Evolution in the understanding of [Fe]-hydrogenase

Hydrogenases catalyse redox reactions with molecular hydrogen, either as substrate or product. The enzymes harness hydrogen as a reductant using metals that are abundant and economical, namely, nickel and iron, and should provide new pointers for the economic use of hydrogen in manmade devices. The most recently discovered and perhaps the most enigmatic of the hydrogenases is the [Fe]-hydrogenase, used by certain microorganisms in the pathway that reduces carbon dioxide to methane. Since its discovery some twenty years ago, [Fe]-hydrogenase has consistently provided structural and mechanistic surprises, often requiring complete re-evaluation of its mechanism of action. This tutorial review combines recent advances in X-ray crystallography and other analytical techniques, as well as in computational studies and in chemical synthesis to provide a platform for understanding this remarkable enzyme type.     

Microporous metal organic materials: promising candidates as sorbents for hydrogen storage

Advancement in hydrogen storage techniques represents one of the most important areas of today's materials research. While extensive efforts have been made to the existing techniques, there is no viable storage technology capable of meeting the DOE cost and performance targets at the present time. New materials with significantly improved hydrogen adsorption capability are needed. Microporous metal coordination materials (MMOM) are promising candidates for use as sorbents in hydrogen adsorption. These materials possess physical characteristics similar to those of single-walled carbon nanotubes (SWNTs) but also exhibit a number of improved features. Here, we report a novel MMOM structure and its room-temperature hydrogen adsorption properties.     

Plasma-chemical processes for the preparation of pure hydrogen

The current status and prospects of the commercial and pilot production of high-purity hydrogen (>99.9999%) in quasi-equilibrium and nonequilibrium plasmas with the extraction of hydrogen using palladium membranes are surveyed. It was concluded that plasma-chemical processes with the use of a quasi-equilibrium arc plasma for the production of hydrogen with acetylene and carbon black or as a syngas constituent are currently most promising.     

Pd基催化剂在分解甲酸析氢中的研究进展

甲酸是最简单的羧酸,无色、低毒,在室温下便于运输和储存。最近,甲酸作为一种最有前景的储氢材料,在室温下采用异相催化剂分解甲酸制氢气引起了科研工作者的广泛关注。和其他催化剂相比,Pd基催化剂在温和条件下催化甲酸分解制备氢气方面表现出优良的性能,是一种非常理想的非均相催化剂。本文介绍了多种Pd基催化剂的性能特点、制备方法和其在催化甲酸分解制备氢气领域的研究进展,并对其未来研究发展方向进行展望。     

An asymmetric synthesis of hamigeran B via a Pd asymmetric allylic alkylation for enantiodiscrimination

A protocol for the asymmetric allylic alkylation of a five-membered ring ketone derivative that employs the lithium enolate in the presence of lithium alkoxides gave high yields and enantioselectivities. This product serves as a versatile intermediate as demonstrated in a convergent total synthesis of the antiviral agent hamigeran B. The sequence involves two unusual observations. In the intramolecular Heck reaction which establishes the complete ring sytem, the beta-H elimination step occurs both endocyclic (as expected) and exocyclic, the latter most surprising since it creates an exocylic tetrasubstituted double bond. In the catalytic hydrogenation, use of Pd/C gives complete selectivity for net delivery of hydrogen to the most hindered face of the substrate, whereas use of Ir black gives complete selectivity for delivery of hydrogen to the least hindered face. Such unusual behavior speaks to the unusual chemical properties associated with hamigeran B which may be relevant to its biological activity.     

嵌碳TiO_2的快速制备及其光催化制氢特性——介绍一个材料综合实验

以TiO_2为代表的半导体光催化剂通过对太阳能的利用,有助于解决当前制约人类社会发展的能源危机与环境污染问题。本实验采用新颖的火焰辅助热解法一步制备出锐钛矿相嵌碳TiO_2微球,该制备方法可避免热处理,实验过程简单快速、无液体废弃物,绿色便捷,利用气相色谱仪测定样品的光催化分解水的产氢量。通过本实验,学生可全面了解半导体光催化剂的光催化原理、制备、表征、功能测试,同时帮助学生认识太阳能、氢能等新能源研究概况,渗透环保的理念。     

Catalytic oxidation of organic substrates by molecular oxygen and hydrogen peroxide by multistep electron transfer--a biomimetic approach

