Biomimetic synthesis of a H2 catalyst using a protein cage architecture

A biomimetic approach has been used to develop an artificial hydrogenase that catalyses the efficient reduction of protons producing hydrogen gas. Analogous to the unique biological metal clusters found in hydrogenase enzymes, the engineered active sites are small, well-defined Pt clusters deposited on the interior of a heat shock protein cage architecture with stoichiometries of 150 to 1000 Pt per protein cage. The proton reduction reaction is driven by visible light through a coupled reaction with Ru(bpy)3(2+) and methyl viologen as an electron-transfer mediator. Hydrogen production rates are comparable to those of hydrogenase on a per protein basis and exceed production rates of other reported Pt-based catalysts. These results demonstrate the utility of a biomimetic approach toward addressing the needs of hydrogen production.     

Electrochemical study of reversible hydrogenase reaction of Desulfovibrio vulgaris cells with methyl viologen as an electron carrier.

An electrode modified with immobilized whole cells of Desulfovibrio vulgaris (Hildenborough) produces an S-shaped voltammogram with both cathodic- and anodic-catalytic-limiting currents in a methyl viologen-containing buffer saturated with H2. Methyl viologen penetrates into the bacterial cells to serve as an electron carrier in the reversible reaction of hydrogenase in the cells and functions as an electron-transfer mediator between the bacterial cells and the electrode, thus producing the catalytic currents for the evolution and consumption of H2. An equation for the catalytic current that takes into account the reversible hydrogenase reaction explains well the shape of the voltammogram. The potential at null current on the voltammogram agrees with the potential determined by potentiometry with the same electrode, which is equal to the redox potential of the H+/H2 couple in the solution--the standard potential of a hydrogen electrode at the pH of the solution. When D. vulgaris cells are suspended in an argon-saturated buffer containing methyl viologen, the suspension produces a catalytic current at a bare glassy carbon electrode for the evolution of H2. Analysis of the current by a theory for a catalytic current for a unidirectional nonlinear enzyme catalysis allows us to determine the kinetic parameters of the reaction between methyl viologen and hydrogenase in intact D. vulgaris cells. Thus we obtain the apparent Michaelis constant for methyl viologen cation radical, K'MV.+ = 0.16 mM, and the apparent catalytic constant (that is, the turnover number per D. vulgaris cell), zkcat,H+ = 1.2 x 10(7) s-1, for the H2 evolution reaction at pH 5.5 and at 25 degrees C, z being the number of hydrogenases contained in a D. vulgaris cell. The bimolecular reaction rate constant, kcat,H+/K'MV.+, of the reaction between methyl viologen cation radical and oxidized hydrogenase in intact D. vulgaris cells is estimated as 4.2 x 10(7) M-1 s-1. Similarly, the bimolecular reaction rate constant, kcat,H2/K'MV2+, of the reaction between methyl viologen and reduced hydrogenase is estimated to be 1.2 x 10(7) M-1 s-1 at pH 9.5 and 25 degrees C. Both rate constants are large enough for the reactions to be diffusion-limited processes.     

Covalent Attachment of FeFe Hydrogenases to Carbon Electrodes for Direct Electron Transfer

Direct electron transfer between enzymes and electrodes is now commonly achieved, but obtaining protein films that are very stable may be challenging. This is particularly crucial in the case of hydrogenases, the enzymes that catalyze the biological conversion between dihydrogen and protons, because the instability of the hydrogenase films may prevent the use of these enzymes as electrocatalysts of H(2) oxidation and production in biofuel cells and photoelectrochemical cells. Here we show that two different FeFe hydrogenases (from Chamydomonas reinhardtii and Clostridium acetobutylicum) can be covalently attached to functionalized pyrolytic graphite electrodes using peptidic coupling. In both cases, a surface patch of lysine residues makes it possible to favor an orientation that is efficient for fast, direct electron transfer. High hydrogen-oxidation current densities are maintained for up to one week, the only limitation being the intrinsic stability of the enzyme. We also show that covalent attachment has no effect on the catalytic properties of the enzyme, which means that this strategy can also used be for electrochemical studies of the catalytic mechanism.     

