Showing papers by "National University of Singapore published in 2004"

Exploration vs. Exploitation: An Empirical Test of the Ambidexterity Hypothesis

Abstract: While exploration and exploitation represent two fundamentally different approaches to organizational learning, recent literature has increasingly indicated the need for firms to achieve a balance between the two. This balanced view is embedded in the concept of ambidextrous organizations. However, there is little direct evidence of the positive effect of ambidexterity on firm performance. This paper seeks to test the ambidexterity hypothesis by examining how exploration and exploitation can jointly influence firm performance in the context of firms' approach to technological innovation. Based on a sample of 206 manufacturing firms, we find evidence consistent with the ambidexterity hypothesis by showing that (1) the interaction between explorative and exploitative innovation strategies is positively related to sales growth rate, and (2) the relative imbalance between explorative and exploitative innovation strategies is negatively related to sales growth rate.

Measuring reactive species and oxidative damage in vivo and in cell culture: how should you do it and what do the results mean?

Abstract: Free radicals and other reactive species (RS) are thought to play an important role in many human diseases. Establishing their precise role requires the ability to measure them and the oxidative damage that they cause. This article first reviews what is meant by the terms free radical, RS, antioxidant, oxidative damage and oxidative stress. It then critically examines methods used to trap RS, including spin trapping and aromatic hydroxylation, with a particular emphasis on those methods applicable to human studies. Methods used to measure oxidative damage to DNA, lipids and proteins and methods used to detect RS in cell culture, especially the various fluorescent ‘probes' of RS, are also critically reviewed. The emphasis throughout is on the caution that is needed in applying these methods in view of possible errors and artifacts in interpreting the results. Keywords: Cell culture, free radical, reactive species, antioxidant, oxidative stress, oxidative damage, fluorescent probe, lipid peroxidation, superoxide Introduction Free radicals and other ‘reactive oxygen (ROS)/nitrogen/chlorine species' (for an explanation of these terms see Table 1) are widely believed to contribute to the development of several age-related diseases, and perhaps, even to the aging process itself (Halliwell & Gutteridge, 1999; Sohal et al., 2002) by causing ‘oxidative stress' and ‘oxidative damage' (terms explained in Table 2). For example, many studies have shown increased oxidative damage to all the major classes of biomolecules in the brains of Alzheimer's patients (Halliwell, 2001; Butterfield, 2002; Liu et al., 2003). Other diseases in which oxidative damage has been implicated include cancer, atherosclerosis, other neurodegenerative diseases and diabetes (Hagen et al., 1994; Chowienczyk et al., 2000; Halliwell, 2000a, 2001, 2002a, 2002b; Parthasarathy et al., 2000). If oxidative damage contributes significantly to disease pathology (Table 3 lists the criteria needed to establish this), then actions that decrease it should be therapeutically beneficial (Halliwell, 2001; Lee et al., 2002a; Liu et al., 2003). If the oxidative damage is involved in the origin of a disease, then successful antioxidant treatment should delay or prevent the onset of that disease (Halliwell, 1991, 2002a, 2002b; Galli et al., 2002; Steinberg & Witztum, 2002). To establish the role of oxidative damage (Table 3), it is therefore essential to be able to measure it accurately. For example, the failure of interventions with antioxidants such as vitamin E, β-carotene or ascorbate to decrease disease incidence in several human intervention trials may have simply been due to the failure of these compounds to decrease oxidative damage in the subjects tested (Halliwell, 1999a, 2000c; Levine et al., 2001; Meagher et al., 2001). In this review, we will examine the methods available to measure reactive species (RS) and oxidative damage, with a particular emphasis on those applicable to human studies. Table 1 Nomenclature of reactive species Table 2 Some key definitions Table 3 Criteria for implicating RS as a significant mechanism of tissue injury in human disease Measuring RS in vivo: basic principles Some fascinating techniques such as L-band electron spin resonance (ESR) with nitroxyl probes and magnetic resonance imaging spin trapping are under development to measure RS directly in whole animals (e.g. Berliner et al., 2001; Han et al., 2001; Utsumi & Yamada, 2003), but no probes are currently suitable for human use. Most RS persist for only a short time in vivo and cannot be measured directly. There are a few exceptions: examples include H2O2 (discussed below), and perhaps, NO•, in the sense that serum levels of NO2− have been claimed to measure vascular endothelial NO• synthesis (Kelm et al., 1999), despite the fact that NO2− is quickly oxidized to NO3− in vivo (Kelm et al., 1999; Oldreive & Rice-Evans, 2001). Essentially, there are two approaches to detecting transient RS: attempting to trap these species and measure the levels of the trapped molecules and measuring the levels of the damage done by RS, that is, the amount of oxidative damage. Sometimes other approaches are used. They include measurements of erythrocyte antioxidant defences and of total antioxidant activity of body fluids; falls in these parameters are often taken as evidence of oxidative stress. Erythrocytes cannot synthesize proteins, however, and their antioxidant enzyme levels may drop as they ‘age' in the circulation (Denton et al., 1975). Thus changes in their levels are more likely to reflect changes in the rates of red blood cell turnover: if this slows down, the circulating erythrocytes will be older on average and so levels of antioxidant enzymes in them will appear to fall. Vice versa, if an intervention accelerates red cell removal or increases erythropoiesis, levels of antioxidants in red cells will seem to rise. Hence, such data should be interpreted with caution. Depending on the method that is used to measure it, the plasma or serum ‘total antioxidant capacity' (TAC) usually involves major contributions from urate, ascorbate and sometimes albumin −SH groups (Benzie & Strain, 1996; Halliwell & Gutteridge, 1999; Prior & Cao, 1999; Rice-Evans, 2000; Bartosz, 2003), although different methods measure different things (Schlesier et al., 2002; Bartosz, 2003). Thus, for example, if plasma albumin levels fall, TAC will fall. If urate levels rise, TAC will rise. The multiple changes in blood chemistry that occur in sick people mean that TAC changes should be interpreted with caution. TAC is also influenced by diet, often because consumption of certain foods may produce changes in plasma ascorbate and/or urate levels (Halliwell, 2003b).

