Increasing Size (increasing + size)

Distribution by Scientific Domains

Selected Abstracts

Chemical amplification in an invaded food web: Seasonality and ontogeny in a high-biomass, low-diversity ecosystem,

Carla A. Ng
Abstract The global spread of invasive species is changing the structure of aquatic food webs worldwide. The North American Great Lakes have proved particularly vulnerable to this threat. In nearshore areas, invasive benthic species such as dreissenid mussels and round gobies (Neogobius melanostomus) have gained dominance in recent years. Such species are driving the flow of energy and material from the water column to the benthic zone, with dramatic effect on nutrient and contaminant cycling. Here, we develop a stage-structured model of a benthified food web in Lake Michigan with seasonal resolution and show how its bioaccumulation patterns differ from expected ones. Our model suggests that contaminant recycling through the consumption of lipid-rich fish eggs and mussel detritus is responsible for these differences. In southern Lake Michigan's Calumet Harbor (Chicago, IL, USA), round gobies have nitrogen isotope signatures with considerable spread, with some values higher than their predators and others lower than their prey. Contrary to patterns observed in linear pelagic systems, we predict that polychlorinated biphenyl (PCB) concentrations in these fish decrease with increasing size due to the lipid- and benthos-enriched diets of smaller fish. We also present here round goby PCB concentrations measured in 2005 after an invasional succession in Calumet Harbor and demonstrate how the change from one invasive mussel species to another may have led to a decrease in round goby PCB accumulation. Our results suggest that benthic-dominated systems differ from pelagic ones chiefly due to the influence of detritus and that these effects are exacerbated in systems with low species diversity and high biomass. [source]

Designing Ionic Liquids: 1-Butyl-3-Methylimidazolium Cations with Substituted Tetraphenylborate Counterions

Joep van den Broeke
Abstract The hydrophobic, low melting, 1-butyl-3-methylimidazolium (BMIm) salts [BMIm][BPh4] (1), [BMIm][B(C6H4Me-4)4] (2), [BMIm][B{C6H4(CF3)-4}4] (3), [BMIm][B{C6H3(CF3)2 -3,5}4] (4), [BMIm][B{C6H4(C6F13)-4}4] (5), [BMIm][B{C6H4(SiMe3)-4}4] (6), [BMIm][B(C6H4{SiMe2(CH2CH2CF3)}-4)4] (7), [BMIm][B{C6H4(SiMe2C8H17}-4}4] (8) and [BMIm][B(C6H4{SiMe2(CH2CH2C6F13)}-4)4] (9) have been prepared. Systematic variation of the substituents on the tetraphenylborate anion allowed an assessment of their influence on the physical properties of the imidazolium salts. Structural investigations using NMR and IR spectroscopy, combined with single crystal X-ray structure determinations for 2, 3, 5 and 6, revealed hydrogen-bonding interactions between the imidazolium ring protons and the borate anion, both in the solid state and in solution. These interactions are weakened upon the introduction of electron-withdrawing substituents in the anion and follow the order 3,5-(CF3)2 < ,C6F13 < ,CF3 < ,SiMe2CH2CH2C6F13 < ,SiMe2CH2CH2CF3 < ,H < ,Me < ,SiMe3. The melting points of the salts depend primarily on the bulk of the lipophilic substituents, and decrease with increasing size. Bulky lipophilic substituents dramatically enhance the solubility of the imidazolium borates 8 and 9 in hexane and reduce their relative polarity. These unique properties make imidazolium borates 8 and 9 interesting as amphiphilic ionic liquids with low polarity. Attempts to crystallise 7 resulted in decomposition. A single-crystal X-ray structure determination of the product, isolated in 6% yield, showed that a carbene,tris[4-{dimethyl(3,3,3-trifluoropropyl)silyl}phenyl]borane adduct was formed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source]