Wiring-up hydrogenase with single-walled carbon nanotubes

Many envision a future where hydrogen is the centerpiece of a sustainable, carbon-free energy supply. For example, the energy in sunlight may be stored by splitting water into H2 and O2 using inorganic semiconductors and photoelectrochemical approaches or with artificial photosynthetic systems that seek to mimic the light absorption, energy transfer, electron transfer, and redox catalysis that occurs in green plants. Unfortunately, large scale deployment of artificial water-splitting technologies may be impeded by the need for the large amounts of precious metals required to catalyze the multielectron water-splitting reactions. Nature provides a variety of microbes that can activate the dihydrogen bond through the catalytic activity of [NiFe] and [FeFe] hydrogenases, and photobiological approaches to water splitting have been advanced. One may also consider a biohybrid approach; however, it is difficult to interface these sensitive, metalloenzymes to other materials and systems. Here we show that surfactant-suspended carbon single-walled nanotubes (SWNTs) spontaneously self-assemble with [FeFe] hydrogenases in solution to form catalytically active biohybrids. Photoluminescence excitation and Raman spectroscopy studies show that SWNTs act as molecular wires to make electrical contact to the biocatalytic region of hydrogenase. Hydrogenase mediates electron injection into nanotubes having appropriately positioned lowest occupied molecular orbital levels when the H2 partial pressure is varied. The hydrogenase is strongly attached to the SWNTs, so mass transport effects are eliminated and the absolute potential of the electronic levels of the nanotubes can be unambiguously measured. Our findings reveal new nanotube physics and represent the first example of "wiring-up" an hydrogenase with another nanoscale material. This latter advance offers a nonprecious metal route to the design of new biohybrid architectures and building blocks for hydrogen-related technologies.     

Photochemical production of hydrogen from water using microporous porphyrin coordination lattices

In this study, we investigated the photochemical production of hydrogen from water using bio-inspired heterogeneous microporous porphyrin coordination lattices (PCLs), [Ru2(MTCPP)BF4] (M = H2 (PCL-1), Zn (PCL-2); TCPP = Tetrakis(4-carboxyphenyl)porphyrin), under visible (380 nm <) and UV (320 nm <) light irradiations. In the presence of Na2EDTA (as a sacrificial donor) and MV2+ (methyl-vilologen; as a electron relay), PCLs exhibits photocatalytic activity for hydrogen evolution; the maximum amounts of turnover numbers (TONs) of PCL-1 and PCL-2 at 24 h irradiation were 20.8 and 29.9, respectively. In the catalytic reactions, the relation between PCLs and MV2+ was similar to the relation between a [cytochrome c3 hydrogenase] pair and lysine residues in enzymatic reactions. By using the hydrogen production rate and the MV+ (methyl-vilologen radical-cation) concentration, kinetic parameters such as affinities between MV+ and PCLs, maximum reaction rate, and total efficiency of the reaction are introduced using the Michaelis-Menten equation. These parameters indicated that PCLs are good artificial enzyme model catalysts. The stability of the PCLs after the catalytic reactions was confirmed by X-ray photoelectron spectroscopy and Fourier transform-infrared spectra. These results indicated that the frameworks of PCLs are stable for this catalytic reaction.     

Electrochemical hydrogen evolution by use of a glass carbon electrode sandwiched with clay,poly(butylviologen) and hydrogenase

Electrochemical hydrogen evolution was investigated by use of a hydrogenase-modified electrode, which was constructed by immobilizing hydrogenase between two layers of montmorillonite clay and poly(butylviologen) mixture. The optimum conditions for hydrogen evolution were found to be in a slightly acid buffer solution (pH 4–6) with the applied biases corresponding to the reduction potentials of viologens (−700 to −800 mV vs. Ag/AgCl) and a moderate temperature.     

Potentiometric and voltammetric investigations of H2/H+ catalysis by periplasmic hydrogenase from Desulfovibrio gigas immobilized at the electrode surface in an amphiphilic bilayer assembly.

Interactions of an enzyme with an organized amphipilic bilayer are explored as a general means of enzyme immobilization in electroenzymatic systems. Immobilization of Desulfovibrio gigas hydrogenase at the electrode surface involves hydrophobic interactions of the enzyme with the bilayer assembly consisting of octadecyltrichlorosilane and octadecylviologen (C18MV2+) molecules. Due to a hydrophobic character of the enzyme, these interactions direct the enzyme to occupy a central position in the bilayer's hydrocarbon region and lead to immobilization of 3 pmol/cm2 of the enzyme in the plane of the bilayer. This corresponds to 50% surface coverage. The immobilized enzyme catalyzes H2 oxidation mediated by the C18MW2+/.+ couple. This electroenzymatic scheme functions under steady-state voltammetric as well as potentiometric conditions in the pH range 3.5-10. Coupling of enzymatic activity to the electrode surface is accomplished via lateral diffusion of the octadecylviologen molecules along the bilayer assembly.     