Transformational leadership and organizational commitment: mediating role of psychological empowerment and moderating role of structural distance

Abstract: Summary Using a sample of 520 staff nurses employed by a large public hospital in Singapore, we examined whether psychological empowerment mediated the effects of transformational leadership on followers’ organizational commitment. We also examined how structural distance (direct and indirect leadership) between leaders and followers moderated the relationship between transformational leadership and organizational commitment. Results from HLM analyses showed that psychological empowerment mediated the relationship between transformational leadership and organizational commitment. Similarly, structural distance between the leader and follower moderated the relationship between transformational leadership and organizational commitment. Implications for research and practice of our findings are discussed. Copyright # 2004 John Wiley & Sons, Ltd.

A Cognitive Hierarchy Model of Games

Abstract: Players in a game are “in equilibrium” if they are rational, and accurately predict other players' strategies. In many experiments, however, players are not in equilibrium. An alternative is “cognitive hierarchy” (CH) theory, where each player assumes that his strategy is the most sophisticated. The CH model has inductively defined strategic categories: step 0 players randomize; and step k thinkers best-respond, assuming that other players are distributed over step 0 through step k − 1. This model fits empirical data, and explains why equilibrium theory predicts behavior well in some games and poorly in others. An average of 1.5 steps fits data from many games.

Southeast Asian biodiversity: an impending disaster

Abstract: Southeast Asia has the highest relative rate of deforestation of any major tropical region, and could lose three quarters of its original forests by 2100 and up to 42% of its biodiversity. Here, we report on the current state of its biota and highlight the primary drivers of the threat of extinction now faced by much of the unique and rich fauna and flora of the region. Furthermore, the known impacts on the biodiversity of Southeast Asia are likely to be just the tip of the iceberg, owing to the paucity of research data. The looming Southeast Asian biodiversity disaster demands immediate and definitive actions, yet such measures continue to be constrained by socioeconomic factors, including poverty and lack of infrastructure. Any realistic solution will need to involve a multidisciplinary strategy, including political, socioeconomic and scientific input, in which all major stakeholders (government, non-government, national and international organizations) must participate.