Oxidation of phenols by laccase and laccase-mediator systems

FEBS JOURNAL, Issue 21 2002
Solubility, steric issues
To investigate how solubility and steric issues affect the laccase-catalysed oxidation of phenols, a series of oligomeric polyphenol compounds, having increasing size and decreasing solubility in water, was incubated with laccase. The extent of substrate conversion, and the nature of the products formed in buffered aqueous solutions, were compared to those obtained in the presence of an organic cosolvent, and also in the presence of two mediating species, i.e. N -hydroxyphthalimide (HPI) and 2,2,6,6-tetramethylpiperidin-1-yloxy (TEMPO). This approach showed not only an obvious role of solubility, but also a significant role of the dimension of the substrate upon the enzymatic reactivity. In fact, reactivity decreases as substrate size increases even when solubility is enhanced by a cosolvent. This effect may be ascribed to limited accessibility of encumbered substrates to the enzyme active site, and can be compensated through the use of the appropriate mediator. While TEMPO was highly efficient at enhancing the reactivity of large, less soluble substrates, HPI proved less effective. In addition, whereas the laccase/HPI system afforded the same products as laccase alone, the use of TEMPO provided a different product with high specificity. These results offer the first evidence of the role of ,oxidation shuttles' that the mediators of laccase may have, but also suggest two promising routes towards an environmentally friendly process for kraft pulp bleaching: (a) the identification of mediators which, once oxidized by laccase, are able to target strategic functional groups present in lignin, and (b) the introduction of those strategic functional groups in an appropriate pretreatment. [source]

Efficient and Long-Living Light-Emitting Electrochemical Cells

Rubén D. Costa
Abstract Three new heteroleptic iridium complexes that combine two approaches, one leading to a high stability and the other yielding a high luminescence efficiency, are presented. All complexes contain a phenyl group at the 6-position of the neutral bpy ligand, which holds an additional, increasingly bulky substituent on the 4-position. The phenyl group allows for intramolecular ,,, stacking, which renders the complex more stable and yields long-living light-emitting electrochemical cells (LECs). The additional substituent increases the intersite distance between the cations in the film, reducing the quenching of the excitons, and should improve the efficiency of the LECs. Density functional theory calculations indicate that the three complexes have the desired ,,, intramolecular interactions between the pendant phenyl ring of the bpy ligand and the phenyl ring of one of the ppy ligands in the ground and the excited states. The photoluminescence quantum efficiency of concentrated films of the complexes improves with the increasing size of the bulky groups indicating that the adopted strategy for improving the efficiency is successful. Indeed, LEC devices employing these complexes as the primary active component show shorter turn-on times, higher efficiencies and luminances, and, surprisingly, also demonstrate longer device stabilities. [source]

The use of fyke nets as a quantitative capture technique for freshwater eels (Anguilla spp.) in rivers

Abstract, Fyke netting is a convenient and effective technique for capturing freshwater eels, and catch-per-unit-effort is usually assumed to be an index of eel abundance. The present study investigated the potential of depletion fishing using baited fyke nets to obtain population estimates of longfin eels Anguilla dieffenbachii Gray, in a river in the South Island of New Zealand. The probability of capture (capture efficiency) of a single night's fishing increased with increasing size of eel, and ranged from 0.2 to 0.9 for eels <400 mm, to 0.7,0.9 for eels ,500 mm. The capture efficiency of baited vs unbaited nets was assessed in a small stream that was electric fished after netting trials were completed. Baited nets proved to be an effective method of assessing abundance of longfin eels (>400 mm) but not shortfins (A. australis Richardson); capture efficiency of the population of longfins (the sum of fyke caught and electric fished eels) was 0.4 for a single night's fyke netting, increasing to 0.8 over four nights. Comparable efficiencies for shortfins were <0.1 and 0.3 respectively. Unbaited nets were markedly less efficient for both species. [source]

Automated application component placement in data centers using mathematical programming

