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Paths
Kinds of Paths Selected AbstractsPATHS IN TRANSNATIONAL TIME-SPACE: REPRESENTING MOBILITY BIOGRAPHIES OF YOUNG SWEDESGEOGRAFISKA ANNALER SERIES B: HUMAN GEOGRAPHY, Issue 1 2008Lotta Frändberg ABSTRACT. This article sets out to capture and describe individual transnational mobility from a long-term, biographical perspective. The purpose is to discuss the use of a time-geographical form of notation to represent people's transnational mobility as paths in time and space, and to demonstrate how such representations can contribute to explaining some of the dynamics of longdistance mobility. An advantage of using time-space paths is that several aspects of an individual's travel biography can be represented in a single image: intensity and extensity are immediately evident, and the temporal and spatial relationships between the various mobility actions are made visible. Using data describing all transnational trips taken during childhood and adolescence by sixty-two Swedish youth with different backgrounds, three aspects of how trajectories develop over time are discussed in more detail. The first concerns overall change in travel pattern with time. A dominant pattern of increase in travel with increasing age is observed, indicating the importance of further investigating how travel behaviour is related to experience and life-course transitions. Second, sequential relationships between migration and temporary mobility are examined. In spite of the relatively small number of respondents, a wide range of such relationships are disclosed in the material. Third, regularity and repetition in long-distance travel patterns is discussed as an increasingly important aspect of contemporary transnational mobility. Among these young people, highly regular travel is often motivated by enduring long-distance social relationships, but is also generated by leisure or holiday travel alone. [source] Determination of Gradient and Curvature Constrained Optimal PathsCOMPUTER-AIDED CIVIL AND INFRASTRUCTURE ENGINEERING, Issue 1 2006Michael J. De Smith Initially, we examine the case of a single (global) gradient constraint and a planar surface, with or without boundaries and obstacles. This leads to a consideration of surface representation using rectangular lattices and procedures for determining shortest gradient-constrained paths across such surfaces. Gradient-constrained distance transforms are introduced as a new procedure to enable such optimal paths to be computed, and examples are provided for a range of landform profiles and gradients. Horizontal and vertical curvature constraints are then analyzed and incorporated into final solution paths at subsequent stages of the optimization process. Such paths may then be used as preanalyzed input to detailed cost and engineering models to speed up, and where possible improve, the quality and cost-effectiveness of route selection. [source] Toward a Reconceptualization of Regional Development Paths: Is Leipzig's Media Cluster a Continuation of or a Rupture with the Past?ECONOMIC GEOGRAPHY, Issue 3 2003Harald Bathelt Abstract: This article develops a model of regional development that is then used to examine the evolution of two media industries in Leipzig, Germany. We note that the city's current media cluster, centered on television/film production and interactive digital media, shares little in common with the city's once-premier book publishing media cluster. Treating interactive learning as the primary causal mechanism that drives economic growth and change, our conceptual framework incorporates both sectoral/technological and political crises as mechanisms that rupture regional development paths. These regional development paths are not homogeneous, but instead consist of bundles of various technological trajectories. Regions recover from crises as their actors continually rebundle local assets until they find a combination that generates growth. As a result of these crises, new opportunities for growth may arise for new and previously marginal industries. In turn, these expanding industries shape the region's development path. [source] Paths forward between prohibition and laissez-faireADDICTION, Issue 6 2003ROBIN ROOM No abstract is available for this article. [source] Alternative Mechanistic Paths in the Hetero-Diels,Alder Reaction of ,-Oxothiones: A Theoretical StudyEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 21 2007Laura Legnani Abstract DFT calculations at the B3LYP/6-311+G(d,p) level for the C, H, and O atoms and at the 6-311+G(2df,p) level for the S atom were used to study the hetero-Diels,Alder reactions between several ,-oxothiones and ethylene or methyl vinyl ether (MVE). All the transition states and the intermediates along the reaction pathways, as well as the reaction products, were located. The reactions with ethylene are all concerted though asynchronous whereas in the reactions with MVE the electron-releasing character of the methoxy substituent lowers the energy barriers and enhances the asynchronicity and the charge transfer process to such an extent that the reaction may become unconcerted and exhibit a two-step mechanism with a zwitterionic intermediate derived from nucleophilic attack of electron-rich MVE to the sulfur atom of the strongly electrophilically activated ,-oxothiones. The reactions are also favored by the conjugation of the newly formed C=C bond. Moreover, the geometric features of the diene exert a nonnegligible role, as dienes that are planar or almost planar in their ground state show a lower energy barrier. Thus, both geometric and electronic features of the dienes as well as of the dienophiles play a significant role in the easiness of the reactions and in their mechanism. