Stepwise Nature Research Articles

Destructive effect of accidental explosions of some mixed compositions

This paper deals with issues related to the dependence of the parameters of an air blast wave on the mass of the explosive charge and explosion conditions. It is established that the explosive yield of mixed explosives changes appreciably with variation in the mass of the charge up to the limiting values. After the attainment of the limiting values of the mass, the TNT equivalent ceases to change. The range of charge masses in which the indicated changes are observed depends on the properties of the explosives and is the smaller the less pronounced the stepwise nature of their detonation. The conclusions drawn agree with experimental data.

Cost-optimal design of VoIP networks using the VPN concept

This paper addresses the issue of cost-optimal voice over IP (VoIP) network design. In the applied model, the whole VoIP network is divided into two logical components: the access network and the transport network. The access network consists of VoIP end-points that connect to the transport network through edge routers serving as gateways. Since multiple edge routers may be available for any given VoIP node, one task of the design process is to assign a particular edge router to every VoIP node. The edge routers have to be connected in a way that security and availability can be assured for the VoIP traffic. One obvious approach to fulfilling these requirements, which is assumed throughout the paper, is to define a virtual private network (VPN). Supposing a large volume of VoIP traffic, the cost of the VPN can be significant; thus, the other task of VoIP network design is to specify the transport VPN in the most economical way. These two tasks of VoIP network design can be solved separately using existing methods; nevertheless, the specification of VoIP regions influences the cost of the final solution to a great extent. Therefore, in this paper a novel approach is proposed in which the edge router assignment process takes the objective function of VPN specification into consideration as well. In order to realize the new approach, multiple methods are introduced which are based on the paradigms of genetic algorithms and simulated annealing. These methods perform a sophisticated optimization of the gateway assignments using various cost calculation methods. To evaluate the new algorithms, a method based on a well-known greedy solution to the problem is used as reference. Moreover, a VPN specification algorithm is presented which utilizes the stepwise nature of the cost functions. The performance of the presented methods is evaluated with the help of simulations. It is shown that the proposed methods outperform the reference algorithm significantly in the simulation scenarios investigated.

Modeling Intrinsic Bioremediation for Interpret Observable Biogeochemical Footprints of BTEX Biodegradation: The Need for Fermentation and Abiotic Chemical Processes

AbstractFor Abstract see ChemInform Abstract in Full Text.

Coupling–Isomerization–Enamine Addition–Cyclocondensation Sequences:A Multicomponent Approach to Substituted and Annelated Pyridines

AbstractAnnelated (dihydropyridines, tetrahydroquinolines, naphthyridines) and substituted pyridines can be synthesized in moderate to good yields in a consecutive one‐pot, four‐component process by a coupling–isomerization–enamine addition–cyclocondensation sequence of an electron poor (hetero)aryl halide, a terminal propargyl alcohol, an enamine, and ammonium chloride. After the coupling–isomerization sequence a Diels–Alder reaction with inverse electron demand of the intermediate chalcone and the enamine furnishes a cycloadduct 6 that was unambiguously characterized by X‐ray structure analysis as well as the 1,5‐diketones 4b and 4g — the corresponding hydrolysis products — and the pyridine derivatives 11e (dihydropyridine), 11i (tetrahydroquinoline), 11o (naphthyridine), and 14b and 14c (ethyl nicotinoates). Additionally, quantum chemical calculations support the stepwise nature of the enamine cycloaddition. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Modeling Intrinsic Bioremediation for Interpret Observable Biogeochemical Footprints of BTEX Biodegradation: The Need for Fermentation and Abiotic Chemical Processes

The intrinsic bioremediation of BTEX must be documented by the stoichiometric consumption and production of several other compounds, called 'footprints' of the biodegradation reaction. Although footprints of BTEX biodegradation are easy to identify from reaction stoichiometry, they can be confounded by the stepwise nature of the biodegradation reactions and by several abiotic chemical reactions that also produce or consume the footprints. In order to track the footprints for BTEX biodegradation, the following reactions need to be considered explicitly: (1) fermentation and methanogenesis as separate processes, (2) precipitation and dissolution of calcite, (3) precipitation and dissolution of amorphous iron monosulfide (FeS), (4) conversion of FeS into the thermodynamically stable pyrite (FeS2) with loss of sulfide and abiotic formation of H2, and (5) reductive dissolution of solid iron(III) by oxidation of sulfide. We critically review the research that underlies why these mechanisms must be included and how to describe them quantitatively. A companion manuscript develops and applies a mathematical model that includes these reactions.