Xiaoyun Zhu
In this article we address the application component placement (ACP) problem for a data center. The problem is defined as follows: for a given topology of a network consisting of switches, servers and storage devices with varying capabilities, and for a given specification of a component-based distributed application, decide which physical server should be assigned to each application component, such that the application's processing, communication and storage requirements are satisfied without creating bottlenecks in the infrastructure, and that scarce resources are used most efficiently. We explain how the ACP problem differs from traditional task assignment in distributed systems, or existing grid scheduling problems. We describe our approach of formalizing this problem using a mathematical optimization framework and further formulating it as a mixed integer program (MIP). We then present our ACP solver using GAMS and CPLEX to automate the decision-making process. The solver was numerically tested on a number of examples, ranging from a 125-server real data center to a set of hypothetical data centers with increasing size. In all cases the ACP solver found an optimal solution within a reasonably short time. In a numerical simulation comparing our solver to a random selection algorithm, our solver resulted in much more efficient use of scarce network resources and allowed more applications to be placed in the same infrastructure. Copyright © 2008 John Wiley & Sons, Ltd. [source]

Immobilisation of the Pyrphos Ligand on Soluble Hyperbranched Supports and Use in Rhodium-Catalysed Hydrogenation in Ionic Liquids

Abstract The immobilisation of the pyrphos ligand has been extended from highly symmetric dendrimers to less regular structured hyperbranched poly(ethylene imines). Cationic dendritic pyrphos-rhodium(norbornadiene) [pyrphos-Rh(NBD)] complexes have been synthesised using poly(propylene imine) (PPI) and poly(amido amine) (PAMAM) dendrimers as supports bearing between 4 and 64 potential catalytically active sites at their periphery as well as pyrphos-Rh(NBD) complexes tethered to hyperbranched poly(ethylene imines) carrying on average 9 to 139 functionalities located throughout the whole supporting structure. These immobilised systems have been used as catalysts for the hydrogenation of Z -methyl ,-acetamidocinnamate. With the PPI, PAMAM, and hyperbranched poly(ethylene imine)-bound pyrphos-Rh(NBD) omplexes as catalysts, hydrogenations were carried out in methanol, and a decrease in activity and selectivity was observed with increasing size of the macromolecular support. Furthermore, the polycationic catalysts were tested in a liquid/liquid-biphasic system consisting of the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate {[BMIM] [BF4]} and isopropyl alcohol. Using the PPI-, PAMAM-, and PEI-immobilised systems, a strong negative "dendritic effect" with increasing molecular size was observed with respect to the catalytic activity, the stereoinduction, and the recyclability. The PPI-bound rhodium(I)-complexes of generations one and two showed good reusability; even after triple recycling, with no loss of activity or selectivity being noted. It has been demonstrated that in this case the catalytic behaviour of hyperbranched systems is essentially similar to that of the related dendritic model system. [source]

Mercury accumulation in the fish community of a sub-Arctic lake in relation to trophic position and carbon sources

M. Power
Summary 1Stable isotope analysis has improved understanding of trophic relationships among biota. Coupled with contaminant analysis, stable isotope analysis has also been used for tracing the pattern and extent of biomagnification of contaminants in aquatic food webs. 2Combined analysis of nitrogen (, 15 N) and carbon (, 13 C) isotopes from fish species in a sub-Arctic lake were related to tissue mercury (Hg) concentrations to assess whether carbon sources influenced Hg accumulation in fish, in addition to trophic position. 3Statistical models were used to estimate Hg biomagnification and uptake, to elucidate Hg accumulation dynamics and to appraise the relative importance of Hg exposure routes for the fish species. 4Species Hg contamination increased as a function of trophic position (, 15 N) and was inversely related to the , 13 C signature. Species connected to the benthic food chain had lower Hg concentrations than species connected to the pelagic food chain. Species undergoing ontogenetic dietary shifts with increasing size, e.g. lake trout Salvelinus namaycush , also showed increased Hg concentrations with increasing reliance on pelagic fish as prey. 5The results indicate that both vertical (trophic) and horizontal (habitat) food web structure influence Hg concentrations in fish tissue. 6The biomagnification and uptake models indicated that contamination at the base of the food chain in the lake exceeded estimates for more southerly environments, thereby demonstrating the importance of dietary and water column Hg exposure routes in the sub-Arctic for determining Hg concentrations in fish. 7Overall, the data reported here demonstrate how a combination of ecological concepts (food webs), developing ecological methods (stable isotopes) and environmental geochemistry can combine profitably to indicate the risks of exposure to environmental contaminants. Additional studies of the dynamics of Hg accumulation in the food webs of sub-Arctic lakes are needed, particularly in the light of the estimated high biomagnification rates and the heavy reliance of Inuit communities on subsistence fish harvests. [source]