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Moderate Bioclogging Leading to Preferential Flow Paths in BiobarriersGROUND WATER MONITORING & REMEDIATION, Issue 3 2006Katsutoshi Seki Permeable reactive barriers (PRBs) are an alternative technique for the biological in situ remediation of ground water contaminants. Nutrient supply via injection well galleries is supposed to support a high microbial activity in these barriers but can ultimately lead to changes in the hydraulic conductivity of the biobarrier due to the accumulation of biomass in the aquifer. This effect, called bioclogging, would limit the remediation efficiency of the biobarrier. To evaluate the effects bioclogging can have on the flow field of a PRB, flow cell experiments were carried out in the laboratory using glass beads as a porous medium. Two types of flow cells were used: a 20- × 1- × 1-cm cell simulating a single injection well in a one-dimensional flow field and a 20- × 10- × 1-cm cell simulating an injection well gallery in a two-dimensional flow field. A mineral medium was injected to promote microbial growth. Results of 9 d of continuous operation showed that conditions, which led to a moderate (50%) reduction of the hydraulic conductivity of the one-dimensional cell, led to a preferential flow pattern within the simulated barrier in the two-dimensional flow field (visualized by a tracer dye). The bioclogging leading to this preferential flow pattern did not change the hydraulic conductivity of the biobarrier as a whole but resulted in a reduced residence time of water within barrier. The biomass distribution measured after 9 d was consistent with the observed clogging effects showing step spatial gradients between clogged and unclogged regions. [source] Reducing cell loss in banyan based ATM switching fabricsINTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 1 2001M. Al-Mouhamed Abstract In this paper, we propose a new technique for reducing cell loss in multi-banyan-based ATM switching fabrics. We propose a switch architecture that uses incremental path reservation based on previously established connections. Path reservation is carried out sequentially within each banyan but multiple banyan planes can be concurrently reserved. We use a conflict resolution approach according to which banyans make concurrent reservation offers of conflict-free paths to head of the line cells waiting in input buffers. A reservation offer from a given banyan is allocated to the cell whose source-to-destination path uses the largest number of partially allocated switching elements which are shared with previously reserved paths. Paths are incrementally clustered within each banyan. This approach leaves the largest number of free switching elements for subsequent reservations which has the effect of reducing the potential of future conflicts and improves throughput. We present a pipelined switch architecture based on the above concept of path-clustering which we call path-clustering banyan switching fabric (PCBSF). An efficient hardware that implements PCBSF is presented together with its theoretical basis. The performance and robustness of PCBSF are evaluated under simulated uniform traffic and ATM traffic. We also compare the cell loss rate of PCBSF to that of other pipelined banyan switches by varying the switch size, input buffer size, and traffic pattern. Copyright © 2001 John Wiley & Sons, Ltd. [source] Policy Paths and Governance BlueprintsINTERNATIONAL STUDIES PERSPECTIVES, Issue 2 2002Kurt Burch No abstract is available for this article. [source] Some Paths Toward Making Praxis Scores More UsefulJOURNAL OF EDUCATIONAL MEASUREMENT, Issue 2 2000Howard Wainer In this study we describe an analytic method for aiding in the generation of subscales that characterize the deep structure of tests. In addition we also derive a procedure for estimating scores for these scales that are much more statistically stable than subscores computed solely from the items that are contained on that scale. These scores achieve their stability through augmentation with information from other related information on the test. These methods were used to complement each other on a data set obtained from a Praxis administration. We found that the deep structure of the test yielded ten subscales and that, because the test was essentially unidimensional, ten subscores could be computed, all with very high reliability. This result was contrasted with the calculation of six traditional subscales based on surface features of the items. These subscales also yielded augmented subscores of high reliability. [source] Optimal Velocity Planning of Wheeled Mobile Robots on Specific Paths in Static and Dynamic EnvironmentsJOURNAL OF FIELD ROBOTICS (FORMERLY JOURNAL OF ROBOTIC SYSTEMS), Issue 12 2003María Prado