Ontology based context modeling and reasoning using OWL

Abstract: Here we propose an OWL encoded context ontology (CONON) for modeling context in pervasive computing environments, and for supporting logic-based context reasoning. CONON provides an upper context ontology that captures general concepts about basic context, and also provides extensibility for adding domain-specific ontology in a hierarchical manner. Based on this context ontology, we have studied the use of logic reasoning to check the consistency of context information, and to reason over low-level, explicit context to derive high-level, implicit context. By giving a performance study for our prototype, we quantitatively evaluate the feasibility of logic based context reasoning for nontime-critical applications in pervasive computing environments, where we always have to deal carefully with the limitation of computational resources.

Evidence of Airborne Transmission of the Severe Acute Respiratory Syndrome Virus

Abstract: background There is uncertainty about the mode of transmission of the severe acute respiratory syndrome (SARS) virus. We analyzed the temporal and spatial distributions of cases in a large community outbreak of SARS in Hong Kong and examined the correlation of these data with the three-dimensional spread of a virus-laden aerosol plume that was modeled using studies of airflow dynamics. methods We determined the distribution of the initial 187 cases of SARS in the Amoy Gardens housing complex in 2003 according to the date of onset and location of residence. We then studied the association between the location (building, floor, and direction the apartment unit faced) and the probability of infection using logistic regression. The spread of the airborne, virus-laden aerosols generated by the index patient was modeled with the use of airflow-dynamics studies, including studies performed with the use of computational fluid-dynamics and multizone modeling. results The curves of the epidemic suggested a common source of the outbreak. All but 5 patients lived in seven buildings (A to G), and the index patient and more than half the other patients with SARS (99 patients) lived in building E. Residents of the floors at the middle and upper levels in building E were at a significantly higher risk than residents on lower floors; this finding is consistent with a rising plume of contaminated warm air in the air shaft generated from a middle-level apartment unit. The risks for the different units matched the virus concentrations predicted with the use of multizone modeling. The distribution of risk in buildings B, C, and D corresponded well with the three-dimensional spread of virus-laden aerosols predicted with the use of computational fluiddynamics modeling. conclusions Airborne spread of the virus appears to explain this large community outbreak of SARS, and future efforts at prevention and control must take into consideration the potential for airborne spread of this virus.

International diversification and firm performance: the s-curve hypothesis

Abstract: A proposed theoretical framework for the study of multinationality and performance includes both benefits and costs of geographic expansion over different phases of internationalization. Data on 1,489 Japanese firms over 12 years show a consistent horizontal X S-shaped X relationship between multinationality and performance. Further, firms investing more heavily in intangible assets, such as technology and advertising, achieved greater profitability gains from growth in foreign direct investment. Our framework and findings highlight complexity and temporal dynamics.

Board Composition: Balancing Family Influence in S&P 500 Firms

Abstract: Recent research indicates that founding families have substantial stakes in roughly one-third of the largest U.S. companies and, in these firms, control nearly twenty percent of all board seats. Burkart, Panunzi, and Shleifer (2003) suggest that a key element in the desirability of family ownership is the ability to limit the family's expropriation of minority shareholders. Consistent with this notion, we find that the most valuable public firms are those in which independent directors balance family board representation. In contrast, in firms with continued founding family ownership and relatively few independent directors, firm performance is significantly worse than in non-family firms. We also document that moderate family board presence provides substantial benefits to the firm. Additional tests suggest that families often seek to minimize the presence of independent directors, while outside shareholders seek independent director representation. These findings highlight the importance of independent directors in mitigating shareholder-shareholder conflicts and suggest that considering shareholder-shareholder conflicts provides a richer setting in which to explore corporate governance. Note: A list of the firms classified as family and non-family firms is available from the authors.

