Possessors and Possessedanalyzes how and why museums-characteristically Western institutions-emerged in the late-nineteenth-century Ottoman Empire. Shaw argues that, rather than directly emulating ...post-Enlightenment museums of Western Europe, Ottoman elites produced categories of collection and modes of display appropriate to framing a new identity for the empire in the modern era. In contrast to late-nineteenth-century Euro-American museums, which utilized organizational schema based on positivist notions of progress to organize exhibits of fine arts, Ottoman museums featured military spoils and antiquities long before they turned to the "Islamic" collections with which they might have been more readily associated. The development of these various modes of collection reflected shifting moments in Ottoman identity production. Shaw shows how Ottoman museums were able to use collection and exhibition as devices with which to weave counter-colonial narratives of identity for the Ottoman Empire. Impressive for both the scope and the depth of its research,Possessors and Possessedlays the groundwork for future inquiries into the development of museums outside of the Euro-American milieu.
Some enzymes, including those that are involved in the activation of small molecules such as H2 or CO2, can be wired to electrodes and function in either direction of the reaction depending on the ...electrochemical driving force and display a significant rate at very small deviations from the equilibrium potential. We call the former property “bidirectionality” and the latter “reversibility”. This performance sets very high standards for chemists who aim at designing synthetic electrocatalysts. Only recently, in the particular case of the hydrogen production/evolution reaction, has it been possible to produce inorganic catalysts that function bidirectionally, with an even smaller number that also function reversibly. This raises the question of how to engineer such desirable properties in other synthetic catalysts. Here we introduce the kinetic modeling of bidirectional two-electron-redox reactions in the case of molecular catalysts and enzymes that are either attached to an electrode or diffusing in solution in the vicinity of an electrode. We emphasize that trying to discuss bidirectionality and reversibility in relation to a single redox potential leads to an impasse: the catalyst undergoes two redox transitions, and therefore two catalytic potentials must be defined, which may depart from the two potentials measured in the absence of catalysis. The difference between the two catalytic potentials defines the reversibility; the difference between their average value and the equilibrium potential defines the directionality (also called “preference”, or “bias”). We describe how the sequence of events in the bidirectional catalytic cycle can be elucidated on the basis of the voltammetric responses. Further, we discuss the design principles of bidirectionality and reversibility in terms of thermodynamics and kinetics and conclude that neither bidirectionality nor reversibility requires that the catalytic energy landscape be flat. These theoretical findings are illustrated by previous results obtained with nickel diphosphine molecular catalysts and hydrogenases. In particular, analysis of the nickel catalysts highlights the fact that reversible catalysis can be achieved by catalysts that follow complex mechanisms with branched reaction pathways.
Conspectus Redox active metalloenzymes play a major role in energy transformation reactions in biological systems. Examples include formate dehydrogenases, nitrogenases, CO dehydrogenase, and ...hydrogenases. Many of these reactions are also of interest to humans as potential energy storage or utilization reactions for photoelectrochemical, electrolytic, and fuel cell applications. These metalloenzymes consist of redox active metal centers where substrates are activated and undergo transformation to products accompanied by electron and proton transfer to or from the substrate. These active sites are typically buried deep within a protein matrix of the enzyme with channels for proton transport, electron transport, and substrate/product transport between the active site and the surface of the protein. In addition, there are amino acid residues that lie in close proximity to the active site that are thought to play important roles in regulating and enhancing enzyme activity. Directly studying the outer coordination sphere of enzymes can be challenging due to their complexity, and the use of modified molecular catalysts may allow us to provide some insight. There are two fundamentally different approaches to understand these important interactions. The “bottom-up” approach involves building an amino acid or peptide containing outer coordination sphere around a functional molecular catalyst, and the “top-down” approach involves attaching molecular catalyst to a structured protein. Both of these approaches have been undertaken for hydrogenase mimics and are the emphasis of this Account. Our focus has been to utilize amino acid or peptide based scaffolds on an active functional enzyme mimic for H2 oxidation and production, Ni(PR 2NR′ 2)22+. This “bottom-up” approach has allowed us to evaluate individual functional group and structural contributions to electrocatalysts for H2 oxidation and production. For instance, using amine, ether, and carboxylic acid functionalities in the outer coordination sphere enhances proton movement and results in lower catalytic overpotentials for H2 oxidation, while achieving water solubility in some cases. Amino acids with acidic and basic side chains concentrate substrate around catalysts for H2 production, resulting in up to 5-fold enhancements in rate. The addition of a structured peptide in an H2 production catalyst limited the structural freedom of the amino acids nearest the active site, while enhancing the overall rate. Enhanced stability to oxygen or extreme conditions such as strongly acidic or basic conditions has also resulted from an amino acid based outer coordination sphere. From the “top-down” approach, others have achieved water solubility and photocatalytic activity by associating this core complex with photosystem-I. Collectively, by use of this well understood core, the role of individual and combined features of the outer coordination sphere are starting to be understood at a mechanistic level. Common mechanisms have yet to be defined to predictably control these processes, but our growing knowledge in this area is essential for the eventual mimicry of enzymes by efficient molecular catalysts for practical use.