This paper deals with optimal temporal-planning of wheeled mobile robots (WMRs) when navigating on predefined spatial paths. A method is proposed to generate a time-optimal velocity profile for any spatial path in static environments or when mobile obstacles are present. The method generates a feasible trajectory to be tracked by fully exploiting velocity, acceleration and deceleration boundaries of the WMR, and by ensuring the continuity of the velocity and acceleration functions. As an additional benefit for the tracking process the jerk is also bounded. The algorithm is not time consuming, since it mostly uses closed mathematical expressions, nonetheless iteration strategies are presented to solve specific situations. However, such situations are not expected to occur when the spatial paths are planned as smooth curves. The success of the algorithm was tested by experimental and simulation results on the WMR "RAM." © 2003 Wiley Periodicals, Inc. [source] Cognitive Dynamics of Capability Development PathsJOURNAL OF MANAGEMENT STUDIES, Issue 6 2009Tomi Laamanen abstract Recent research on capability dynamics has increasingly turned its attention to the cognitive microfoundations of capability development. On the basis of a longitudinal case study of the evolution of three network security software firms, we find that the effects of managerial cognition can be detected at three distinct levels of capability development. At the level of operational capabilities, instrumental cognition affects the way in which capabilities are developed. At the level of a firm's capability portfolio, shifts in management's attention regarding capability development cause different evolutionary paths to emerge. Finally, at the extended enterprise level, managerial foresight influences the way in which a firm's capability constellation morphs over time. Our findings provide novel empirical evidence and contribute to an improved understanding of the role of managerial cognition in capability development. [source] Finding New Paths to Family Scholarship: A Response to James White and Sheila MarshallJOURNAL OF MARRIAGE AND FAMILY, Issue 3 2001Katherine Allen No abstract is available for this article. [source] Paths to nursing leadershipJOURNAL OF NURSING MANAGEMENT, Issue 5 2006TERESE BONDAS PhD Aim, The aim was to explore why nurses enter nursing leadership and apply for a management position in health care. The study is part of a research programme in nursing leadership and evidence-based care. Background, Nursing has not invested enough in the development of nursing leadership for the development of patient care. There is scarce research on nurses' motives and reasons for committing themselves to a career in nursing leadership. Method, A strategic sample of 68 Finnish nurse leaders completed a semistructured questionnaire. Analytic induction was applied in an attempt to generate a theory. Findings, A theory, Paths to Nursing Leadership, is proposed for further research. Four different paths were found according to variations between the nurse leaders' education, primary commitment and situational factors. They are called the Path of Ideals, the Path of Chance, the Career Path and the Temporary Path. Conclusion, Situational factors and role models of good but also bad nursing leadership besides motivational and educational factors have played a significant role when Finnish nurses have entered nursing leadership. The educational requirements for nurse leaders and recruitment to nursing management positions need serious attention in order to develop a competent nursing leadership. [source] Paths to deutero-learning through successive process simulations: a case studyKNOWLEDGE AND PROCESS MANAGEMENT: THE JOURNAL OF CORPORATE TRANSFORMATION, Issue 4 2004Päivi Haho This paper discusses the dynamic interaction between organizational learning processes and their outcomes in the context of innovative business process development and change projects in a pharmaceutical company. Through the answers to the research questions, I wish to demonstrate the paths to deutero-learning, which seldom can be empirically identified in an organization. The paper uses notions of strategic, operational and cultural outcomes,including their intangible and tangible manifestations,to explain different results in organizational learning processes. From 1998 to 1999, the pharmaceutical case company applied an evolutionary, process simulation-based business process development method. This method was used to invent and implement business process innovations in the New Product Development process, to shorten the time-to-market of its new medical entities. Successive process simulations guided and focused the business process development and actions on the strategically most valuable areas. The process simulations prepared the organization for the change, and promoted the implementation of the process outcomes. The successive simulations have triggered and thereafter sustained individual and organizational learning. Thus, they have accelerated organizational learning processes and the development of knowledge and innovations. The case demonstrates efficient deutero-learning, enabled through empowered successive process simulations. The results indicate that development projects