Historical biogeography of scarabaeine dung beetles

Adrian L. V. Davis
Abstract Aim, (1) To review briefly global biogeographical patterns in dung beetles (Coleoptera: Scarabaeidae: Scarabaeinae), a group whose evolutionary history has been dominated by ecological specialization to vertebrate dung in warmer climates. (2) To develop hypotheses accounting for the evolution of these patterns. Location, Six principal biogeographical regions: Palaearctic, Oriental, Afrotropical, Australasia, Neotropical, Nearctic and five outlying islands or island groups harbouring endemic genera: Caribbean, Madagascar, Mauritius, New Caledonia, New Zealand. Methods, Major patterns of tribal, generic and species distribution are investigated using cluster analysis, ordination, parsimony analysis of endemism and track analysis. Attempts are made to resolve biogeographical patterns with findings in the fields of plate tectonics, fossil and evolutionary history, plus phylogeny of both mammals and dung beetles. Results, Because of conflict between published findings, it is uncertain at what point in time density of dinosaur dung, mammal dung or both became sufficiently great to select for specialized habits in dung beetles. However, biogeographical evidence would suggest a Mesozoic origin followed by further taxonomic radiation during the Cenozoic, possibly in response to the increasing size and diversity of mammalian dung types in South America and Afro-Eurasia. Proportional generic distribution in fourteen tribes and subtribes showed four principal biogeographical patterns: (1) southerly biased Gondwanaland distribution, (2) Americas or (3) Madagascar endemism, and (4) northerly biased, Afro-Eurasian-centred distribution with limited numbers of genera also widespread in other regions. Proportional composition of faunas in eleven geographical regions indicated three principal distributional centres, East Gondwanaland fragments, Afro-Eurasia and the Americas. These patterns probably result from three principal long-term range expansion and vicariance events (Mesozoic: Gondwanaland interchange and fragmentation, Cenozoic: Afro-Eurasian/Nearctic interchange and the Great American interchange). It is suggested that old vicariance caused by the Mesozoic fragmentation of Gondwanaland leads to a high degree of regional endemism at generic or tribal level across one or more Gondwanaland tracks. In contrast, it is suggested that the more recent Cenozoic range expansions occurred primarily towards northern regions leading to endemism primarily at species level. These Cenozoic radiations were facilitated by the re-linking of continents, either because of tectonic plate movements (Africa to Eurasia in Miocene), climatically induced sea-level change (Afro-Eurasia to Nearctic in Miocene and Pleistocene), or similar coupled with orogenics (Nearctic to Neotropical in Pliocene). Speciation has followed vicariance either because of climatic change or physical barrier development. These recent range expansions probably occurred principally along an Afro-Eurasian land track to the Nearctic and Neotropical and an Americas land track northwards from the Neotropics to the Nearctic, with limited dispersal from Eurasia to Australia, probably across a sea barrier. This accounts for the overall, spatially constrained, biogeographical pattern comprising large numbers of species-poor genera endemic to a single biogeographical region and fewer more species-rich genera, many of which show wider biogeographical distributions. In most southerly regions (Australasia, Madagascar, Neotropical), faunal composition and generic endemism is primarily dominated by elements with Gondwanaland ancestry, which is consistent with the Gondwanaland origin claimed for Scarabaeinae. In Afro-Eurasia (Palaearctic, Oriental, Afrotropical), generic endemism of monophyletically derived Afro-Eurasian and widespread lineages is centred in the Afrotropical region and faunal composition is numerically dominated by Afro-Eurasian and widespread elements. In the Nearctic region, the fauna is jointly dominated by widespread elements, derived from Afro-Eurasia, and Gondwanaland and Americas elements derived from the Neotropical region. Main conclusions, Global biogeographical patterns in scarabaeine dung beetles primarily result from Mesozoic and Cenozoic range expansion events followed by vicariance, although recent dispersal to Australia may have occurred across sea barriers. Detailed phylogenetics research is required to provide data to support dispersal/vicariance hypotheses. [source]

New and old trends in chemometrics.

