K , the reaction rate for urea-SCR of NO 2 in nanocrystalline NaY zeolite was significantly greater than that in commercial NaY zeolite with a larger crystal size. In addition, a dramatic decrease in the concentration of undesirable surface species, including biuret and cyanuric acid, was observed in nanocrystalline NaY compared with commercial NaY after urea-SCR of NO 2 at T = 473 K . The increased reactivity for urea-SCR of NO 2 was attributed to silanol groups and extra-framework aluminum species located on the external surface of nanocrystalline NaY. Specifically, NO x storage as nitrate and nitrite on the internal zeolite surface was coupled to reactive deNO x sites on the external surface. Isotopic labeling combined with IR analysis suggest that N N bond formation involved both an N-atom originating from NO 2 and an N-atom originating from urea. This is the first clear example demonstrating that the increased external surface area (up to ∼40% of total surface area) of nanocrystalline zeolites can be used as a reactive surface with unique active sites for catalysis.

The surface sites of sulfated zirconia studied in situ by laser-induced fluorescence spectroscopy

The surface sites of sulfated zirconia were investigated in situ by laser-induced fluorescence spectroscopy using aniline as the probe molecule. Different from the cases for many other oxides, the aniline adsorbed on the unique active sites of sulfated zirconia at r.t. is changed into another species, which emits a characteristic fluorescence band at 422 nm. The results illustrate that the sulfate groups in sulfated zirconia are favorable for the generation of these unique active sites, which also rarely exist on pure zirconia composed of tetragonal and monoclinic phases but do not exist on pure zirconia composed of monoclinic phase.

A structure model as the origin of catalytic properties of metal-doped MgO systems

The local structures of Fe 3+ — and Cr 3+ — doped MgO catalysts which show high activities for the cross-coupling reaction between CH 3OH and RCH 2Z(R=H and CH 3, Z=CN, COR’, and COOR”) to produce the α β-unsaturated compounds were studied by means of EXAFS and XANES. For comparison the Ni 2+-, Co 2+-, and Cu 2+-doped MgO catalysts which are practically inactive were also investigated. Small Fe 2MgO 4 spinel cluster structures in MgO lattice were found for Fe 3+-MgO. Cr 3+ in Cr 3+-MgO was also considered to form the similar cluster structure. The ensemble composed of the coordinatively unsaturated basic O 2− site and acidic Fe 3+ site in the Fe 3+-MgO system exists at the interface between Fe 2MgO 4 and the parent MgO lattice. Such ensembles of the basic and acidic sites having cation defects provide the unique active sites for the cross-coupling reaction. On the contrary, Ni 2+, Co 2+ and Cu 2+ ions were observed to randomly dissolve into the MgO lattice.

The binding of ethacrynic acid to rabbit kidney cortex

1. 1. The uptake of labeled ethacrynic acid by slices of rabbit kidney cortex at 25 °C approached a steady state level by 1 h of incubation, was linearly dependent on the external concentration of the compound, and was not affected by metabolic inhibitors. 2. 2. The apparent steady state level of ethacrynic acid in the tissue considerably exceeded the external concentration. Evidence for binding of ethacrynic acid by tissue components was obtained as follows: (a) 40 % of the label could not be washed out; (b) the same fraction of the label was found in a well washed tissue solid sedimenting at up to 18 000 × g. 3. 3. The presence of the thiol reagents N-ethylmaleimide, HgCl 2, and iodoacetic acid did not diminish binding of ethacrynic acid. 4. 4. Neither the uptake into nor the efflux from rabbit kidney cortex of ethacrynic acid was modified by the presence, in various concentrations, of materials presumably associated with sites of active transport of ions and sugars, e.g. ouabain, normally actively transported sugar, and Na +. 5. 5. It was concluded that the major part of the binding of ethacrynic acid by rabbit kidney cortex was not specific for either tissue sulfhydryl groups nor unique membrane active transport sites.