1,529 research outputs found
Origin of Enantioselection in Chiral Alcohol Oxidation Catalyzed by Pd[(-)-sparteine]Cl2
A kinetic investigation into the origin of enantioselectivity for the Pd[(-)-sparteine]Cl2-catalyzed aerobic oxidative kinetic resolution (OKR) is reported. A mechanism to account for a newly discovered chloride dissociation from Pd[(-)-sparteine]Cl2 prior to alcohol binding is proposed. The mechanism includes (1) chloride dissociation from Pd[(-)-sparteine]Cl2 to form cationic Pd(-)-sparteine]Cl, (2) alcohol binding, (3) deprotonation of Pd-bound alcohol to form a Pd-alkoxide, and (4) Ăą-hydride elimination of Pd-alkoxide to form ketone product and a Pd-hydride. Utilizing the addition of (-)-sparteine HCl to control the [Cl-] and [H+] and the resulting derived rate law, the key microscopic kinetic and thermodynamic constants were extracted for each enantiomer of sec-phenethyl alcohol. These constants allow for the successful simulation of the oxidation rate in the presence of exogenous (-)-sparteine HCl. A rate law for oxidation of the racemic alcohol was derived that allows for the successful prediction of the experimentally measured krel values when using the extracted constants. Besides a factor of 10 difference between the relative rates of Ăą-hydride elimination for the enantiomers, the main enhancement in enantiodetermination results from a concentration effect of (-)-sparteine HCl and the relative rates of reprotonation of the diastereomeric Pd-alkoxides
Local and Remote Forcing of Denitrification in the Northeast Pacific for the Last 2,000 Years
Sedimentary ÎŽ15N (ÎŽ15Nsed) has been widely applied as a proxy for water column denitrification. When combined with additional productivity proxies, it provides insights into the driving forces behind longâterm changes in water column oxygenation. Highâresolution (~2 years) ÎŽ15Nsed and productivity proxy records (total organic carbon [TOC], Si/Ti, and Ca/Ti) from Santa Barbara Basin, California, were generated from a wellâdated Kasten core (SPR0901â03KC). These records reveal the relationship between Southern California upwelling and oxygenation over the past 2,000 years. Inconsistencies between Si/Ti (coastal upwelling proxy) and TOC (total export productivity proxy) suggest wind curl upwelling influenced Southern California primary productivity, especially during intervals of weak coastal upwelling. Coherence between ÎŽ15Nsed, TOC, and drought indicators supports a local control of ÎŽ15Nsed by atmospheric circulation, as persistent northerly winds associated with an intensified North Pacific High pressure cell lead to enhanced coastal upwelling. In the northeast Pacific, ÎŽ15Nsed is used as a water mass tracer of denitrification signals transported north from the eastern tropical North Pacific (ETNP) via the California Undercurrent. A 1,200âyear ÎŽ15Nsed record from the Pescadero slope, Gulf of California, lies between denitrifying subsurface waters in the ETNP and Southern California. During the Medieval Climate Anomaly, coherence between Pescadero and Santa Barbara Basin ÎŽ15Nsed indicates connections between ETNP and Southern California on centennial timescales. Yet an outâofâphase relationship occurred when the Aleutian Low was anomalously strong during the Little Ice Age. We suggest intensified nutrientârich subarctic water advection might have transported highâ15N nitrate into Southern California when the California Undercurrent and ETNP denitrification weakened.Key PointsWind curl upwelling contributes to Southern California primary productivity, especially during weak coastal upwelling intervalsIntensified NPH leads to stronger denitrification through enhanced coastal upwelling and reduced rainfallCalifornia receives relatively more tropical water during the Medieval Climate Anomaly and more subarctic water during the Little Ice AgePeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151806/1/palo20779_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151806/2/palo20779.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151806/3/palo20779-sup-0001-2019PA003577-SI.pd
Avoiding catastrophic failure in correlated networks of networks
Networks in nature do not act in isolation but instead exchange information,
and depend on each other to function properly. An incipient theory of Networks
of Networks have shown that connected random networks may very easily result in
abrupt failures. This theoretical finding bares an intrinsic paradox: If
natural systems organize in interconnected networks, how can they be so stable?