are more successful, if there are intangible learning outcomes and systemic process learning at the early stages of the project. This also supports double-loop learning in the business process development project and assists changes in norms to occur. Copyright © 2004 John Wiley & Sons, Ltd. [source] Paths to the Triune God: An Encounter between Aquinas and Recent Theologies , By Anselm K. MinMODERN THEOLOGY, Issue 2 2007Matthew Levering First page of article [source] Paths to Negotiation SuccessNEGOTIATION AND CONFLICT MANAGEMENT RESEARCH, Issue 2 2010Jane A. Halpert Abstract This article presents a multi-variable model of the negotiation process and tests it via a series of meta-analyses and follow-up path analyses. Negotiator goals, relationships, expectations, and behavior are tested as predictors of (a) the profit-or-loss outcome of the negotiation, (b) the negotiator's perceptions of the other party, and (c) the negotiator's satisfaction with the negotiation. A path model was tested based on separate meta-analyses to create the correlation table of the seven variables. The findings demonstrate that to be successful, negotiations should focus on goals and cooperation within the negotiation. High goals and positive relationships started negotiators on the path to successful outcomes. [source] Civic Education in Post-Apartheid South Africa: Alternative Paths to the Development of Political Knowledge and Democratic ValuesPOLITICAL PSYCHOLOGY, Issue 3 2005Steven E. Finkel Despite the proliferation of civic education programs in the emerging democracies of Latin America, Africa, and Eastern Europe, there have been few recent evaluations of the effectiveness of civics instruction in achieving changes in democratic orientations among student populations. We present findings from a study conducted in 1998 that examined the impact of democratic civic education among South African high school students. Using a battery of items to gauge democratic orientations, including measures of political knowledge, civic duty, tolerance, institutional trust, civic skills, and approval of legal forms of political participation, we find that civic education had the largest effects on political knowledge, with the magnitude of the effect being approximately twice as large as the recent Niemi and Junn (1998) finding for the United States. Exposure to civic education per se had weaker effects on democratic values and skills; for these orientations, what matters are specific factors related to the quality of instruction and the use of active pedagogical methods employed by civics instructors. Further, we find that civic education changed the structure of students' orientations: a "democratic values" dimension coalesces more strongly, and in greater distinction, from a "political competence" dimension among students exposed to civic education than among those with no such training. We discuss the implications of the findings for our theoretical understanding of the role of civic education in fostering democratic attitudes, norms, and values, as well as the practical implications of the results for the implementation and funding of civic education programs in developing democracies in the future. [source] Two Parties, Two Types of Nominees, Two Paths to Winning a Presidential Nomination, 1972-2004PRESIDENTIAL STUDIES QUARTERLY, Issue 2 2007D. JASON BERGGREN Contrary to findings that show the contemporary nomination process, regardless of party, favoring early frontrunners, this article shows that the eventual Democratic nominee is typically different from and often travels a different path to victory than the eventual Republican nominee. Since 1972, the eventual Democratic winners began as relatively unknown candidates with single-digit support who emerge as the frontrunner late in the process, sometimes just before the voting begins in Iowa and New Hampshire and sometimes just after the first votes are cast. John Kerry is only the latest Democratic example. In contrast, Republican winners have been national figures and have consistently been the early favorites a year before any votes were cast or large sums of money raised. To date, the accuracy of partyless models is driven largely by Republican successes. These models may be better at predicting Republican nominees than predicting Democratic nominees. [source] Performance Improvement Paths in the U.S. Airline Industry:Linking Trade-offs to Asset FrontiersPRODUCTION AND OPERATIONS MANAGEMENT, Issue 2 2004Michael A. Lapré Several articles have been written during the past few years examining performance improvement paths and various forms of efficiency frontiers in operations strategy. These articles focus primarily on defining and describing these frontiers and raise questions concerning how to improve operations. In this paper, we provide one of the first empirical studies aimed at validating these earlier studies. Using a database on the 10 largest U.S. airlines for a period of 11 years, we test and validate some of the models presented in the operations literature. The 10 major airlines are separated into 2 groups for analysis: geographic specialists and geographic generalists. Our analysis shows that better performing airlines (in terms of cost-quality position) in both groups confirm the