立式弹性支承滑动轴承系统的上稳定区理论

研究了一种滑动轴承的油膜稳定性理论——上稳定区理论。首先建立了立式弹性支承滑动轴承系统的力学模型和运动微分方程；通过对该模型的油膜稳定性分析，确定了两个稳定性界限，并由此划分了稳定区。研究结果证实：弹性支承滑动轴承系统的油膜失稳区具有上限，即在通常的失稳区上方存在一个新的稳定区，本文称它为上稳定区；此外，在上稳定区的上方还存在有第二失稳区。本文还讨论了通过选择不同的支承参数和轴承参数来扩大上稳定区的方法。     

The energy characteristics and structure of carbon nanoscrolls

The structure and energy characteristics of carbon nanoscrolls formed from a rectangular graphene ribbon have been studied using semianalytical and numerical calculations. Geometric parameters of the initial graphene ribbon are determined for which stable and energetically favorable nanoscrolls can be obtained. Energy barriers for rolling the graphene ribbon up into a nanoscroll and unrolling it back, as well as the nanoscroll lifetime in a stable state have been estimated. It is established that nanoscrolls with layer overlap lengths of about 1 nm are stable at room temperature.     

Hydrogen sensing by enzyme-catalyzed electrochemical detection

Hydrogen (H2) is a possible future alternative to current fossil-based transportation fuels; however, its lower explosive limit in air requires a reliable sensor to detect leaks wherever H2 is produced, stored, or used. Most current H2 sensors employ palladium or its alloy as the sensing element, featuring high operating temperature and limited selectivity. In this study, we report using soluble hydrogenase (SH) of aerobic beta-proteobacterium Ralstonia eutropha strain H16 to accomplish ambient, electrochemical detection of H2. Gas samples were collected in a solution containing SH that catalyzed the oxidation of H2. The electrons released during the H2 oxidation reaction were accepted by benzyl viologen (BV2+). The product of the redox reaction, BV+, was then detected using chronoamperometry. Using this sensing scheme, we demonstrate detection of H2 ranging from 1 to 100%. In addition, enzyme kinetics and the effect of oxygen on signal response were studied. Our results indicate that it is feasible to develop a sensor to detect H2 in the atmosphere that is based on enzyme-catalyzed electrochemical detection.     

Stable supervisory control of flexible manufacturing systems with fixed supply and demand rates

In this article, we develop a method for computing a stable supervisory control for a class of manufacturing systems (consisting of reconfigurable processors and buffers with limited capacities) with fixed supply and demand rates. It is shown that, with this method, we are able to (i) decide constructively whether a stable supervisory control exists for a given manufacturing system, and (ii) compute the stable supervisory control if it exists. Furthermore, the stable supervisory control thus obtained is complete in the sense that it contains all 'safe' sequences (of possibly concurrent buffer and processor activities) which guarantee the stability of the system. An example is presented to illustrate the proposed method. Implications and computational complexity associated with this method are discussed.     

On crack path stability in a finite body

Theoretical and numerical investigations are made for crack path stability in a finite brittle solid under a predominantly Mode I loading condition. Based upon a first order perturbation solution, an“intermediate” range of stability (as compared with the “local” and “global” ranges) is introduced for the crack growth path. This theory is an extension of the stability concept proposed by Cotterell and Rice, but includes the effect of the change of the stability with increasing crack length along the curved trajectory. Four kinds of crack paths are identified: (i) stable, (ii) initially unstable but intermediately stable, (iii) initially stable but intermediately unstable, and (iv) unstable ones. The first two conditions may lead to stable (i.e. straight) crack growth, while the remaining conditions may lead to an unstable (i.e. sharply curved) crack path. Crack path stability is examined for a biaxially stressed Griffith crack. Then numerical results are given for compact tension and double-cantilever beam specimens for various initial crack lengths. In the numerical examples of compact tension specimens, crack paths are sometimes predicted as unstable by the Cotterell-Rice theory, while stable crack paths are always obtained by using the present theory. On the other hand. unstable crack paths are predicted for double-cantilever beam specimens, except for the eases with extremely long initial cracks. The numerical estimates of crack path stability presented here are in good agreement with experimental observations.     

Coherent structures near the boundary between excitable and oscillatory media

We investigate reaction–diffusion systems near parameter values that mark the transition from an excitable to an oscillatory medium. We analyse the existence and stability of travelling waves near a steep pulse that arises as the limit of excitation pulses when parameters cross into the oscillatory regime. Travelling waves near this limiting profile are obtained by analysing a codimension-two homoclinic saddle-node/orbit-flip bifurcation. The main result shows that there are precisely two generic scenarios for such a transition, distinguished by the sign of an interaction coefficient between pulses. In both scenarios, we find stable fast fronts, unstable slow fronts, stable excitation pulses, and trigger and phase waves. Both trigger and phase waves are stable for repulsive interaction and both are unstable for attractive interaction.     

