Showing papers by "University of Delaware published in 2003"

A point-charge force field for molecular mechanics simulations of proteins based on condensed-phase quantum mechanical calculations.

Abstract: Molecular mechanics models have been applied extensively to study the dynamics of proteins and nucleic acids. Here we report the development of a third-generation point-charge all-atom force field for proteins. Following the earlier approach of Cornell et al., the charge set was obtained by fitting to the electrostatic potentials of dipeptides calculated using B3LYP/cc-pVTZ//HF/6-31G** quantum mechanical methods. The main-chain torsion parameters were obtained by fitting to the energy profiles of Ace-Ala-Nme and Ace-Gly-Nme di-peptides calculated using MP2/cc-pVTZ//HF/6-31G** quantum mechanical methods. All other parameters were taken from the existing AMBER data base. The major departure from previous force fields is that all quantum mechanical calculations were done in the condensed phase with continuum solvent models and an effective dielectric constant of e = 4. We anticipate that this force field parameter set will address certain critical short comings of previous force fields in condensed-phase simulations of proteins. Initial tests on peptides demonstrated a high-degree of similarity between the calculated and the statistically measured Ramanchandran maps for both Ace-Gly-Nme and Ace-Ala-Nme di-peptides. Some highlights of our results include (1) well-preserved balance between the extended and helical region distributions, and (2) favorable type-II poly-proline helical region in agreement with recent experiments. Backward compatibility between the new and Cornell et al. charge sets, as judged by overall agreement between dipole moments, allows a smooth transition to the new force field in the area of ligand-binding calculations. Test simulations on a large set of proteins are also discussed. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 1999–2012, 2003

Energy, water, and broad-scale geographic patterns of species richness

Abstract: It is often claimed that we do not understand the forces driving the global diversity gradient. However, an extensive literature suggests that contemporary climate constrains terrestrial taxonomic richness over broad geographic extents. Here, we review the empirical literature to examine the nature and form of the relationship between climate and richness. Our goals were to document the support for the climatically based energy hypothesis, and within the constraints imposed by correlative analyses, to evaluate two versions of the hypothesis: the productivity and ambient energy hypotheses. Focusing on studies extending over 800 km, we found that measures of energy, water, or water-energy balance explain spatial variation in richness better than other climatic and non-climatic variables in 82 of 85 cases. Even when considered individually and in isolation, water/ energy variables explain on average over 60% of the variation in the richness of a wide range of plant and animal groups. Further, water variables usually represent the strongest predictors in the tropics, subtropics, and warm temperate zones, whereas energy variables (for animals) or water-energy variables (for plants) dominate in high latitudes. We conclude that the interaction between water and energy, either directly or indirectly (via plant productivity), provides a strong explanation for globally extensive plant and animal diversity gradients, but for animals there also is a latitudinal shift in the relative importance of ambient energy vs. water moving from the poles to the equator. Although contemporary climate is not the only factor influencing species richness and may not explain the diversity pattern for all taxonomic groups, it is clear that understanding water-energy dynamics is critical to future biodiversity research. Analyses that do not include water-energy variables are missing a key component for explaining broad-scale patterns of diversity.

Multi-Agent Systems for the Simulation of Land-Use and Land-Cover Change: A Review

Abstract: This paper presents an overview of multi-agent system models of land-use/cover change (MAS/LUCC models). This special class of LUCC models combines a cellular landscape model with agent-based representations of decisionmaking, integrating the two components through specification of interdependencies and feedbacks between agents and their environment. The authors review alternative LUCC modeling techniques and discuss the ways in which MAS/LUCC models may overcome some important limitations of existing techniques. We briefly review ongoing MAS/LUCC modeling efforts in four research areas. We discuss the potential strengths of MAS/LUCC models and suggest that these strengths guide researchers in assessing the appropriate choice of model for their particular research question. We find that MAS/LUCC models are particularly well suited for representing complex spatial interactions under heterogeneous conditions and for modeling decentralized, autonomous decision making. We discuss a range of possible roles for MAS/LUCC models, from abstract models designed to derive stylized hypotheses to empirically detailed simulation models appropriate for scenario and policy analysis. We also discuss the challenge of validation and verification for MAS/LUCC models. Finally, we outline important challenges and open research questions in this new field. We conclude that, while significant challenges exist, these models offer a promising new tool for researchers whose goal is to create fine-scale models of LUCC phenomena that focus on human-environment interactions.