predictions of the sand cone model when operating further away from their asset frontiers, although trade-offs do occur when operating close to asset frontiers. [source] Career Paths and Choices in a Highly Differentiated Profession: The Position of Newly Qualified SolicitorsTHE MODERN LAW REVIEW, Issue 4 2001Andrew Boon Differences in the working lives of solicitors have become increasingly marked in recent years. Growing numbers of lawyers are employed in the public and corporate sectors and, with the increasing size and wealth of City of London commercial firms, there are significant differences between these firms and those ,high-street' firms that serve local communities. These differences impact on lawyers throughout training and beyond, both in terms of rites of passage into the profession and in conditions of employment. This research, the final stage in a longitudinal survey spanning the 1990s, combines quantitative and qualitative methods to explore the reactions of newly qualified solicitors to their work. Building on the project's previous surveys, which charted the nature of disadvantage suffered by many prospective entrants to the legal profession, the research finds a large measure of satisfaction regarding careers. It also identifies causes for concern, including increasing specialisation in legal education and the potential separation of the intrinsic and extrinsic rewards of professional practice. [source] Adjustment of the atmospheric circulation to tropical Pacific SST anomalies: Variability of transient eddy propagation in the Pacific,North America sector,THE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 647 2010R. Seager Abstract El Niño,Southern Oscillation (ENSO) related precipitation anomalies in North America are related to changes in the paths of storm systems across the Pacific Ocean, with a more southern route into southwestern North America during El Niños and a more northern route into the Pacific Northwest during La Niñas. Daily reanalysis data are analyzed to confirm these changes. Seasonal mean upper tropospheric eddy statistics show, for El Niños (La Niñas), a pattern that is shifted southward (northward) compared with climatology. Paths of coherent phase propagation of transient eddies and of the propagation of wave packets are analyzed. A coherent path of propagation across the Pacific towards North America is identified that is more zonal during El Niño winters and, during La Niñas, has a dominant path heading northeastward to the Pacific Northwest. A second path heading southeastward from the central Pacific to the tropical east Pacific is more accentuated during La Niñas than El Niños. These changes in wave propagation are reproduced in an ensemble of seasonal integrations of a general circulation model forced by a tropical Pacific sea-surface temperature pattern, confirming that the changes are forced by changes in the mean atmospheric state arising from changes in tropical sea-surface temperature. A simplified model with a specified basic state is used to model the storm tracks for El Niño and La Niña winters. The results suggest that the changes in transient eddy propagation and the eddy statistics can be understood in terms of the refraction of transient eddies within different basic states. Copyright © 2010 Royal Meteorological Society [source] Supply and Operations: Parallel Paths and Integrated StrategiesBRITISH JOURNAL OF MANAGEMENT, Issue 4 2004Steve Brown This paper provides evidence of links between the nature of strategic formulation within firms and subsequent plant performance. The research focuses on how some firms have managed to link operations and supply into an integrated set of mutually supportive strategies. In the current competitive arena firms can no longer rely wholly on internal resource-based capabilities but must instead form strategic partnerships within what are often complex networks. The paper explores how operations and supply functions have pursued parallel paths but have often not been fully integrated within firms. It is argued that such integration, which forms part of what has been termed strategic resonance, can reap enormous benefits in terms of plant-level capabilities for firms. [source] Academic Paths, Ageing and the Living Conditions of Students in the Late 20th Century,CANADIAN REVIEW OF SOCIOLOGY/REVUE CANADIENNE DE SOCIOLOGIE, Issue 2 2001Arnaud Sales Le monde étudiant est profondément marqué par la diversité des tra-jectoires individuelles, qui sont souvent très éloignées d'un déroule-ment linéaire des études. Les conséquences sur l'âge des étudiants sont majeures, et la condition étudiante ne peut plus être définie comme une expérience strictement juvénile. Or, entre 20 et 30 ans, l'âge engendre des impératifs différentiels sur le plan des conditions et des modes de vie qui ne sont pas toujours compatibles avec la condition étudiante classique. Cette étude des parcours et de la situation financière des étudiants des universités québécoises de langue françhise et anglaise montre comment s'opère la déconnexion entre jeunesse et condition étudiante, et comment cette déconnexion influe sur la différenciation des conditions de vie et de financement des études. Student life is profoundly marked by the diversity of individual trajectories, which