How Cytochrome c Folds, and Why: Submolecular Foldon Units and their Stepwise Sequential Stabilization

Native state hydrogen exchange experiments have shown that the cytochrome c (Cyt c) protein consists of five cooperative folding-unfolding units, called foldons. These are named, in the order of increasing unfolding free energy, the nested-Yellow, Red, Yellow, Green, and Blue foldons. Previous results suggest that these units unfold in a stepwise sequential way so that each higher energy partially unfolded form includes all of the previously unfolded lower free energy units. If this is so, then selectively destabilizing any given foldon should equally destabilize each subsequent unfolding step above it in the unfolding ladder but leave the lower ones before it unaffected. To perform this test, we introduced the mutation Glu62Gly, which deletes a salt link in the Yellow unit and destabilizes the protein by 0.8 kcal/mol. Native state hydrogen exchange and other experiments show that the stability of the Yellow unit and the states above it in the free energy ladder are destabilized by about the same amount while the lower lying states are unaffected. These results help to confirm the sequential stepwise nature of the Cyt c unfolding pathway and therefore a similar refolding pathway. The steps in the pathway are dictated by the concerted folding-unfolding property of the individual unit foldons; the order of steps is determined by the sequential stabilization of progressively added foldons in the native context. Much related information for Cyt c strongly conforms with this mechanism. Its generality is supported by available information for other proteins.

Thermal chemistry of diiodomethane on Ni(1 1 0) surfaces I. Clean and hydrogen-covered

The thermal chemistry of diiodomethane on Ni(1 1 0) single-crystal surfaces was studied by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Diiodomethane was chosen as a precursor for the formation of methylene surface species. I 3d and C 1s XPS data indicated that, indeed, adsorbed diiodomethane undergoes the C–I bond dissociations needed for that transformation, and detection of iodomethane production in TPD experiments pointed to the stepwise nature of those reactions. Significant amounts of methane are produced from further thermal activation of the chemisorbed methylene groups. This involves surface hydrogen, both coadsorbed from background gases and produced by dehydrogenation of some of the adsorbed diiodomethane, and can be induced at temperatures as low as about 160 K, right after the C–I bond breaking steps. Unique to this system is the detection of significant amounts, up to 10% of the total CH 2I 2 adsorbed, of heavier hydrocarbons, including ethene, ethane, propene, propane, and butene. Deuterium labeling experiments were used to provide support for a mechanism where the initial hydrogenation of some adsorbed methylene to methyl moieties is followed by a rate-limiting methylene insertion step to yield ethyl intermediates. Facile subsequent β-hydride elimination and reductive elimination with coadsorbed hydrogen account for the formation of ethene and ethane, respectively, while a second and third methylene insertions lead to C 3 and C 4 production. Based on the final product distribution, the methylene insertion was estimated to be approximately 20 times slower than the following hydrogenation–dehydrogenation reactions. Normal kinetic isotope effects were observed for most of the hydrogenation and dehydrogenation reactions involved.

Crystal Structure of S-ovalbumin as a Non-loop-inserted Thermostabilized Serpin Form

Ovalbumin, a non-inhibitory member of serine proteinase inhibitors (serpin), is transformed into a heat-stabilized form, S-ovalbumin, under elevated pH conditions. The structural mechanism for the S-ovalbumin formation has long been a puzzling question in food science and serpin structural biology. On the basis of the commonly observed serpin thermostabilization by insertion of the reactive center loop into the proximal beta-sheet, the most widely accepted hypothetical model has included partial loop insertion. Here we demonstrate, for the first time, the crystal structure of S-ovalbumin at 1.9-A resolution. This structure unequivocally excludes the partial loop insertion mechanism; the overall structure, including the reactive center loop structure, is almost the same as that of native ovalbumin, except for the significant motion of the preceding loop of strand 1A away from strand 2A. The most striking finding is that Ser-164, Ser-236, and Ser-320 take the d-amino acid residue configuration. These chemical inversions can be directly related to the irreversible and stepwise nature of the transformation from native ovalbumin to S-ovalbumin. As conformational changes of the side chains, significant alternations are found in the values of the chi 1 of Phe-99 and the chi 3 of Met-241. The former conformational change leads to the decreased solvent accessibility of the hydrophobic core around Phe-99, which includes Phe-180 and Phe-378, the highly conserved residues in serpin. This may give a thermodynamic advantage to the structural stability of S-ovalbumin.