Oxidation reactions are of fundamental importance in nature, and are key transformations in organic synthesis. The development of new processes that employ transition metals as substrate-selective catalysts and stoichiometric environmentally friendly oxidants, such as molecular oxygen or hydrogen peroxide, is one of the most important goals in oxidation chemistry. Direct oxidation of the catalyst by molecular oxygen or hydrogen peroxide is often kinetically unfavored. The use of coupled catalytic systems with electron-transfer mediators (ETMs) usually facilitates the procedures by transporting the electrons from the catalyst to the oxidant along a low-energy pathway, thereby increasing the efficiency of the oxidation and thus complementing the direct oxidation reactions. As a result of the similarities with biological systems, this can be dubbed a biomimetic approach.     

利用乙醇重整制氢进行硝基苯原位液相加氢合成苯胺

 利用乙醇液相催化重整制得的氢直接进行硝基苯原位液相加氢合成苯胺. 考察了不同催化剂、反应温度及反应时间等因素的影响. 在以Pt/Al2O3为催化剂,反应温度220 ℃和反应时间3 h的条件下,硝基苯的转化率可达99.3%, 苯胺的选择性为99.8%, 催化剂表现出较高的加氢活性和选择性.     

Isosteric enthalpies for hydrogen adsorbed on nanoporous materials at high pressures

A sound understanding of any sorption system requires an accurate determination of the enthalpy of adsorption. This is a fundamental thermodynamic quantity that can be determined from experimental sorption data and its correct calculation is extremely important for heat management in adsorptive gas storage applications. It is especially relevant for hydrogen storage, where porous adsorptive storage is regarded as a competing alternative to more mature storage methods such as liquid hydrogen and compressed gas. Among the most common methods to calculate isosteric enthalpies in the literature are the virial equation and the Clausius–Clapeyron equation. Both methods have drawbacks, for example, the arbitrary number of terms in the virial equation and the assumption of ideal gas behaviour in the Clausius–Clapeyron equation. Although some researchers have calculated isosteric enthalpies of adsorption using excess amounts adsorbed, it is arguably more relevant to applications and may also be more thermodynamically consistent to use absolute amounts adsorbed, since the Gibbs excess is a partition, not a thermodynamic phase. In this paper the isosteric enthalpies of adsorption are calculated using the virial, Clausius–Clapeyron and Clapeyron equations from hydrogen sorption data for two materials—activated carbon AX-21 and metal-organic framework MIL-101. It is shown for these two example materials that the Clausius–Clapeyron equation can only be used at low coverage, since hydrogen’s behaviour deviates from ideal at high pressures. The use of the virial equation for isosteric enthalpies is shown to require care, since it is highly dependent on selecting an appropriate number of parameters. A systematic study on the use of different parameters for the virial was performed and it was shown that, for the AX-21 case, the Clausius–Clapeyron seems to give better approximations to the exact isosteric enthalpies calculated using the Clapeyron equation than the virial equation with 10 variable parameters.     

Use of crude extract of lentil plant (Lens culinaris Medikus) in peroxidase-based analyses: fast kinetic determination of hydrogen peroxide and sarcosine in urine

Peroxidase-catalysed reactions are used in a wide variety of analytical applications, most of them based on the final quantification of hydrogen peroxide. Clinical tests for glucose, cholesterol, creatine, creatinine or uric acid in blood or urine and enzyme-linked immunosorbent assays for pesticides, hepatitis or acquired immune deficiency syndrome are good examples of such applications. The most widely used and commercially available peroxidase for biotechnological processes and analytical applications is horseradish peroxidase followed, although in much lower proportion, by soybean peroxidase. The high commercial interest in peroxidases has led to the search for new sources of these enzymes. This work describes the analytical use of lentil plant peroxidase (LPP), which is a new peroxidase extracted from lentil plants (Lens culinaris Medikus); an abundant post-harvest agricultural waste in the area of Castilla y León (Spain). A procedure for the quantification of hydrogen peroxide in urine is first proposed using crude extract of lentil plant instead of the purified enzyme. This procedure is then applied to the determination of sarcosine; a natural amino acid that has attracted considerable interest in clinical diagnostics since urinary sarcosine was proposed and later questioned as a biomarker for prostate cancer. Under the action of sarcosine oxidase, sarcosine is oxidized by molecular oxygen to give glycine, formaldehyde and hydrogen peroxide that is quantified according to the previously proposed procedure. The limit of detection for both hydrogen peroxide and sarcosine is around 5?×?10(-7)?M. In the determination of sarcosine, the high selectivity of the overall enzymatic reaction, the simple sample treatment and instrumentation, the high-sample throughput and the use of LPP in the plant extract instead of the purified enzyme provide a rapid and inexpensive procedure with characteristics very suitable for routine analysis in a clinical laboratory.     