How to deal with the increasing data volumes in R&D&P (research, development, process modeling, production), with examples from pharmaceutical research
Abstract Chemometrics was started around 30 years ago to cope with and utilize the rapidly increasing volumes of data produced in chemical laboratories. The methods of early chemometrics were mainly focused on the analysis of data, but slowly we came to realize that it is equally important to make the data contain reliable information, and methods for design of experiments (DOE) were added to the chemometrics toolbox. This toolbox is now fairly adequate for solving most R&D problems of today in both academia and industry, as will be illustrated with a few examples. However, with the further increase in the size of our data sets, we start to see inadequacies in our multivariate methods, both in their efficiency and interpretability. Drift and non-linearities occur with time or in other directions in data space, and models with masses of coefficients become increasingly difficult to interpret and use. Starting from a few examples of some very complicated problems confronting chemical researchers today, possible extensions and generalizations of the existing chemometrics methods, as well as more appropriate preprocessing of the data before the analysis, will be discussed. Criteria such as scalability of methods to increasing size of problems and data, increasing sophistication in the handling of noise and non-linearities, interpretability of results, and relative simplicity of use will be held as important. The discussion will be made from a perspective of the evolution of the scientific methodology as driven by new technology, e.g. computers, and constrained by the limitations of the human brain, i.e. our ability to understand and interpret scientific and data analytical results. Quilt-PCA and Quilt-PLS presented here address and offer a possible solution to these problems. Copyright © 2002 John Wiley & Sons, Ltd. [source]

The evolutionary ecology of vegetative dormancy in mature herbaceous perennial plants

Richard P. Shefferson
Summary 1.,I present an evolutionary ecology interpretation of vegetative dormancy in mature herbaceous perennials. This kind of vegetative dormancy has been noted for at least 40 years, but has only recently become a topic of study. 2.,Vegetative dormancy may be considered in a life-history context. Both vegetative dormancy and mortality typically decrease with increasing size. Vegetative dormancy's relationship to reproduction is more complex, because some species increase flowering and fruiting after dormancy while others do the opposite. 3.,If vegetative dormancy is adaptive, then it is most likely a bet-hedging trait. Dormancy-prone plants are often long-lived, and in such organisms, bet-hedging traits should counter the effects of environmental stochasticity on adult survival. This adaptive context may vary by life span, because in shorter-lived plants, fitness is most sensitive to changes in reproduction rather than survival. 4.,Vegetative dormancy could evolve if the costs of sprouting ever outweigh the benefits. The benefits of sprouting include: (i) photosynthesis and (ii) the opportunity to flower and reproduce. The costs include: (i) greater chance of herbivory, (ii) greater need for limiting nutrients, and (iii) greater maintenance costs. The many losses of photosynthesis among plants suggest that these benefits may not always outweigh the costs. 5.,Vegetative dormancy may be an evolutionary step towards the loss of photosynthesis. Many non-photosynthetic plants acquire carbon from their mycorrhizal fungi. Many autotrophic, dormancy-prone plants also acquire some carbon from their mycorrhizal fungi. Further, non-photosynthetic plants often become dormant to an even greater extent than autotrophic, dormancy-prone plants. 6.Synthesis,Vegetative dormancy often occurs in clades with non-photosynthetic, myco-heterotrophic plants, with implications for the evolution of traits involved in carbon nutrition. The links between vegetative dormancy, other life-history traits, mycorrhizas and the loss of photosynthesis should provide exciting directions for further research in plant evolutionary ecology. Particularly needed is an assessment of the physiology of vegetative dormancy, including whether the mycorrhiza is a carbon source in all dormancy-prone plant species. Equally important is a better understanding of the genetic relationships among photosynthesis, myco-heterotrophy and dormancy. [source]