The Field-Assisted Stepwise Dissociation of Acetone in an Intense Femtosecond Laser Field

The dissociation of acetone in an intense (1013−1014 W/cm2) femtosecond laser field has been investigated. The stepwise nature of the dissociation has been verified by analyzing the time-of-flight mass spectroscopic patterns at different laser intensities. The stepwise nature was interpreted using a quasi-diatomic field-assisted dissociation model and the QCT calculations. Both the experimental identification and the theoretical predictions show that if the laser intensity is gradually increased, the sequence of the primary dissociation is C−C, C−O, and C−H bonds in the acetone ion. The preference of breaking the C−C and C−O bond over the H-elimination of the C−H bond is also the case in the secondary dissociation.

Frogs and a Cloud‐Forest Edge in Ecuador

Abstract: Edge effects on tropical faunas may differ between highland and lowland sites and may therefore require different conservation strategies. In northwestern Ecuador, we examined the frog fauna on both sides of a boundary between wet Andean forest and agricultural lands, asking the following questions: ( 1 ) What is the nature of the response of frogs to edge? ( 2 ) Does the response change with land use? ( 3 ) What conservation measures are suggested by the first two questions? We set up 12 study plots along 16 km of convoluted boundary in one watershed. Each plot contained an agricultural parcel bordering a rectangle extending into the forest. In the dry season of 1996, following local tradition each agricultural parcel was cleared of second‐growth vegetation and then burned, planted, cultivated, and ( in six cases ) harvested 1 year later. Half the parcels were planted in pasture grasses; the others were planted with corn and beans. We randomly assigned each treatment to two sites within each of three blocks. At each site, parallel to the boundary we set up eight all transects, four on the forest side of the boundary and four on the agricultural side. We censused frogs in plots with visual‐encounter surveys along all transects in four different seasons from just after planting to just before crop harvest. Regardless of land use or survey date, frog‐capture frequencies changed abruptly at the forest‐agriculture boundary and then remained constant regardless of distance into the forest or into the cultivated parcels. Forest transects provided 95% of the 2111 captures; all but 5 captures were of Eleutherodactylus spp. ( Leptodactylidae). Eleutherodactylus leoni dominated the forest frog fauna but was rare throughout agricultural parcels. The second most common species, E. w‐nigrum, displayed the opposite pattern. The stepwise nature of these responses differs from results of studies in tropical lowlands and suggests that edge effects that might penetrate from highland agriculture into cloud forest may be of far less conservation concern than the absolute loss of habitat as the agricultural boundary creeps upslope. This suggestion should be viewed with caution because the scope of our study included only one watershed, two idiosyncratic land‐use treatments, and a unique vertebrate taxon.

ERGOtool for the integral improvement of ergonomics and process flow in assembly

Manufacturers of manually assembled products are increasingly being forced to improve the flow of assembly orders, together with having more efficient employment, in order to meet increasing demands on short delivery times, high product variety, good quality and low manufacturing costs. At the same time, the human factor is increasingly important--a well-motivated workforce is a necessity these days. To improve assembly processes, layout and workstation design, a participatory approach was developed by TNO and applied in various assembly companies. The goals of this approach are the involvement of assembly staff, lead time reduction and an improvement of the assembly tasks in an ergonomic sense. The main elements of the approach are the active participation of the company, the integration of two basic disciplines: assembly engineering and ergonomics, and its stepwise nature. In March 2000, the European project PSIM (Participative Simulation environment for Integral Manufacturing enterprise renewal) was launched. The goal of the project, in which the employees play a central role, is to enable the ongoing and integral improvement of company assembly processes using advanced simulation software. To support this participatory and integral approach to improve production lines, a software tool, the ERGOtool, is developed within the PSIM project. ERGOtool is a software tool that helps in the description and evaluation of current and future assembly processes. It takes factors such as physical load, ergonomic hazards, mental load, and process flow characteristics into consideration.