are in stark contrast with the linear path traditionally taken by students. The impact on the age of the student population is significant: indeed, student life can no longer be qualified as strictly for the young. Between the ages of 20 and 30 years, different imperatives come into play in terms of living conditions and lifestyle. These imperatives are not always compatible with the conditions of classic student life. This study of the academic paths and the financial situation of Quebec university students shows how the disconnection between student condition and youth occurs and how this disconnection impacts the differentiation of student's living conditions and modes of financing university studies. [source] Productivity and Career Paths of Previous Recipients of Society for Academic Emergency Medicine Research Grant AwardsACADEMIC EMERGENCY MEDICINE, Issue 6 2008Kelly D. Young MD Abstract Objectives:, The objective was to assess productivity of previous recipients of Society for Academic Emergency Medicine (SAEM) grant awards. Methods:, All previous recipients of SAEM Research Training Grants, Neuroscience Research Awards, Scholarly Sabbatical Awards, and Emergency Medical Services (EMS) Research Fellowship awards funded through 2004 were identified through SAEM's records and surveyed. Award categories assessed were those still offered by SAEM at the time of the survey and therefore excluded the Geriatric Research Award. The 2005,2006 SAEM Grants Committee developed a survey using previous publications assessing productivity of training grants and fellowship awards and refined it through consensus review and limited pilot testing. We assessed measures of academic productivity (numbers of publications and additional grants awarded), commitment to an academic career, satisfaction with the SAEM award, and basic demographic information. Results:, Overall response rate was 70%; usable data were returned by all seven Research Training Grant awardees, both Neuroscience awardees, four of five Scholarly Sabbatical awardees, and six of 14 EMS Research Fellowship awardees. Of those who gave demographic information, 78% (14/18) were male and 94% (16/17) were non-Hispanic white. All the respondents remained in academics, and 14 of 19 felt that they will definitely be in academics 5 years from the time of the survey. They have a median of 1.8 original research publications per year since the end of their grant period, and 74% (14/19) have received subsequent federal funding. All found the SAEM award to be helpful or very helpful to their careers. Conclusions:, Previous recipients of the SAEM grant awards show evidence of academic productivity in the form of subsequent grant funding and research publications, and the majority remain committed to and satisfied with their academic research careers. [source] BiCl3 -Mediated Opening of Epoxides, a Facile Route to Chlorohydrins or Amino Alcohols: One Reagent, Two Paths.CHEMINFORM, Issue 8 2006Adam McCluskey Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access the actual ChemInform Abstract, please click on HTML or PDF. [source] Ligand Association/Dissociation Paths and Ill-Defined Coordination NumbersCHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2010Antonio Ruiz-Martínez Abstract The continuous shape measures approach provides a means for handling the common situation in which a metal atom presents an ill-defined coordination number. Those cases are characterized by the presence of secondary interactions to Lewis bases at distances significantly longer than those expected for a chemical bond. Systematic ways of analyzing ligand association/dissociation pathways that describe such structures and their application to a variety of specific cases are presented. The concepts and methodology presented here apply to molecules and extended solids as well and provide, when needed, a more flexible and precise stereochemical description of the metal coordination sphere than that of an integer coordination number and the associated polyhedral shape. El mètode de les mesures contínues de forma ens permet tractar la situació trobada freqüentment, en què un àtom metàl,lic presenta un nombre de coordinació no ben definit, caracteritzat per la presència d,interaccions secundàries amb bases de Lewis a distàncies significativament més llargues que les que correspondrien a un enllaç químic. Aquí es presenten formes sistemàtiques d,analitzar els camins d,associació/dissociació de lligands que descriuen aquestes estructures, així com llur aplicació a diverses famílies de compostos. Els conceptes i la metodologia presentats en aquest article s'apliquen tant a molècules com a sòlids infinits i ens proporcionen una descripció estereoquímica més flexible i alhora precisa de les esferes de coordinació que no pas un nombre de coordinació enter i la forma polièdrica associada. [source] The Implications of Symmetry of the External Potential on Bond PathsCHEMISTRY - A EUROPEAN JOURNAL, Issue 33 2008Erick Cerpa Sixty-coordinate atom? Using a set of molecules in which the number of "bond" paths terminating at an atom is chemically meaningless (an example is shown here); the implications of symmetry of the external potential on the presence of bond paths are discussed. [source] Analysis of Classical