Long-range polar and steric effects in propionate-SG1-type alkoxyamines (SG1-CHMeCOOX): a multiparameter analysis,

Gennady Ananchenko
Abstract The effects of the substituent X on the homolysis rate constants (kd) of SG1-propionate type alkoxyamines (SG1-CHMeCOOX) are analyzed by a multiparametric equation with ,, the steric constant and ,I, the polar inductive/field Hammett constant of X. An influence of long-range polar and steric effects on kd was observed, that is, decrease in kd with increasing size of the X group and increase in kd with increasing polarity of the X group. Copyright © 2006 John Wiley & Sons, Ltd. [source]

Factors Controlling the Abundance and Size Distribution of the Phototrophic Ciliate Myrionecta rubra in Open Waters of the North Atlantic

ABSTRACT. Myrionecta rubra, a ubiquitous planktonic ciliate, has received much attention due to its wide distribution, occurrence as a red tide organism, and unusual cryptophyte endosymbiont. Although well studied in coastal waters, M. rubra is poorly examined in the open ocean. In the Irminger Basin, North Atlantic, the abundance of M. rubra was 0,5 cells/ml, which is low compared with that found in coastal areas. Distinct patchiness (100 km) was revealed by geostatistical analysis. Multiple regression indicated there was little relationship between M. rubra abundance and a number of environmental factors, with the exception of temperature and phytoplankton biomass, which influenced abundance in the spring. We also improve on studies that indicate distinct size classes of M. rubra; we statistically recognise four significantly distinct width classes (5,16, 12,23, 18,27, 21,33 ,m), which decrease in abundance with increasing size. A multinomial logistic regression revealed the main variable correlated with this size distribution was ambient nitrate concentration. Finally, we propose a hypothesis for the distribution of sizes, involving nutrients, feeding, and dividing of the endosymbiont. [source]

Career Paths and Choices in a Highly Differentiated Profession: The Position of Newly Qualified Solicitors

Andrew 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]

Survival of very small macropod pouch young followingshort-term isolation from the pouch at various environmental temperatures

David A. Taggart
There is considerable interest in the use of assisted breeding techniques as a component of conservation programmes for a variety of macropods threatened with extinction. While cross-fostering is being conducted in some programmes in captivity, little is known about the best techniques for carrying this procedure out from wild populations. In this study we compare the success of various procedures for isolating small tammar wallaby (Macropus eugenii) and brush-tailed rock wallaby (Petrogale penicillata) pouch young (0.4 g,410 g) for short periods (6 hours) as a first step towards establishing a methodology for recruiting endangered macropod pouch young directly from the wild for foster-rearing in captivity. Pouch temperature and humidity were determined. Pouch young from females in experimental groups were weighed, measured and then isolated, at 30°C, 27°C or 23°C, and 100% humidity. After isolation the weight of pouch young maintained at 30°C and 27°C had declined significantly (2,17% of body weight), whereas those held at 23°C and 100% humidity lost less than 0.5% of body weight. All young were reattached to the active teat of their mother following pouch isolation and their survival and growth monitored. Pouch young from the control group and those isolated at 23°C and 100% humidity survived and grew at normal rates. Only the largest pouch young maintained in isolation at either 30°C or 27°C survived to day 7 following reattachment. Data indicated that the proportion of weight lost by pouch young as a result of isolation decreased with increasing size and age of the pouch young. These trials indicate that it would be possible to harvest very small macropod pouch young from a species in the wild with a very high probability that they would survive short-term isolation from the pouch and reattachment to the teat. The use of this procedure in combination with cross-fostering techniques will allow small pouch young from threatened macropod species to be recruited directly from the wild into captivity, for captive breeding or to improve genetic diversity in captive colonies. It should also act to reactivate diapause embryos in these animals and hence may accelerate breeding in wild-based mothers. [source]