Information theoretic clustering

Clustering is an important topic in pattern recognition. Since only the structure of the data dictates the grouping (unsupervised learning), information theory is an obvious criteria to establish the clustering rule. The paper describes a novel valley seeking clustering algorithm using an information theoretic measure to estimate the cost of partitioning the data set. The information theoretic criteria developed here evolved from a Renyi entropy estimator (A. Renyi, 1960) that was proposed recently and has been successfully applied to other machine learning applications (J.C. Principe et al., 2000). An improved version of the k-change algorithm is used in optimization because of the stepwise nature of the cost function and existence of local minima. Even when applied to nonlinearly separable data, the new algorithm performs well, and was able to find nonlinear boundaries between clusters. The algorithm is also applied to the segmentation of magnetic resonance imaging data (MRI) with very promising results.

Synthesis, structure, and properties of azatriangulenlenium salts.

A general synthetic route to novel nitrogen-bridged heterocyclic carbenium ions of the acridinium and triangulenium type has been developed and investigated. The synthetic method is based on nucleophilic aromatic substitution (SNAr) on the tris(2,6-dimethoxyphenyl)carbenium ion (1) with primary amines and, by virtue of its stepwise and irreversible nature, provides a powerful tool for the preparation of a wide variety of new heterocyclic carbenium salts. Several derivatives of the three new oxygen- and/or nitrogen-bridged triangulenium salts, azadioxa- (6), diazaoxa- (7), and triazatriangulenium (4), have been synthesized and their physicochemical properties have been investigated. Crystal structures for compounds 2 b-PF6, 2 d-PF6, 4b-BF4, 4c-BF4, 6e-BF4, and 8 are reported. The different packing modes found for the triazatriagulenium salts are discussed in relation to the electrostatic and space-filling requirements of the ions. The stabilities of the cations 6a, 7b, and 4a, as expressed by their pKR+ values, have been determined in strongly basic nonaqueous solution by use of the C_ acidity function; the values obtained were 14.5, 19.4, and 23.7, respectively. This study further implied that the C_ scale in its present form is unsuitable for the precise determination of pKR+ values beyond 22.

Some new features of interface roughening dynamics in paper-wetting experiments

Some new observations concerning the spatial-temporal dynamics of selfaffine interfaces formed in paper-wetting experiments are reported. We find that the motion of the wet front in a porous medium has a stepwise nature. The height of the wetted area, as a function of time, displays a Devil's-staircase-like behaviour with scaling exponent delta, whereas the front width oscillates erratically with time. These erratic oscillations possess a statistical self-affine invariance in time with the scaling exponent chi, which is found to be equal to the growth exponent beta. We also note that the values of chi, beta and delta, as well as the interface roughness (Hurst) exponent H vary from one experiment to another in wide ranges, and that their distributions obey a normal distribution. The mean values of all the exponents are dependent on the paper structure, but not on the environmental conditions. Furthermore, we find that the mean value of beta depends on the interface front orientation with respect to the fibre direction in the paper, whereas the mean values of H and delta do not depend on it.

A Participatory and Integrative Approach to Increase Productivity and Comfort in Assembly

Manufacturers of manually assembled products are more and more forced to improve the flow of assembly orders together with a more efficient employment, in order to meet increasing demands on short delivery times, high product variety, good quality and low manufacturing costs. At the same time, the human factor is increasingly important, for a well-motivated work force is needed more than ever. To improve assembly processes, an approach was developed and applied in various assembly companies. Goals of this approach are both a lead time reduction and an improvement of the assembly tasks in an ergonomic sense. Main elements of the approach are the active participation of the companies, the integration of two basic disciplines: assembly engineering and ergonomics, and its stepwise nature. The initial steps of the approach include the analysis of the assembly process by setting up an ‘assembly process scheme’, and the analysis of bottlenecks with regard to the flow of orders and ergonomics. Latter steps include the selection of improving measures, the implementation of these measures and the evaluation of the effects. This paper describes this approach and its application in two assembly companies: one producing relatively small products (magnetic stop valves) and the other making relatively large products (office furniture). In both companies the approach lead to a new design of the assembly process, a new factory lay-out including new work places, and another involvement of people in the assembling process.