and Quantum Paths for Deprotonation of Methylamine by Methylamine DehydrogenaseCHEMPHYSCHEM, Issue 12 2007Kara E. Ranaghan Abstract The hydrogen-transfer reaction catalysed by methylamine dehydrogenase (MADH) with methylamine (MA) as substrate is a good model system for studies of proton tunnelling in enzyme reactions,an area of great current interest,for which atomistic simulations will be vital. Here, we present a detailed analysis of the key deprotonation step of the MADH/MA reaction and compare the results with experimental observations. Moreover, we compare this reaction with the related aromatic amine dehydrogenase (AADH) reaction with tryptamine, recently studied by us, and identify possible causes for the differences observed in the measured kinetic isotope effects (KIEs) of the two systems. We have used combined quantum mechanics/molecular mechanics (QM/MM) techniques in molecular dynamics simulations and variational transition state theory with multidimensional tunnelling calculations averaged over an ensemble of paths. The results reveal important mechanistic complexity. We calculate activation barriers and KIEs for the two possible proton transfers identified,to either of the carboxylate oxygen atoms of the catalytic base (Asp428,),and analyse the contributions of quantum effects. The activation barriers and tunnelling contributions for the two possible proton transfers are similar and lead to a phenomenological activation free energy of 16.5±0.9 kcal,mol,1 for transfer to either oxygen (PM3-CHARMM calculations applying PM3-SRP specific reaction parameters), in good agreement with the experimental value of 14.4 kcal,mol,1. In contrast, for the AADH system, transfer to the equivalent OD1 was found to be preferred. The structures of the enzyme complexes during reaction are analysed in detail. The hydrogen bond of Thr474,(MADH)/Thr172,(AADH) to the catalytic carboxylate group and the nonconserved active site residue Tyr471,(MADH)/Phe169,(AADH) are identified as important factors in determining the preferred oxygen acceptor. The protein environment has a significant effect on the reaction energetics and hence on tunnelling contributions and KIEs. These environmental effects, and the related clearly different preferences for the two carboxylate oxygen atoms (with different KIEs) in MADH/MA and AADH/tryptamine, are possible causes of the differences observed in the KIEs between these two important enzyme reactions. [source] The Influence of Solid-State Molecular Organization on the Reaction Paths of Thiyl RadicalsCHEMPHYSCHEM, Issue 6 2005Antonio Faucitano Prof. Abstract Electron paramagnetic resonance (EPR) spectroscopy has been employed to investigate the effect of solid-state molecular organization on the reaction of thiyl radicals with thiols. In an irradiated C18H37SH/thiourea clathrate, the conversion of thiyl to perthiyl radicals is substantial, due to the head-to-head arrangement of the reactants within the channels and the suppression of other possible competing reactions due to hindrance by the clathrate walls. The perthiyl radical was identified using EPR analysis of its molecular dynamics within the clathrate channels. Irradiated polyethylene film containing 30,% C18H37SH afforded a negligible conversion of thiyl to perthiyl radicals because of the random distribution of reactants. These results suggest that in the absence of favorable structure-control effects, the reaction between RS. and RSH is unimportant with respect to other competing reactions. Perthiyl radicals are also the major product in the vacuum solid-state radiolysis of lysozyme. A proposal of the mechanism involved in all cases is based on the equilibrium RS.+RSH,RSS.(H)R, followed by the irreversible conversion of the sulfuranyl radical to the perthiyl radical. As the equilibrium is strongly shifted to the left, the intermediate sulfuranyl radicals were not detected, but the lack of other competing reactions for the thiyl radicals caused the formation of perthiyl radicals to become the major path in the clathrate and in solid lysozyme radiolysis. [source] Paths of Effects of Early Childhood Intervention on Educational Attainment and Delinquency: A Confirmatory Analysis of the Chicago Child-Parent CentersCHILD DEVELOPMENT, Issue 5 2004Arthur J. Reynolds This study investigated the contributions of 5 mechanisms to the effects of preschool participation in the Child-Parent Centers for 1,404 low-income children in the Chicago Longitudinal Study. Based on a matched-group design, preschool participation was associated with significantly higher rates of educational attainment and lower rates of juvenile arrest. LISREL analysis revealed that the primary mediators of effects for both outcomes were attendance in high-quality elementary schools and lower mobility (school support hypothesis), literacy skills in kindergarten and avoidance of grade retention (cognitive advantage hypothesis), and parent involvement in school and avoidance of child maltreatment (family support hypothesis). The model accounted for 58% and 79% of the preschool links with school completion and juvenile arrest, respectively. The maintenance early intervention effects are influenced by many alterable factors. [source] | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||