Beating the superparamagnetic limit with exchange bias

Abstract: Interest in magnetic nanoparticles has increased in the past few years by virtue of their potential for applications in fields such as ultrahigh-density recording and medicine. Most applications rely on the magnetic order of the nanoparticles being stable with time. However, with decreasing particle size the magnetic anisotropy energy per particle responsible for holding the magnetic moment along certain directions becomes comparable to the thermal energy. When this happens, the thermal fluctuations induce random flipping of the magnetic moment with time, and the nanoparticles lose their stable magnetic order and become superparamagnetic. Thus, the demand for further miniaturization comes into conflict with the superparamagnetism caused by the reduction of the anisotropy energy per particle: this constitutes the so-called 'superparamagnetic limit' in recording media. Here we show that magnetic exchange coupling induced at the interface between ferromagnetic and antiferromagnetic systems can provide an extra source of anisotropy, leading to magnetization stability. We demonstrate this principle for ferromagnetic cobalt nanoparticles of about 4 nm in diameter that are embedded in either a paramagnetic or an antiferromagnetic matrix. Whereas the cobalt cores lose their magnetic moment at 10 K in the first system, they remain ferromagnetic up to about 290 K in the second. This behaviour is ascribed to the specific way ferromagnetic nanoparticles couple to an antiferromagnetic matrix.

The influence of ethnic discrimination and ethnic identification on African American adolescents' school and socioemotional adjustment.

Abstract: Do experiences with racial discrimination at school predict changes in African American adolescents' academic and psychological functioning? Does African American ethnic identity buffer these relations? This paper addresses these two questions using two waves of data from a longitudinal study of an economically diverse sample of African American adolescents living in and near a major East Coast metropolis. The data were collected at the beginning of the 7th grade and after the completion of the 8th grade. As expected, experiences of racial discrimination at school from one's teachers and peers predicts declines in grades, academic ability self-concepts, academic task values, mental health (increases in depression and anger, decreases in self-esteem and psychological resiliency), and increases in the proportion of one's friends who are not interested in school and who have problem behaviors. A strong, positive connection to one's ethnic group (our measure of ethnic identity) reduced the magnitude of the association of racial discrimination experiences with declines in academic self-concepts, school achievement, and perception of friends' positive characteristics, as well as the association of the racial discrimination experiences with increases in problem behaviors.

Handbook of Zeolite Science and Technology

Abstract: The Handbook of Zeolite Science and Technology offers effective analyses ofsalient cases selected expressly for their relevance to current and prospective research. Presenting the principal theoretical and experimental underpinnings of zeolites, this international effort is at once complete and forward-looking, combining fundamental concepts with the most sophisticated data for each scientific subtopic and budding technology. Supplying over 750 figures, and 350 display equations, this impressive achievement in zeolite science observes synthesis through the lens of MFI (ZSM-5 and silicalite). Chapters progress from conceptual building blocks to complex research presentations.

Preference for Online Social Interaction A Theory of Problematic Internet Use and Psychosocial Well-Being

Abstract: The model introduced and tested in the current study suggests that lonely and depressed individuals may develop a preference for online social interaction, which, in turn, leads to negative outcomes associated with their Internet use. Participants completed measures of preference for online social interaction, depression, loneliness, problematic Internet use, and negative outcomes resulting from their Internet use.Results indicated that psychosocial health predicted levels of preference for online social interaction, which, in turn, predicted negative outcomes associated with problematic Internet use. In addition, the results indicated that the influence of psychosocial distress on negative outcomes due to Internet use is mediated by preference for online socialization and other symptoms of problematic Internet use.T he results support the current hypothesis that that individuals’ preference for online, rather than face-to-face, social interaction plays an important role in the development of negative con...

Perceived organizational support and psychological contracts: a theoretical integration

Abstract: Summary Although organizational support theory and psychological contract theory both stress social exchange processes in the establishment and maintenance of the employee–employer relationship, they have focused on different aspects of this relationship. We suggest that, far from being independent, the different parts of the employee–employer association considered by the two theories are mutually interdependent. Further, key processes identified by each theory influence the relationships described by the other theory. To further the understanding of the employee–employer relationship, we provide an integrated account that emphasizes the interdependence of perceived organizational support and the psychological contract. Copyright # 2003 John Wiley & Sons, Ltd.