Origins of the Loss of Concertedness in Pericyclic Reactions: Theoretical Prediction and Direct Observation of Stepwise Mechanisms in [3 + 2] Thermal Cycloadditions

Several [3 + 2] thermal cycloadditions between azomethine ylides and nitroalkenes have been studied both theoretically and experimentally. When the N-metalated 1,3-dipoles are used, the reaction is stepwise. The corresponding zwitterionic intermediates have been located computationally and observed by NMR monitoring. In the case of N-unsubstituted azomethine ylides, the reaction can be concerted or stepwise, depending upon the ability of the substituents to stabilize zwitterionic intermediates. A general model is proposed to explain the observed phenomena. This simple model can be extended to other thermal cycloadditions to predict the stepwise or concerted nature of their mechanisms without computing complete reaction coordinates.

Intramolecular [3 + 2]Cycloaddition Reaction of Dipolar Trimethylenemethane

Abstract A variety of alkylidenecyclopropanone acetals possessing a terminal diylophile were allowed to undergo an intramolecular [3 + 2]cycloaddition via a dipolar trimethylenemethane to obtain bicyclo[4.3.0] and [3.3.0] carbo- and heterocycles in good yields. The product selectivities were rigorously controlled by the concerted or stepwise nature of the cycloaddition reaction.

On the thermal unfolding character of globular proteins.

A theoretical model is presented to study the stepwise thermal unfolding of globular proteins using the stabilizing/destabilizing characters of amino acid residues in protein crystals. A multiple regression relation connecting the melting temperature and the amounts of stabilizing and destabilizing groups of residues in a protein, when used for the thermal behavior of peptide segments, provides reliable results on the stepwise unfolding nature of the protein. In ribonuclease A, the shell residues 16-22 are predicted to unfold earlier in the temperature range 30-45 degrees C; the beta-sheet structures undergo thermal denaturation as a single cooperative unit and there is evidence indicating the segment 106-118 as a nucleation site. In ribonuclease S, the S-peptide unfolds earlier than S-protein. The predicted average and the range of melting temperatures, and the folding pathways of a set of globular proteins, agree very well with the experimental results. The results obtained in the present study indicate that (i) most of the nucleation parts possess high relative thermal stability, (ii) the unfolded state retains some residual structure, and (iii) some segments undergo gradual and overlapping thermal denaturation.

Interfaces between nanostructures and stepwise creep in highly oriented polymers

A comparative laser-interferometric study of steady creep in oriented ultrahigh-molecular-weight polyethylene films differing in the structure of interfaces between nanosized structural units has been carried out to gain a better understanding of the creep mechanism in oriented polymer materials. In contrast to conventional methods, laser interferometry permits measurement of creep rates from very small strain increments (0.3 µm) to within 1%. This technique made it possible to detect the stepwise nature of plastic deformation in creep. The data obtained suggest that the creep rate and its periodic changes are controlled by the structure of the interfaces, and that the plastic deformation itself occurs to a considerable extent through shear of nanosized structural units relative to one another by an “acceleration-deceleration” type. It is proposed that the “deceleration” phase is due to a glide resistance created by some “stoppers” having either physical or chemical nature, which become destroyed and reappear again in the course of creep.

Cases in stress prevention: the success of a participative and stepwise approach

Stress prevention programmes are predominantly reactive and aimed at individuals. Four factors that may contribute to this current status are discussed: the opinions and interests of company management, the nature of psychology, the difficulty of conducting methodologically ‘sound’ intervention studies and the denominational segregation of stress research. To increase the impact of organizational level interventions, the effects need to be demonstrated on matters that appeal to company management, such as quality of products and services, organizational flexibility, productivity and sickness absence rates. The demonstration of examples of good preventive practice is considered as a conditio sine qua non for developing effective stress prevention procedures and for the involvement of both social partners in this field. Therefore, 10 Dutch projects from several branches of industry, aimed at the reduction of work stress, physical workload and sickness absenteeism, were selected, analysed and compared. The results show that in most cases sickness absenteeism was reduced and that the benefits exceeded the costs of the interventions. Five factors seem to be at the heart of a successful approach: (1) its stepwise and systematic nature, (2) an adequate diagnosis or risk analysis, (3) a combination of measures (i.e. both work-directed and person-directed), (4) a participative approach (i.e. worker involvement) and (5) top management support. In conclusion, the projects suggest that stress prevention may be beneficial to both the employee and the organization. © 1998 John Wiley & Sons, Ltd.