Sorption of lead, copper, cadmium, zinc, and nickel by marine algal biomass: characterization of biosorptive capacity and investigation of mechanisms

Abstract: The marine algae Sargassum sp, Padina sp, Ulva sp, and Gracillaria sp, harvested locally, were investigated for their biosorption performance in the removal of lead, copper, cadmium, zinc, and nickel from dilute aqueous solutions It was found that the biosorption capacities were significantly affected by solution pH, with higher pH favoring higher metal-ion removal Kinetic and isotherm experiments were carried out at the optimal pH: at pH 50 for lead and copper, and at pH 55 for cadmium, zinc, and nickel The metal removal rates were rapid, with 90% of the total adsorption taking place within 60 min Sargassum sp and Padina sp showed the highest potential for the sorption of the metal ions, with the maximum uptake capacities ranging from 061 to 116 mmol/g for Sargassum sp and 063 to 125 mmol/g for Padina sp The general affinity sequence for Padina sp was Pb>Cu>Cd>Zn>Ni, while that for Sargassum sp was Pb>Zn>Cd>Cu>Ni XPS and FTIR analysis of Sargassum sp and Padina sp revealed the chelating character of the ion coordination to carboxyl groups It was confirmed that carboxyl, ether, alcoholic, and amino groups are responsible for the binding of the metal ions

‘Globalizing’ regional development: a global production networks perspective

Abstract: Recent literature concerning regional development has placed significant emphasis on local institutional structures and their capacity to ‘hold down’ the global. Conversely, work on inter-firm networks – such as the global commodity chain approach – has highlighted the significance of the organizational structures of global firms’ production systems and their relation to industrial upgrading. In this paper, drawing upon a global production networks perspective, we conceptualize the connections between ‘globalizing’ processes, as embodied in the production networks of transnational corporations, and regional development in specific territorial formations. We delimit the ‘strategic coupling’ of the global production networks of firms and regional economies which ultimately drives regional development through the processes of value creation, enhancement and capture. In doing so, we stress the multi-scalarity of the forces and processes underlying regional development, and thus do not privilege one particular geographical scale. By way of illustration, we introduce an example drawn from recent research into global production networks in East Asia and Europe. The example profiles the investments of car manufacturer BMW in Eastern Bavaria, Germany and Rayong, Thailand, and considers their implications for regional development.

Role of inflammatory mediators in the pathophysiology of acute respiratory distress syndrome

Abstract: Inflammatory response leading to organ dysfunction and failure continues to be the major problem after injury in many clinical conditions such as sepsis, severe burns, acute pancreatitis, haemorrhagic shock, and trauma. In general terms, systemic inflammatory response syndrome (SIRS) is an entirely normal response to injury. Systemic leukocyte activation, however, is a direct consequence of a SIRS and if excessive, can lead to distant organ damage and multiple organ dysfunction syndrome (MODS). When SIRS leads to MODS and organ failure, the mortality becomes high and can be more than 50%. Acute lung injury that clinically manifests as acute respiratory distress syndrome (ARDS) is a major component of MODS of various aetiologies. Inflammatory mediators play a key role in the pathogenesis of ARDS, which is the primary cause of death in these conditions. This review summarizes recent studies that demonstrate the critical role played by inflammatory mediators such as tumour necrosis factor (TNF)-alpha, interleukin (IL)-1beta, IL-6, platelet activating factor (PAF), IL-10, granulocyte macrophage-colony stimulating factor (GM-CSF), C5a, intercellular adhesion molecule (ICAM)-1, substance P, chemokines, VEGF, IGF-I, KGF, reactive oxygen species (ROS), and reactive nitrogen species (RNS) in the pathogenesis of ARDS. It is reasonable to speculate that elucidation of the key mediators in ARDS coupled with the discovery of specific inhibitors would make it possible to develop clinically effective anti-inflammatory therapy.

Perfect State Transfer in Quantum Spin Networks

Abstract: We propose a class of qubit networks that admit the perfect state transfer of any quantum state in a fixed period of time. Unlike many other schemes for quantum computation and communication, these networks do not require qubit couplings to be switched on and off. When restricted to N-qubit spin networks of identical qubit couplings, we show that 2log3N is the maximal perfect communication distance for hypercube geometries. Moreover, if one allows fixed but different couplings between the qubits, then perfect state transfer can be achieved over arbitrarily long distances in a linear chain.

Preparation of Hollow Anatase TiO2 Nanospheres via Ostwald Ripening

Abstract: In this work, we report a simple “one pot” approach to prepare hollow anatase TiO2 nanospheres via Ostwald ripening under hydrothermal conditions. Inner nanospace and highly organized crystallites in the shell structure and surface regions can be created with a wide range of controlling parameters. The formation mechanism has been investigated with TEM/ED/SEM/EDX/XRD/XPS methods. The approach shows a high versatility for structural engineering of various targeted morphological products, including inner material refills.