The ballistic impact characteristics of Kevlar® woven fabrics impregnated with a colloidal shear thickening fluid

Abstract: This study reports the ballistic penetration performance of a composite material composed of woven Kevlar® fabric impregnated with a colloidal shear thickening fluid (silica particles (450 nm) dispersed in ethylene glycol). The impregnated Kevlar fabric yields a flexible, yet penetration resistant composite material. Fragment simulation projectile (FSP) ballistic penetration measurements at ∼244 m/s have been performed to demonstrate the efficacy of the novel composite material. The results demonstrate a significant enhancement in ballistic penetration resistance due to the addition of shear thickening fluid to the fabric, without any loss in material flexibility. Furthermore, under these ballistic test conditions, the impregnated fabric targets perform equivalently to neat fabric targets of equal areal density, while offering significantly less thickness and more material flexibility. The enhancement in ballistic performance is shown to be associated with the shear thickening response, and possible mechanisms of fabric-fluid interaction during ballistic impact are identified.

Virtualness and knowledge in teams: managing the love triangle of organizations, individuals, and information technology

Abstract: Information technology can facilitate the dissemination of knowledge across the organization-even to the point of making virtual teams a viable alternative to face-to-face work. However, unless managed, the combination of information technology and virtual work may serve to change the distribution of different types of knowledge across individuals, teams, and the organization. Implications include the possibility that information technology plays the role of a jealous mistress when it comes to the development and ownership of valuable knowledge in organizations; that is, information technology may destabilize the relationship between organizations and their employees when it comes to the transfer of knowledge. The paper advances theory and informs practice by illustrating the dynamics of knowledge development and transfer in more and less virtual teams.

The Association Between Cardiovascular Autonomic Neuropathy and Mortality in Individuals With Diabetes: A meta-analysis

Abstract: OBJECTIVE —To examine by meta-analysis the relationship between cardiovascular autonomic neuropathy (CAN) and risk of mortality in individuals with diabetes. RESEARCH DESIGN AND METHODS —We searched Medline for English-language articles published from 1966 to 2001. Fifteen studies having a baseline assessment of cardiovascular autonomic function and mortality follow-up were identified. The analyses were stratified according to whether a single abnormality or two or more measures of cardiovascular autonomic function were used to define CAN. A global measure of association (i.e., relative risk) was generated for each group by pooling estimates across the studies using the Mantel-Haenszel procedure. RESULTS —CAN was significantly associated with subsequent mortality in both groups, although the magnitude of the association was stronger for those studies for which two or more measures were used to define CAN. The pooled relative risk for studies that defined CAN with the presence of two or more abnormalities was 3.45 (95% CI 2.66–4.47; P P = 0.03) for studies that used one measure. CONCLUSIONS —These results support an association between CAN and increased risk of mortality. The stronger association observed in studies defining CAN by the presence of two or more abnormalities may be due to more severe autonomic dysfunction in these subjects or a higher frequency of other comorbid complications that contributed to their higher mortality risk. Future studies should evaluate whether early identification of subjects with CAN can lead to a reduction in mortality.

On the elastic properties of carbon nanotube-based composites: modelling and characterization

Abstract: The exceptional mechanical and physical properties observed for carbon nanotubes has stimulated the development of nanotube-based composite materials, but critical challenges exist before we can exploit these extraordinary nanoscale properties in a macroscopic composite. At the nanoscale, the structure of the carbon nanotube strongly influences the overall properties of the composite. The focus of this research is to develop a fundamental understanding of the structure/size influence of carbon nanotubes on the elastic properties of nanotube-based composites. Towards this end, the nanoscale structure and elastic properties of a model composite system of aligned multi-walled carbon nanotubes embedded in a polystyrene matrix were characterized, and a micromechanical approach for modelling of short fibre composites was modified to account for the structure of the nanotube reinforcement to predict the elastic modulus of the nanocomposite as a function of the constituent properties, reinforcement geometry and nanotube structure. The experimental characterization results are compared with numerical predictions and highlight the structure/size influence of the nanotube reinforcement on the properties of the nanocomposite. The nanocomposite elastic properties are particularly sensitive to the nanotube diameter, since larger diameter nanotubes show a lower effective modulus and occupy a greater volume fraction in the composite relative to smaller-diameter nanotubes.