Determination of anabolic steroids with gas chromatography-ion trap mass spectrometry using hydrogen as carrier gas.

Helium is considered to be the ideal carrier gas for gas chromatography/mass spectrometry (GC/MS) in general, and for use with an ion trap in particular. Helium is an inert gas, can be used without special precautions for security and, moreover, it is needed as a damping gas in the trap. A disadvantage of helium is the high viscosity resulting in long GC run times. In this work hydrogen was tested as an alternative carrier gas for GC in performing GC/MS analyses. A hydrogen generator was used as a safe source of hydrogen gas. It is demonstrated that hydrogen can be used as a carrier gas for the gas chromatograph in combination with helium as make-up gas for the trap. The analysis time was thus shortened and the chromatographic performance was optimized. Although hydrogen has proven useful as a carrier gas in gas chromatography coupled to standard detectors such as ECD or FID, its use is not mentioned extensively in the literature concerning gas chromatography-ion trap mass spectrometry. However, it is worth considering as a possibility because of its chromatographic advantages and its advantageous price when using a hydrogen generator.     

Chemistry of ethanediyl S,S-acetals 6- An example of vicarious nucleophilic substitution of hydrogen in 1,4-benzodithians

1,4-Benzodithians, when treated with bromine in anhydrous chloroform, undergo very fast monobromination at the aromatic ring. By the use of quantum mechanical semiempirical calculations, the reaction is shown to proceed most likely via a vicarious nucleophilic substitution of hydrogen.     

A counter-intuitive approach to calculating non-exchangeable 2H isotopic composition of hair: treating the molar exchange fraction fE as a process-related rather than compound-specific variable

Hair is a keratinous tissue that incorporates hydrogen from material that an animal consumes but it is metabolically inert following synthesis. The stable hydrogen isotope composition of hair has been used in ecological studies to track migrations of mammals as well as for forensic and archaeological purposes to determine the provenance of human remains or the recent geographic life trajectory of living people. Measurement of the total hydrogen isotopic composition of a hair sample yields a composite value comprised of both metabolically informative, non-exchangeable hydrogen and exchangeable hydrogen, with the latter reflecting ambient or sample preparation conditions. Neither of these attributes is directly measurable, and the non-exchangeable hydrogen composition is obtained by estimation using a commonly applied mathematical expression incorporating sample measurements obtained from two distinct equilibration procedures. This commonly used approach treats the fraction of exchangeable hydrogen as a mixing ratio, with a minimal procedural fractionation factor assumed to be close or equal to 1. Instead, we propose to use full molar ratios to derive an expression for the non-exchangeable hydrogen composition explicitly as a function of both the procedural fractionation factor α and the molar hydrogen exchange fraction f(E). We apply these derivations in a longitudinal study of a hair sample and demonstrate that the molar hydrogen exchange fraction f(E) should, like the procedural fractionation factor α, be treated as a process-dependent parameter, i.e. a reaction-specific constant. This is a counter-intuitive notion given that maximum theoretical values for the molar hydrogen exchange fraction f(E) can be calculated that are arguably protein-type specific and, as such, f(E) could be regarded as a compound-specific constant. We also make some additional suggestions for future approaches to determine the non-exchangeable hydrogen composition of hair and the use of standards.     

Influence of Swelling and Disruption of the Starch Granule on the Composition of the Starch-Polyacrylonitrile Copolymer

Abstract

The results of an evaluation of polymers as cryogenic adhesives are presented. It was observed that polyether-based polyurethanes provided the most attractive capability as adhesives for use at liquid hydrogen temperatures.     

Uricase based methods for determination of uric acid in serum

The most common methods for determination of uric acid in serum are based on the use of the enzyme uricase. Uric acid is enzymatically oxidized by oxygen to produce hydrogen peroxide, allantoin, and carbon dioxide. The four most often applied uricase methods are critically compared in this review.     