Thermal diode: rectification of heat flux.

Abstract: By coupling two nonlinear one dimensional lattices, we demonstrate a thermal diode model that works in a wide range of system parameters. We provide numerical and analytical evidence for the underlying mechanism which allows heat flux in one direction while the system acts like an insulator when the temperature gradient is reversed. The possible experimental realization in nanoscale systems is briefly discussed.

The enzymes and control of eukaryotic mRNA turnover.

Abstract: The degradation of eukaryotic mRNAs plays important roles in the modulation of gene expression, quality control of mRNA biogenesis and antiviral defenses. In the past five years, many of the enzymes involved in this process have been identified and mechanisms that modulate their activities have begun to be identified. In this review, we describe the enzymes of mRNA degradation and their properties. We highlight that there are a variety of enzymes with different specificities, suggesting that individual nucleases act on distinct subpopulations of transcripts within the cell. In several cases, translation factors that bind mRNA inhibit these nucleases. In addition, recent work has begun to identify distinct mRNP complexes that recruit the nucleases to transcripts through different mRNA-interacting proteins. These properties and complexes suggest multiple mechanisms by which mRNA degradation could be regulated.

Mesoscale Organization of CuO Nanoribbons: Formation of “Dandelions”

Abstract: A two-tiered organizing scheme with multiple-length scales for construction of dandelion-like hollow CuO microspheres has been elucidated: (1) mesoscale formation of rhombic building units from smaller nanoribbons via oriented aggregation and (2) macroscopic organization of these units into the CuO microspheres. This self-assembly concept may also be applicable to other metal oxides by creating geometric constraints for constructional units.

Adaptive neural control of uncertain MIMO nonlinear systems

Abstract: In this paper, adaptive neural control schemes are proposed for two classes of uncertain multi-input/multi-output (MIMO) nonlinear systems in block-triangular forms. The MIMO systems consist of interconnected subsystems, with couplings in the forms of unknown nonlinearities and/or parametric uncertainties in the input matrices, as well as in the system interconnections without any bounding restrictions. Using the block-triangular structure properties, the stability analyses of the closed-loop MIMO systems are shown in a nested iterative manner for all the states. By exploiting the special properties of the affine terms of the two classes of MIMO systems, the developed neural control schemes avoid the controller singularity problem completely without using projection algorithms. Semiglobal uniform ultimate boundedness (SGUUB) of all the signals in the closed-loop of MIMO nonlinear systems is achieved. The outputs of the systems are proven to converge to a small neighborhood of the desired trajectories. The control performance of the closed-loop system is guaranteed by suitably choosing the design parameters. The proposed schemes offer systematic design procedures for the control of the two classes of uncertain MIMO nonlinear systems. Simulation results are presented to show the effectiveness of the approach.

Uptake and Cytotoxicity of Chitosan Molecules and Nanoparticles: Effects of Molecular Weight and Degree of Deacetylation

Abstract: Purpose. To evaluate the effects of molecular weight (Mw) and degree of deacetylation (DD) on the cellular uptake and in vitro cytotoxicity of chitosan molecules and nanoparticles.

Carbon-Supported Pt and PtRu Nanoparticles as Catalysts for a Direct Methanol Fuel Cell

Abstract: Nanosized Pt and PtRu colloids were prepared by a microwave-assisted polyol process and transferred to a toluene solution of decanthiol Vulcan XC-72 was then added to the toluene solution to adsorb the thiolated Pt and PtRu colloids TEM examinations showed nearly spherical particles and narrow size distributions for both supported and unsupported metals The carbon-supported Pt and PtRu nanoparticles were activated by thermal treatment to remove the thiol stabilizing shell All Pt and PtRu catalysts (except Pt23Ru77) showed the X-ray diffraction pattern of a face-centered cubic (fcc) crystal structure, whereas the Pt23Ru77 alloy was more typical of the hexagonal close-packed (hcp) structure The electro-oxidation of liquid methanol on these catalysts was investigated at room temperature by cyclic voltammetry and chronoamperometry The results showed that the alloy catalyst was catalytically more active than pure platinum The heat-treated catalyst was also expectedly more active than the non-heat-treate