Updated emissions from ocean shipping

Abstract: [1] Marine vessel inventories demonstrate that ship emissions cannot be neglected in assessing environmental impacts of air pollution, although significant uncertainty in these inventories remains. We address this uncertainty by employing a bottom-up estimate of fuel consumption and vessel activity for internationally registered fleets, including cargo vessels, other commercial vessels, and military vessels. We identify model bias in previous work, which assumed internationally registered ships primarily consume international marine fuels. Updated results suggest fuel consumption is ∼289 million metric tons per year, more than twice the quantity reported as international fuel. According to our analysis, fuel used by internationally registered fleets is apparently allocated to both international and domestic fuel statistics; this implies either that ships operate along domestic routes much of the time or that marine fuel sales to these ships may be misassigned. If the former is true, then allocation of emissions to international shipping routes may underestimate near-coastal emissions from ships. Our updated inventories increases previous ship emissions inventories for all pollutants; for example, global NOx emissions (∼6.87 Tg N) are more than doubled. This work also produces detailed sensitivity analyses of inputs to these estimates, identifying uncertainty in vessel duty-cycle as critical to overall emissions estimates. We discuss implications for assessing ship emissions impacts.

To err is autonomic: error-related brain potentials, ANS activity, and post-error compensatory behavior.

Abstract: A two-component event-related brain potential consisting of an error-related negativity (ERN/Ne) and positivity (Pe) has been associated with response monitoring and error detection. Both the ERN and Pe have been source-localized to the anterior cingulate cortex (ACC)Fa frontal structure implicated in both cognitive and affective processing, as well as autonomic nervous system (ANS) modulation. The current study sought to examine the relationships among the ERN, the Pe, two autonomic measures, and behavior. Electroencephalogram (EEG), heart rate (HR), and skin conductance (SC) were recorded while subjects performed a two-choice reaction-time task. In addition to the characteristic ERN-Pe complex, errors were associated with larger SCRs and greater HR deceleration. The ERN correlated with the number of errors, but was unrelated to ANS activity and compensatory behavior. Pe, on the other hand, was correlated significantly with SCR, and both SCR and Pe were significantly correlated with post-error slowing. Descriptors: Error-related negativity (ERN), Event-related brain potential (ERP), Error processing, Heart rate (HR), Skin conductance response (SCR) The error-related negativity (ERN/Ne) and subsequent error positivity (Pe) is a two-component event-related brain potential (ERP) complex that has been associated with monitoring actions and detecting errors (Gehring, Coles, Meyer, & Donchin, 1990;

Bacteriocins and their Food Applications

Abstract: Over the last 2 decades, a variety of bacteriocins, produced by bacteria that kill or inhibit the growth of other bacteria, have been identified and characterized biochemically and genetically. This review article focuses on the ecology of bacteriocins, determination of bacteriocin activity, biosynthesis of bacteriocins, and mode of action. Bacteriocin production and modeling are discussed in the article. Nisin is discussed in some detail in this article since it is currently the only purified bacteriocin approved for food use in the U.S. and has been successfully used for several decades as a food preservative in more than 50 countries. For activity spectra and food applications, the review article focuses primarily on class I and class IIa bacteriocins produced by lactic acid bacteria (LAB) given their development as food preservatives.

Experience-Based Top Management Team Competence and Sustained Growth

Abstract: This paper develops and tests a model of multilevel experience-based top management team competence and its effects on a firm's capacity of entrepreneurial growth. The model incorporates the individual and additive effects of firm, team, and industry levels of managerial experience and the conflict effects of combining multiple levels of experience. Theoretical arguments are tested in a longitudinal sample of entrepreneurial firms from the medical and surgical instruments industry. The results indicate that founders' participation in the top management team and managers' past experience in the industry contribute to the competence of the team in seizing new growth opportunities. The results also show that, because of conflict effects, the positive effect of founders' participation in the management team on the rate of growth weakens as either the shared team-specific experience or industry-specific managerial experience in the team increases. For practitioners, the most important implication is that for sustained growth, entrepreneurial firms should learn to balance different levels of managerial experience in the top management team. One way to achieve this balance is to retain valuable founder resources in the team while avoiding high levels of shared team-specific experience and industry-specific managerial experience.