Comparison of the swimming performance of farmed and wild gilthead sea bream, Sparus aurata

Fatih Basaran
Abstract Farmed gilthead sea bream, Sparus aurata, frequently escape from the sea cages and interact with wild populations. The impact of these interactions on the wild populations will depend, in part, on differences in performance of the bream. This study compared the swimming performance of the wild and farmed fish in a current channel. The absolute critical swimming speed (Ucrit) increased with increasing size while the relative Ucrit decreased. Even at the same length there were noticeable performance differences between the individuals. The wild sea bream have significantly higher (P<0.05) absolute Ucrit performance (0.86±0.01 m s,1) than the farmed fish (0.79±0.01 m s,1) and significantly higher (P<0.05) relative Ucrit performance (4.52±0.05 BL s,1) than the farmed fish (4.21±0.05 BL s,1). The present study suggests that cultured sea bream may not have the ability to compete with wild sea bream in native seawaters. [source]

New Polyalkynyl Dendrons and Dendrimers: "Click" Chemistry with Azidomethylferrocene and Specific Anion and Cation Electrochemical Sensing Properties of the 1,2,3-Triazole-Containing Dendrimers

Jérémy Camponovo
Abstract Dendrimers for ion sensing: The synthesis and use of new tris-alkynyl dendrons are reported. So-called "click" reactions of the dendrimers described with azidomethylferrocene give 27-ferrocenyl, 81-ferrocenyl, and 243-ferrocenyl dendrimers. Electrochemical recognition of oxo-anions and Pd2+ cations has been compared using the three polyferrocenyl dendrimers. The synthesis and use of the new tris-alkynyl dendrons 2 to 5 are reported, including the Williamson reaction of 5 with 9-iodo (9), 27-iodo (11), and 81-iodo (12) dendritic cores to yield 27-alkynyl (13), 81-alkynyl (14), and 243-alkynyl (15) dendrimers. So-called "click" reactions of these three dendrimers with azidomethylferrocene (20) give 27-ferrocenyl (16), 81-ferrocenyl (17), and 243-ferrocenyl (18) dendrimers. Electrochemical recognition of oxo-anions (H2PO4, and ATP2,) and Pd2+ cation has been compared using the three polyferrocenyl dendrimers. Derivatization of Pt electrodes with the dendrimers for recognition becomes more facile with increasing size of the dendrimer. This first "click" dendrimer bearing 243-ferrocenyl groups is the best one in the series to obtain robust, recyclable modified Pt electrodes, whereas previous "click" ferrocenyl dendrimers have not been suitable for this purpose. Nous reportons ici la synthèse et l'utilisation de nouveaux dendrons tris-alcynes (composés 2 à 5). La réaction de Williamson entre 5 et les c,urs dendritiques polyiodés comportant 9, 27 ou 81 branches (composés 9, 11 et 12) conduit aux dendrimères poly-alcynes à 27, 81 et 243 branches respectivement (composés 13 à 15). La réaction "click" de ces dendrimères avec l'azidométhylferrocène (20) permet d'obtenir des dendrimères polyferrocéniques à 27, 81 et 243 branches (composés 16 à 18). La reconnaissance électrochimique d'oxo-anions (H2PO4,et ATP2,) et du cation Pd2+est comparée avec trois dendrimères polyferrocéniques, et l'obtention d'électrodes de Pt modifiées à l'aide de ces dendrimères pour cette reconnaissance est de plus en plus facile lorsque la taille du dendrimère augmente. Le premier dendrimère "click" comportant 243 ferrocènes est le meilleur de la série pour la modification d'électrodes de Pt. Ces électrodes sont robustes et recyclables avec ce dendrimère, alors que les dendrimères "click" précédemment publiés n'étaient pas utilisables pour cette fonction. [source]