Titrations of weak acids in 1,1,3,3-tetramethylguanidine

The use of 1,1,3,3-tetramethylguamdine (TMG) as a medium for the titrations of weak acids has been investigated. The hydrogen electrode behaves reversibly in this solvent and can serve as an indicator electrode in titration reactions. The titrant was a o.1 M solution of tetrabutylammonium hydroxide in a 90-10% mixture of TMG and methanol. A hydrogen electrode dipping into a TMG solution saturated with benzoic acid served as reference electrode. Potentiometric titrations of a number of weak acids gave results accurate to at least ±0.5%. It was found that in most cases curcumin could be used as an end-point indicator with an accuracy comparable to that of the potentiometric titration.     

Confined high-pressure chemical deposition of hydrogenated amorphous silicon

Hydrogenated amorphous silicon (a-Si:H) is one of the most technologically important semiconductors. The challenge in producing it from SiH(4) precursor is to overcome a significant kinetic barrier to decomposition at a low enough temperature to allow for hydrogen incorporation into a deposited film. The use of high precursor concentrations is one possible means to increase reaction rates at low enough temperatures, but in conventional reactors such an approach produces large numbers of homogeneously nucleated particles in the gas phase, rather than the desired heterogeneous deposition on a surface. We report that deposition in confined micro-/nanoreactors overcomes this difficulty, allowing for the use of silane concentrations many orders of magnitude higher than conventionally employed while still realizing well-developed films. a-Si:H micro-/nanowires can be deposited in this way in extreme aspect ratio, small-diameter optical fiber capillary templates. The semiconductor materials deposited have ~0.5 atom% hydrogen with passivated dangling bonds and good electronic properties. They should be suitable for a wide range of photonic and electronic applications such as nonlinear optical fibers and solar cells.     

Alkylation of nitroaromatics with tetraalkylborate ion via electrochemical oxidation

Aromatic nucleophilic substitution reaction (S(N)Ar) is one of the most thoroughly studied reactions. Alkylation of nitroaromatics with Grignard reagents via chemical oxidation of the sigma(H)-complexes is the most general method to introduce an alkyl group into a nitroaromatic compound. This approach has considerable drawbacks, especially when more than one nitro group are present in the aromatic ring. In this article, we present an electrochemical approach, which offers a new very selective methodology for obtaining alkyl polynitroaromatic compounds. Different strategies based on the use of tetralkylborate anion as nucleophiles are used so as to increase efficiency and to reduce the drawbacks associated with this reaction. A wide list of dinitro- and trinitro-aromatic compounds are studied, the range of yields obtained being from fair (40%) to excellent (85%). The key to improvement in the process is the use of electrochemical techniques for the oxidation of the mixture sigma(H)-complexes/tetrabutylborate ion. The electroactive character of the nucleophile, which can be oxidized to an alkyl radical, means that the S(N)Ar of the hydrogen polar mechanism is not the only mechanism operating during the electroxidation process, since the hydrogen radical S(N)Ar mechanism is running at the same time. Electrochemical mechanistic studies allow the participation of each mechanism in the global product yield obtained to be quantified.     

Aliphatic peptidyl hydroperoxides as a source of secondary oxidation in hydroxyl radical protein footprinting

Hydroxyl radical footprinting is a technique for studying protein structure and binding that entails oxidizing a protein system of interest with diffusing hydroxyl radicals, and then measuring the amount of oxidation of each amino acid. One important issue in hydroxyl radical footprinting is limiting amino acid oxidation by secondary oxidants to prevent uncontrolled oxidation, which can cause amino acids to appear more solvent accessible than they really are. Previous work suggested that hydrogen peroxide was the major secondary oxidant of concern in hydroxyl radical footprinting experiments; however, even after elimination of all hydrogen peroxide, some secondary oxidation was still detected. Evidence is presented for the formation of peptidyl hydroperoxides as the most abundant product upon oxidation of aliphatic amino acids. Both reverse phase liquid chromatography and catalase treatment were shown to be ineffective at eliminating peptidyl hydroperoxides. The ability of these peptidyl hydroperoxides to directly oxidize methionine is demonstrated, suggesting the value of methionine amide as an in situ protectant. Hydroxyl radical footprinting protocols require the use of an organic sulfide or similar peroxide scavenger in addition to removal of hydrogen peroxide to successfully eradicate all secondary oxidizing species and prevent uncontrolled oxidation of sulfur-containing residues.