Nonlinear collisionless perpendicular diffusion of charged particles

Abstract: A nonlinear theory of the perpendicular diffusion of charged particles is presented, including the influence of parallel scattering and dynamical turbulence. The theory shows encouraging agreement with numerical simulations.

Elements of resilience after the World Trade Center disaster: reconstituting New York City's Emergency Operations Centre.

Abstract: In this paper we examine the reconstitution of the Emergency Operations Centre (EOC) after its destruction in the World Trade Center attack, using that event to highlight several features of resilience. The paper summarises basic EOC functions, and then presents conceptions of resilience as understood from several disciplinary perspectives, noting that work in these fields has sought to understand how a natural or social system that experiences disturbance sustains its functional processes. We observe that, although the physical EOC facility was destroyed, the organisation that had been established to manage crises in New York City continued, enabling a response that drew on the resources of New York City and neighbouring communities, states and the federal government. Availability of resources — which substituted for redundancy of personnel, equipment and space — pre-existing relationships that eased communication challenges as the emergency developed and the continuation of organisational patterns of response integration and role assignments were among the factors that contributed to resilience following the attack.

Binarity in Brown Dwarfs: T Dwarf Binaries Discovered with the Hubble Space Telescope Wide Field Planetary Camera 2

Abstract: We present the discovery of two T dwarf binaries, 2MASS 1225-2739AB and 2MASS 1534-2952AB, identified in a sample of 10 T dwarfs imaged with the Hubble Space Telescope (HST) Wide Field Planetary Camera 2. Companionship is established by the uniquely red F814W-F1042M colors of the binary components, caused by heavily pressure-broadened K I absorption centered at 7665 and 7699 A. The separations of the two binary systems are 0."282 ± 0."005 and 0."065 ± 0."007, implying projected separations of 3.17 ± 0.14 and 1.0 ± 0.3 AU, respectively. These close separations are similar to those found in previous brown dwarf binary searches and permit orbital mapping over the coming decade. 2MASS 1225-2739AB has a substantially fainter secondary, with ΔM_(F814W) = 1.59 ± 0.04 and ΔM_(F1042M) = 1.05 ± 0.03; this system is likely composed of a T6 primary and T8 secondary with mass ratio 0.7-0.8. The observed binary fraction of our HST sample, 20^(+17%)_(-1), is consistent with results obtained for late-type M and L field dwarfs and implies a bias-corrected binary fraction of 9^(+15%)_(-4) for α ≳ 1 AU and q ≳ 0.4, significantly lower than the binary fractions of F-G and early-type M dwarf stars. Neither of the T binaries have separations α ≳ 10 AU, consistent with results from other brown dwarf binary searches. Using the statistical models of Weinberg, Shapiro, & Wasserman, we conclude that tidal disruption by passing stars or giant molecular clouds, which limits the extent of wide stellar binaries, plays no role in eliminating wide brown dwarf binaries, implying either disruption very early in the formation process (ages ≾ 1-10 Myr) or a formation mechanism that precludes such systems. We find that the maximum binary separation in the brown dwarf regime appears to scale as M^2_(total), a possible clue to the physical mechanism that restricts wide substellar systems.

A longitudinal study of mathematical competencies in children with specific mathematics difficulties versus children with comorbid mathematics and reading difficulties.

Abstract: Mathematical competencies of 180 children were examined at 4 points between 2nd and 3rd grades (age range between 7 and 9 years). Children were initially classified into one of 4 groups: math difficulties but normal reading (MD only), math and reading difficulties (MD-RD), reading difficulties but normal math (RD only), and normal achievement in math and reading (NA). The groups did not differ significantly in rate of development. However, at the end of 3rd grade the MD only group performed better than the MD-RD group in problem solving but not in calculation. The NA and RD only groups performed better than the MD-RD group in most areas. Deficiencies in fact mastery and calculation fluency, in particular, are defining features of MD, with or without RD.