Polyphosphazenes for the Stille reaction: A new type of recyclable stannyl reagents

Producción CientíficaThe random phosphazene copolymer {[N=P((CH2)7-Br)Ph]0,5[N=PMePh]0,5}n (2) and the block copolyphosphazene {[N=P((CH2)7-Br)Ph]25[N=PMePh]20}-b-[N=P(O2C12H8)]55 (5), having a branch with two randomly distributed units, have been synthesized and used as precursors for the stannyl derivatives {[N=P((CH2)7-SnBu2An)Ph]0,5[N=PMePh]0,5}n (3) and {[N=P((CH2)7-SnBu2An)Ph]25[N=PMePh]20}-b-[N=P(O2C12H8)]55 (6, An = p-MeOC6H4). Polymers 3 and 6 were tested as recyclable tin reagents in the Stille cross-coupling reaction with C6H5I, using various Pd catalysts and different experimental conditions. Polymer 6 can be recycled without significant release of tin, but its efficiency decreased after three consecutive cycles. This effect was explained by studying the self-assembly of the polymer under the same conditions used for the catalytic experiments, which evidenced the progressive coalescence of the polymeric vesicles (polymersomes) leading to stable and bigger core-shell aggregates by the attraction of the [N=P(O2C12H8)] rich membranes, thus decreasing the accessibility of the tin active centers.2017-12-15Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. VA302U13)Ministerio de Economía, Industria y Competitividad (CTQ2014-56345)Ministerio de Economía, Industria y Competitividad (CTQ2013-48406-P)Universidad de Oviedo (Project UNOV-13-EMERG-GIJON-08

Ligand exchange in gold-coated FePt nanoparticles

In this work we present the magnetic properties of gold coated FePt nanoparticles and the study of stable aqueous dispersions of FePt@Au and FePt synthesized after ligand exchange with mercaptoundecanoic acid. The particle size determined from TEM micrographs goes from 4 nm for the uncoated nanoparticles to a maximum of 10 nm for the gold coated ones indicating that the thickness of the shell ranges from 1 to 3 nm. The magnetic characterization consists in hysteresis cycles at 10 and 300 K. The results show that, at low field and room temperature, the magnetic behavior of uncoated and coated nanoparticles are surprisingly quite similar. Since the gold coated nanoparticles keep the magnetic properties of FePt and the presence of gold improves the functionalization of nanoparticles, the system is suitable for biological application. Mercaptoundecanoic ligand transfer was used to render water stable nanoparticles in a wide pH range. Transmission electron microscopy and dynamic light scattering results show the nanoparticles slightly agglomerate after ligand exchange. Fourier transform infrared spectroscopy results suggest that thiol bind to the gold atoms of the surface.Comment: Intermag 2008, conference pape

Vasculo-neuronal coupling and neurovascular coupling at the neurovascular unit: impact of hypertension

Components of the neurovascular unit (NVU) establish dynamic crosstalk that regulates cerebral blood flow and maintain brain homeostasis. Here, we describe accumulating evidence for cellular elements of the NVU contributing to critical physiological processes such as cerebral autoregulation, neurovascular coupling, and vasculo-neuronal coupling. We discuss how alterations in the cellular mechanisms governing NVU homeostasis can lead to pathological changes in which vascular endothelial and smooth muscle cell, pericyte and astrocyte function may play a key role. Because hypertension is a modifiable risk factor for stroke and accelerated cognitive decline in aging, we focus on hypertension-associated changes on cerebral arteriole function and structure, and the molecular mechanisms through which these may contribute to cognitive decline. We gather recent emerging evidence concerning cognitive loss in hypertension and the link with vascular dementia and Alzheimer’s disease. Collectively, we summarize how vascular dysfunction, chronic hypoperfusion, oxidative stress, and inflammatory processes can uncouple communication at the NVU impairing cerebral perfusion and contributing to neurodegeneration.Fil: Presa, Jessica Lorena. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Biología y Medicina Experimental. Fundación de Instituto de Biología y Medicina Experimental. Instituto de Biología y Medicina Experimental; Argentina. Augusta University Medical Center. Medical College of Georgia; Estados UnidosFil: Saravia, Flavia Eugenia. Consejo Nacional de Investigaciones Científicas y Técnicas. Instituto de Biología y Medicina Experimental. Fundación de Instituto de Biología y Medicina Experimental. Instituto de Biología y Medicina Experimental; ArgentinaFil: Bagi, Zsolt. Augusta University Medical Center. Medical College of Georgia; Estados UnidosFil: Filosa, Jessica A.. Augusta University Medical Center. Medical College of Georgia; Estados Unido