Stereoselective Allylation of Ketones: Explanation for the Unusual Inversion of the Induced Stereochemistry in the Auxiliary-Mediated Crotylation and Pentenylation of Butanone by DFT Calculations


Abstract The solvent makes the difference: While attack at the Re face of the intermediate oxocarbenium ion becomes less energy demanding with increasing size of R,, it is the solvent effect that makes this attack the main trajectory, leading to inversed stereochemistry (see picture). Auxiliary-mediated domino crotylations and pentenylations of butanone yield homoallylic ethers with two newly formed stereogenic centers. With our norpseudoephedrine-derived auxiliary, we observed the formation of anti isomers exclusively, and the nature of the major isomer was independent of the substrate double bond geometry. Interestingly, there is a switch in induced selectivity when going from crotylation to pentenylation. Here, we present the computational rationalization for this behavior by identification of the relevant transition states (TSs), the energies of which were determined by using the B3LYP/6-31+G(d) level of theory in combination with the PCM/UAKS method to include the effects exerted by the solvent dichloromethane. To quickly narrow down the number of potentially relevant TSs from the whole set of 288 and 864 TSs for the crotylation and pentenylation, respectively, we employed a screening process based on B3LYP//AM1 energies. The predicted selectivities are in good agreement with experimentally determined ones. Furthermore, the obtained results also facilitate an explanation of the selectivities obtained in hexenylations and heptenylations. Finally, activation energies were determined that account for the significantly longer reaction times than those for the domino allylation with unsubstituted trimethylallylsilane. [source]

Current,Voltage Characteristics of a Homologous Series of Polycyclic Aromatic Hydrocarbons

Thilo Böhme Dr.
Abstract A novel alkyl-substituted polycyclic aromatic hydrocarbon (PAH) with D2h symmetry and 78 carbon atoms in the aromatic core (C78) was synthesized, thereby completing a homologous series of soluble PAH compounds with increasing size of the aromatic , system (42, 60, and 78 carbon atoms). The optical band gaps were determined by UV/Vis and fluorescence spectroscopy in solution. Scanning tunneling microscopy (STM) and spectroscopy (STS) revealed diode-like current versus voltage (I,V) characteristics through individual aromatic cores in monolayers at the interface between the solution and the basal plane of graphite. The asymmetry of the current,voltage (I,V) characteristics increases with the increasing size of the aromatic core, and the concomitantly decreasing HOMO,LUMO gap. This is attributed to resonant tunneling through the HOMO of the adsorbed molecule, and an asymmetric position of the molecular species in the tunnel junction. Consistently, submolecularly resolved STM images at negative substrate bias are in good agreement with the calculated pattern for the electron densities of the HOMOs. The analysis provides the basis for tailoring rectification with a single molecule in an STM junction. [source]

Understanding High-Resolution Spectra of Nonrigid Molecules Using Group Theory

CHEMPHYSCHEM, Issue 4 2010
Melanie Schnell Dr.
Abstract Permutation-inversion group theory has developed to become an important tool in the high-resolution spectroscopy of nonrigid molecules. This large class of molecules is very intriguing to study. Small molecules such as ammonia or Na3 are known to be nonrigid. With increasing size, however, several large-amplitude motions are possible in a molecule, and can even interact with each other. The high-resolution spectra of nonrigid molecules are known to be quite complicated and very rich in information. Details about the molecule and its internal dynamics can be extracted, such as the molecular structure, the character of the chemical bonds, and the barrier heights to internal rotation and their dependence on the chemical bonds. However, due to the nonrigidity of the molecule and the complexity of such spectra, their analysis is usually quite challenging. Theoretical methods are needed for their prediction and analysis. This Review concentrates on permutation-inversion group theory and its usefulness for the analysis of high-resolution spectra of nonrigid molecules, which is examined in more detail using different examples. In a separate section, a special aspect of molecular symmetry is discussed: the breakdown of symmetry principles. Special emphasis is placed on the breakdown of space inversion symmetry (parity violation) in chiral molecules and its possible implications in high-resolution spectroscopy. [source]