Stable time step estimates for mesh‐free particle methods

Real‐time computational biomechanics for medicine usually uses explicit time integration, because of its efficiency and suitability for parallel implementation. Explicit time integration is only conditionally stable and therefore requires an estimation of the maximum stable time step that can be used. In this paper we develop a method of estimating the stable time step for mesh‐free particle methods for a specific case of mass lumping: the mass associated with an integration point is distributed equally to all nodes found in the support domain of that integration point. Two estimates of the stable time step for each integration point are developed: one that is very accurate but more costly to compute and one less accurate but easier to implement. The results are also valid for the FEM and beyond computational biomechanics for medicine. Copyright © 2012 John Wiley & Sons, Ltd.     

压杆稳定可靠性灵敏度设计

在稳定可靠性设计理论与灵敏度分析方法的基础上,研究了压杆稳定可靠性灵敏度设计问题。提出了稳定可靠性灵敏度设计的计算方法,给出了压杆可靠性灵敏度的变化规律,研究了设计参数的改变对压杆稳定可靠性的影响,为压杆稳定可靠性设计提供了理论依据。     

Electronic and atomic shell structure in aluminium nanowires

We report experiments on aluminium nanowires in ultra-high vacuum at room temperature that reveal a periodic spectrum of exceptionally stable structures. Two 'magic' series of stable structures are observed: at low conductance, the formation of stable nanowires is governed by electronic shell effects whereas for larger contacts atomic packing dominates. The crossover between the two regimes is found to be smooth. A detailed comparison of the experimental results to a theoretical stability analysis indicates that, while the main features of the observed electron-shell structure are similar to those of alkali and noble metals, a sequence of extremely stable wires plays a unique role in aluminium. This series appears isolated in conductance histograms and can be attributed to 'superdeformed' non-axisymmetric nanowires.     

Analysis of retailers’ coalition stability for supply chain based on LCS and stable set

To attain the general form of stable coalition structure, this paper addressed the problem of retailers’ coalition stability in a two-stage supply chain consisting of one supplier and multiple retailers. A profit gain function was established via introducing market gain coefficient and coalition cost coefficient for different coalition structures. Based on the function, the profit of each retailer in all kinds of coalition structures was analysed, and the general feature of a stable coalition structure was attained by the largest consistent set method and the stable set method. Furthermore, some insights were obtained. For example, stable coalition structures are equidistributed or approximate equidistributed; with supplier’s cost increasing, the size of the retailers’ coalition increases. Finally, the above conclusions are verified by numerical simulation. The results of this paper provide a reference for retailers’ coalition in a supply chain, such as automobile or Information Technology supply chain.     

带外挂后掠机翼极限环颤振的分析与实验研究

从理论及实验两方面，研究了带外挂后掠机翼的极限环颤振分叉现象，得到了稳定、半稳定及不稳定极限环。理论分析采用谐波平衡法，并通过在时域拟合非定常空气动力后，用数值积分法对所得结果进行了比较。风洞实验结果验证了理论分析结果的正确性。     

A stability analysis of the uniaxial viscoplasticity theory based on overstress

In the development of a viscoplasticity theory without a yield surface and without a loading and unloading condition, the properties of a uniaxial constitutive model consisting of two coupled nonlinear differential equations are examined. The critical points of the system of differential equations are evaluated for monotonic loading, creep and relaxation conditions and are shown to be stable. The conditions necessary for the elimination of stable but oscillatory solutions are given. Also given are the asymptotic solutions valid near the critical points. The analytical predictions are confirmed by numerical results.     

ATP‐Stabilized Amorphous Calcium Carbonate Nanospheres and Their Application in Protein Adsorption

Calcium carbonate is a common substance found in rocks worldwide, and is the main biomineral formed in shells of marine organisms and snails, pearls and eggshells. Amorphous calcium carbonate (ACC) is the least stable polymorph of calcium carbonate, which is so unstable under normal conditions that it is difficult to be prepared in vitro because it rapidly crystallizes to form one of the more stable polymorphs in aqueous solution. Herein, we report the successful synthesis of highly stable ACC nanospheres in vitro using adenosine 5′‐triphosphate disodium salt (ATP) as a stabilizer. The effect of ATP on the stability of ACC nanospheres is investigated. Our experiments show that ATP plays an unique role in the stabilization of ACC nanospheres in aqueous solution. Moreover, the as‐prepared ACC nanospheres are highly stable in phosphate buffered saline for a relatively long period of time (12 days) even under relatively high concentrations of calcium and phosphate ions. The cytotoxicity tests show that the as‐prepared highly stable ACC nanospheres have excellent biocompatibility. The highly stable ACC nanospheres have high protein adsorption capacity, implying that they are promising for applications in biomedical fields such as drug delivery and protein adsorption.