Temperature-dependence of the hyperfine interaction in the cubic phase of BaHfO₃

The temperature dependence of the hyperfine interaction in the paraelectric phase of BaHfO₃ in the temperature range from liquid-nitrogen temperature to 1350 K has been studied. The experimental results show an Abragam-Pound mechanism for the attenuation of the angular correlation. In this cubic structure the attenuation would be consistent with a relaxation mechanism that involves O-ion hopping between vacant sites in the lattice. Using the point-charge model for the estimation of the electric field gradient produced by an O vacancy in the coordination sphere of the probe ion, the process is characterized by a correlation time τ_(c) = 12 ± 1 ps at RT

Spectrum of magnetic hyperfine fields at ^111 C probe nuclei in the pseudobinary rare-earth Laves-phase compounds R_(1-x)Y_xCo₂

The spectrum of the magnetic hyperfine fields at the closed-shell probe nucleus ^111 Cd on the rare earth (R) site of the pseudobinary Laves-phase compounds R_81-x)Y_xCo₂ has been investigated by perturbed angular correlation (PAC) spectroscopy at 10 K for the rare earth R=Tb and Ho at various Y concentrations x ≤ 0.8 and for R=Gd, Dy, Er at the concentration x=0.3. Up to four components with different magnetic interaction frequencies ν^(i) _(M) could be resolved from the PAC spectra. The relative intensities of these components are in fair agreement with those of a binomial distribution of Y atoms on the four nearest neighbor (NN) R sites of the probe nucleus. For all R constituents, one finds a strictly linear relation between the number n_(R) of NN R atoms and the magnetic hyperfine frequencies: ν^(i) _(M) =ν_(M)(4Y)+∆ ν(M) Xn_(R).The frequency ν_(M)(4Y)=35(2) MHz is independent of the R constituent and of the Y concentration up to x ≤ 0.6. These properties identify ν_(M)(4Y) as the contribution of the Co 3d moments to the hyperfine interaction at the ^111 Cdsite. The frequency steps ∆ ν_(M)[≤ 0.1 ν_(M)(4Y)] reflect the spin polarization directly induced by the 4f spins at the probe nucleus. From Gd to Er, the spin polarization decreases much stronger than expected from the linear variation of the 4f spin in the heavy R series. An indirect 4f contribution caused by a dependence of the Co 3d moment on the number of R neighbors can be excluded. The relation ν_(M)^(i) = ν_(M)(4Y) + ∆ν_(M) X n_(R) then implies that the contributions of the 3d and 4f spins to the magnetic hyperfine field in RCo₂ have the same relative sign

Mexican Banking Regulation: Evidence of the Appropriateness of Government Intervention

In this paper we investigate the validity of two assumptions frequently used to justify the transformation of the Mexican banking system through federal regulation. These assumptions are: (i) the alleged existence of economies of scale, and (ii) a homogeneous structure of the Mexican banking system. Econometric evidence from banking data over the period 1980 - 1982 suggests that neither of these assumptions are likely valid. The Mexican banking system is subject to constant rather than increasing returns to scale

Plankton Taxonomic and Size Diversity of Mediterranean Brackish Ponds in Spring: Influence of Abiotic and Biotic Factors

In this study, performed in Mediterranean brackish ponds during spring season, we assessed the effects of biotic interactions and abiotic factors on the size and taxonomic structure of the phytoplankton and zooplankton. We used a taxonomic and a size diversity index as a descriptor of the community structure. We predicted that the size diversity of each trophic level would be mainly related to biotic interactions, such as size-based fish predation (in the case of zooplankton) and food resource availability (in the case of phytoplankton), whereas taxonomic diversity would be more affected by abiotic variables (e.g., conductivity, pond morphology). Our results showed a negative relationship between phytoplankton size diversity and food resource availability leading to low size diversities under food scarcity due to dominance of small species. Conductivity also negatively affected the phytoplankton size diversity, although slightly. Regarding zooplankton size diversity, none of predictors tested seemed to influence this index. Similar fish size diversities among ponds may prevent a significant effect of fish predation on size diversity of zooplankton. As expected, taxonomic diversity of phytoplankton and zooplankton was related to abiotic variables (specifically pond morphometry) rather than biotic interactions, which are usually body size dependent, especially in these species-poor brackish environments