Pendant amines play an invaluable role in chemical reactivity, especially for molecular catalysts based on earth-abundant metals. As inspired by FeFe-hydrogenases, which contain a pendant amine ...positioned for cooperative bifunctionality, synthetic catalysts have been developed to emulate this multifunctionality through incorporation of a pendant amine in the second coordination sphere. Cyclic diphosphine ligands containing two amines serve as the basis for a class of catalysts that have been extensively studied and used to demonstrate the impact of a pendant base. These 1,5-diaza-3,7-diphosphacyclooctanes, now often referred to as “P2N2” ligands, have profound effects on the reactivity of many catalysts. The resulting Ni(PR 2NR′ 2)22+ complexes are electrocatalysts for both the oxidation and production of H2. Achieving the optimal benefit of the pendant amine requires that it has suitable basicity and is properly positioned relative to the metal center. In addition to the catalytic efficacy demonstrated with Ni(PR 2NR′ 2)22+ complexes for the oxidation and production of H2, catalysts with diphosphine ligands containing pendant amines have also been demonstrated for several metals for many different reactions, both in solution and immobilized on surfaces. The impact of pendant amines in catalyst design continues to expand.
Significance Enzymes achieve rapid and reversible H ₂ oxidation catalysis by cooperative behavior between the active site and the protein scaffold. To better understand the role of the enzyme ...scaffold, we have attached amino acids (glycine, arginine, and arginine methyl ester) to an active functional mimic of hydrogenase to give Formula. The resulting complexes are fully reversible catalysts with the arginine complex exhibiting high activity for both H ₂ oxidation/production, functionality achieved by the addition of an outer coordination sphere.
Hydrogenases interconvert H ₂ and protons at high rates and with high energy efficiencies, providing inspiration for the development of molecular catalysts. Studies designed to determine how the protein scaffold can influence a catalytically active site have led to the synthesis of amino acid derivatives of Formula complexes, Formula (CyAA). It is shown that these CyAA derivatives can catalyze fully reversible H ₂ production/oxidation at rates approaching those of hydrogenase enzymes. The reversibility is achieved in acidic aqueous solutions (pH = 0–6), 1 atm 25% H ₂/Ar, and elevated temperatures (tested from 298 to 348 K) for the glycine (CyGly), arginine (CyArg), and arginine methyl ester (CyArgOMe) derivatives. As expected for a reversible process, the catalytic activity is dependent upon H ₂ and proton concentrations. CyArg is significantly faster in both directions (∼300 s ⁻¹ H ₂ production and 20 s ⁻¹ H ₂ oxidation; pH = 1, 348 K, 1 atm 25% H ₂/Ar) than the other two derivatives. The slower turnover frequencies for CyArgOMe (35 s ⁻¹ production and 7 s ⁻¹ oxidation under the same conditions) compared with CyArg suggests an important role for the COOH group during catalysis. That CyArg is faster than CyGly (3 s ⁻¹ production and 4 s ⁻¹ oxidation) suggests that the additional structural features imparted by the guanidinium groups facilitate fast and reversible H ₂ addition/release. These observations demonstrate that outer coordination sphere amino acids work in synergy with the active site and can play an important role for synthetic molecular electrocatalysts, as has been observed for the protein scaffold of redox active enzymes.
A biomimetic nickel bis‐diphosphine complex incorporating the amino acid arginine in the outer coordination sphere was immobilized on modified carbon nanotubes (CNTs) through electrostatic ...interactions. The functionalized redox nanomaterial exhibits reversible electrocatalytic activity for the H2/2 H+ interconversion from pH 0 to 9, with catalytic preference for H2 oxidation at all pH values. The high activity of the complex over a wide pH range allows us to integrate this bio‐inspired nanomaterial either in an enzymatic fuel cell together with a multicopper oxidase at the cathode, or in a proton exchange membrane fuel cell (PEMFC) using Pt/C at the cathode. The Ni‐based PEMFC reaches 14 mW cm−2, only six‐times‐less as compared to full‐Pt conventional PEMFC. The Pt‐free enzyme‐based fuel cell delivers ≈2 mW cm−2, a new efficiency record for a hydrogen biofuel cell with base metal catalysts.
A dual‐purpose catalyst: A carbon‐nanotube‐supported, biomimetic nickel catalyst was developed for efficient platinum‐free H2 oxidation. The high activity of the complex over a wide pH range allows its integration in an enzymatic fuel cell together with a multicopper oxidase at the cathode, or in a proton exchange membrane fuel cell using Pt/C at the cathode.