Here we provide a solution to this conundrum, showing that the stability of a
system of networks relies on the relation between the internal structure of a
network and its pattern of connections to other networks. Specifically, we
demonstrate that if network inter-connections are provided by hubs of the
network and if there is a moderate degree of convergence of inter-network
connection the systems of network are stable and robust to failure. We test
this theoretical prediction in two independent experiments of functional brain
networks (in task- and resting states) which show that brain networks are
connected with a topology that maximizes stability according to the theory.Comment: 40 pages, 7 figure
Noise in neurons is message-dependent
Neuronal responses are conspicuously variable. We focus on one particular
aspect of that variability: the precision of action potential timing. We show
that for common models of noisy spike generation, elementary considerations
imply that such variability is a function of the input, and can be made
arbitrarily large or small by a suitable choice of inputs. Our considerations
are expected to extend to virtually any mechanism of spike generation, and we
illustrate them with data from the visual pathway. Thus, a simplification
usually made in the application of information theory to neural processing is
violated: noise {\sl is not independent of the message}. However, we also show
the existence of {\sl error-correcting} topologies, which can achieve better
timing reliability than their components.Comment: 6 pages,6 figures. Proceedings of the National Academy of Sciences
(in press
Learning from the children : exploring preschool children's encounters with ICT at home
This paper is an account of our attempts to understand preschool children's experiences with information and communication technologies (ICT) at home. Using case study data, we focus on what we can learn from talking directly to the children that might otherwise have been overlooked and on describing and evaluating the methods we adopted to ensure that we maximised the children's contributions to the research. By paying attention to the children's perspectives we have learned that they are discriminating users of ICT who evaluate their own performances, know what gives them pleasure and who differentiate between operational competence and the substantive activities made possible by ICT
Nitrogen isotopic composition as a gauge of tumor cell anabolism-to-catabolism ratio.
Studies have suggested that cancerous tissue has a lower <sup>15</sup> N/ <sup>14</sup> N ratio than benign tissue. However, human data have been inconclusive, possibly due to constraints on experimental design. Here, we used high-sensitivity nitrogen isotope methods to assess the <sup>15</sup> N/ <sup>14</sup> N ratio of human breast, lung, and kidney cancer tissue at unprecedented spatial resolution. In lung, breast, and urothelial carcinoma, <sup>15</sup> N/ <sup>14</sup> N was negatively correlated with tumor cell density. The magnitude of <sup>15</sup> N depletion for a given tumor cell density was consistent across different types of lung cancer, ductal in situ and invasive breast carcinoma, and urothelial carcinoma, suggesting similar elevations in the anabolism-to-catabolism ratio. However, tumor <sup>15</sup> N depletion was higher in a more aggressive metaplastic breast carcinoma. These findings may indicate the ability of certain cancers to more effectively channel N towards growth. Our results support <sup>15</sup> N/ <sup>14</sup> N analysis as a potential tool for screening biopsies and assessing N metabolism in tumor cells
Epitaxially strained [001]-(PbTiO)(PbZrO) superlattice and PbTiO from first principles
The effect of layer-by-layer heterostructuring and epitaxial strain on
lattice instabilities and related ferroelectric properties is investigated from
first principles for the [001]-(PbTiO)(PbZrO) superlattice and
pure PbTiO on a cubic substrate. The results for the superlattice show an
enhancement of the stability of the monoclinic r-phase with respect to pure
PbTiO. Analysis of the lattice instabilities of the relaxed centrosymmetric
reference structure computed within density functional perturbation theory
suggests that this results from the presence of two unstable zone-center modes,
one confined in the PbTiO layer and one in the PbZrO layer, which
produce in-plane and normal components of the polarization, respectively. The
zero-temperature dielectric response is computed and shown to be enhanced not
only near the phase boundaries, but throughout the r-phase. Analysis of the
analogous calculation for pure PbTiO is consistent with this
interpretation, and suggests useful approaches to engineering the dielectric
properties of artificially structured perovskite oxides.Comment: 8 pages, 5 figure
Helping and Cooperation in Children with Autism
Helping and cooperation are central to human social life. Here, we report two studies investigating these social behaviors in children with autism and children with developmental delay. In the first study, both groups of children helped the experimenter attain her goals. In the second study, both groups of children cooperated with an adult, but fewer children with autism performed the tasks successfully. When the adult stopped interacting at a certain moment, children with autism produced fewer attempts to re-engage her, possibly indicating that they had not formed a shared goal/shared intentions with her. These results are discussed in terms of the prerequisite cognitive and motivational skills and propensities underlying social behavior
Nudging Cooperation in a Crowd Experiment
We examine the hypothesis that driven by a competition heuristic, people don't even reflect or consider whether a cooperation strategy may be better. As a paradigmatic example of this behavior we propose the zero-sum game fallacy, according to which people believe that resources are fixed even when they are not. We demonstrate that people only cooperate if the competitive heuristic is explicitly overridden in an experiment in which participants play two rounds of a game in which competition is suboptimal. The observed spontaneous behavior for most players was to compete. Then participants were explicitly reminded that the competing strategy may not be optimal. This minor intervention boosted cooperation, implying that competition does not result from lack of trust or willingness to cooperate but instead from the inability to inhibit the competition bias. This activity was performed in a controlled laboratory setting and also as a crowd experiment. Understanding the psychological underpinnings of these behaviors may help us improve cooperation and thus may have vast practical consequences to our society.Fil: Niella, Tamara. Universidad Torcuato di Tella; ArgentinaFil: Stier, Nicolas. Universidad Torcuato di Tella; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; ArgentinaFil: Sigman, Mariano. Universidad Torcuato di Tella; Argentina. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas; Argentin
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