Oxygen-containing carbons are promising supports and metal-free catalysts for many reactions. However, distinguishing the role of various oxygen functional groups and quantifying and tuning each ...functionality is still difficult. Here we investigate the role of Brønsted acidic oxygen-containing functional groups by synthesizing a diverse library of materials. By combining acid-catalyzed elimination probe chemistry, comprehensive surface characterizations,
N isotopically labeled acetonitrile adsorption coupled with magic-angle spinning nuclear magnetic resonance, machine learning, and density-functional theory calculations, we demonstrate that phenolic is the main acid site in gas-phase chemistries and unexpectedly carboxylic groups are much less acidic than phenolic groups in the graphitized mesoporous carbon due to electron density delocalization induced by the aromatic rings of graphitic carbon. The methodology can identify acidic sites in oxygenated carbon materials in solid acid catalyst-driven chemistry.
An issue of growing importance within the field of drowning prevention is the undertaking of aquatic rescues by bystanders, who sometimes drown in the process. The main objectives of this study were ...to describe characteristics of bystanders making rescues in different Australian aquatic environments, identify the role of prior water safety training in conducting bystander rescues and provide insights into future public education strategies relating to bystander rescue scenarios. An online survey was disseminated via various social media platforms in 2017 and gathered a total of 243 complete responses. The majority of bystander rescues described took place in coastal waterways (76.5%; n = 186), particularly beaches (n = 67), followed by pools (17.3%; n = 42) and inland waterways (6.2%; n = 15). The majority of respondents were males (64.2%; n = 156) who rescued on average approximately twice as many people in their lifetime (6.5) than female respondents (3.6). Most rescues occurred more than 1 km from lifeguard/lifesaver services (67%; n = 163), but in the presence of others (94.2%; n = 229). The majority of bystander rescuers had water safety training (65.8%; n = 160), self-rated as strong swimmers (68.3%; n = 166), conducted the rescue without help from others (60%; n = 146), did not use a flotation device to assist (63%; n = 153), but were confident in their ability to make the rescue (76.5%; n = 186). However, most considered the situation to be very serious (58%; n = 141) and felt they had saved a life (70.1%; n = 172). With the exception of pools, most bystanders rescued strangers (76.1%; n = 185).While Australia clearly benefits from having a strong water safety culture, there is no clear consensus on the most appropriate actions bystanders should take when confronted with a potential aquatic rescue scenario. In particular, more research is needed to gather information regarding bystander rescues undertaken by those without prior water safety training.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
As the saying goes, one person’s trash is another’s treasure. Analyses of current reuse movements focus generally on a politics of uncoupling from capitalist consumption traps and commodity ...fetishism. The perspective presented here considers other motivations by tracing desires for specific kinds of objects, from the past. I consider current reuse debates from a subcultural perspective, of inner-urban living in the late 1970s and 1980s. With the assistance of autoethnography, I delve into this urban subculture, known for its reliance on Do-It-Yourself. This included practices of Do-It-Yourself housing, furnishings, clothing and music, and the reliance on the reuse of preowned materials which, in turn, were often also discarded as part of this transient way of living. I therefore highlight the practice of disposable fast-fashion enabled through reuse. This included the display of ‘tasteless’ object d’art. With my personal history as a backdrop, I highlight the complexity of reuse politics that sometimes reaches beyond anti-consumerist imperatives. Specific reference is made to now highly collectable objects such as ‘Aboriginalia’ and other pieces emblazoned with caricatures of ‘noble savages’. The current collectability of these now rare objects, points to a subtle and often hidden politics of racialised nostalgia.
The active site of hydrogenases has been a source of inspiration for the development of molecular catalysts. However, direct comparisons between molecular catalysts and enzymes have not been possible ...because different techniques are used to evaluate both types of catalysts, minimizing our ability to determine how far we have come in mimicking the enzymatic performance. The catalytic properties of the Ni(PCy2NGly2)22+ complex with the NiFe‐hydrogenase from Desulfovibrio vulgaris immobilized on a functionalized electrode were compared under identical conditions. At pH 7, the enzyme shows higher activity and lower overpotential with better stability, while at low pH, the molecular catalyst outperforms the enzyme in all respects. This is the first direct comparison of enzymes and molecular complexes, enabling a unique understanding of the benefits and detriments of both systems, and advancing our understanding of the utilization of these bio‐inspired complexes in fuel cells.
Inspirational performance: The H2 oxidation performance of a surface‐immobilized bio‐inspired Ni‐based complex is directly compared with NiFe‐hydrogenase. The enzyme outperforms the Ni complex at high pH, while the Ni complex outperforms the enzyme at acidic pH and in the presence of CO. Both show competitive functionality compared to Pt. These results suggest benefits for both synthetic and natural catalysts in practical applications.
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