Catalytic Effect of Electric Fields on the Kemp Elimination Reactions with Neutral Bases

The effect of solvent reaction fields and oriented electric fields on the Kemp elimination reaction between methylamine or imidazole and 5-nitrobenzisoxazole has been theoretically studied. The Kemp reaction is the most widely used for the design of new enzymes. Our results, using the SMD continuous model for solvents, are in quite good agreement with the experimental fact that the rate of the analogous reaction with butylamine is one order of magnitude smaller in water than in acetonitrile. In the case of external electric fields, our results show that they can increase or decrease the energy barrier depending on the magnitude and orientation of the field. A duly oriented electric field may have a notable catalytic effect on the reaction. So, external electric fields and reaction fields due to the medium can contribute to the design of new enzymes. Several factors that must be taken into account to increase the catalytic effect are discussed.     

Synthesis of Enantiomeric Pure E-Nor-15-azayohimbines

A stereodivergent synthesis of enantiomerically pure E-nor-15-azayohimbines via condensation of tryptamine with derivatives of pyroglutamate 4 is reported. When the Pictet-Spengler reaction was induced in refluxing aqueous AcOH a nearly equimolar ratio of lactams 7 and 8 was obtained, whereas under kinetic control (TFA, room temperature) the trans-derivative 8 was the major product. In contrast, cyclization of amido acetal 12 with TsOH gave the cis-derivative 7 as preponderant component.     

Electromechanical phase transition in hexacyanometallate nanostructure (Prussian Blue)

This paper demonstrates the importance of the structural changeover in controlling the physical-chemical properties of hexacyanometalate-like materials (Prussian Blue). A meticulous in situ study of compositional variations using electroacoustic impedance techniques associated to electrogravimetric techniques in hexacyanoferrates containing K+ alkali metals reveals the existence of a nanostructural changeover coupled to a change of the magnetic properties of these electromagnetic materials. In the same way, the electroacoustic impedance techniques can be useful both in the understanding and in the in situ monitoring of the structural changeovers and the magnetic behavior of all kinds of materials.     

Quantification of the effect of nonphotochemical quenching on the determination of in vivo chl a from phytoplankton along the water column of a freshwater reservoir

Nonphotochemical quenching (NPQ) is a well-known collection of different photoprotective mechanisms of plants and algae to avoid photodamage under an excess of light energy. In order to evaluate the overall effect of NPQ processes on the fluorometric determination of in vivo Chl a from a phytoplankton community dominated by diatoms, we compared the results obtained by two different fluorometric field devices with the total concentration of extracted Chl a measured by HPLC ( in vitro Chl a ). A different set of measurements were made to assess the performance of these fluorometers at high, moderate and low irradiance conditions. The Fbbe fluorometer, which is capable of distinguishing different algal groups according to their pigment content, allowed a better determination of in vivo Chl a under high irradiance conditions, with only a 10% mean difference from the in vitro Chl a concentration. In turn, the FMII fluorometer underestimated by as much as 50% the in vitro Chl a concentration under the same light conditions. As data from both fluorometers were in accordance with the in vitro Chl a values at moderate irradiance levels, the differences observed at high irradiances were attributed to the decrease in the yield of Chl a fluorescence caused by photoprotective NPQ processes. Accordingly, we estimated the effect of NPQ processes on the in vivo Chl a determination and the results allow us to provide an equation to correct this effect when in situ fluorometric measurements are carried out under high irradiance regimes. Our results demonstrate that under certain circumstances NPQ seriously compromises the results obtained by in situ fluorometric probes and highlight the need for a cautious interpretation of field data under such environmental conditions.     

Influence of surface heterogeneity on hydrogen adsorption on activated carbons

This study presents an experimental and theoretical analysis of the effect of surface heterogeneity on the capacity of 20 commercial activated carbons to adsorb hydrogen at 77 and 258 K and for maximum pressures of 20 bar. Some of the samples have been subjected to surface modification by impregnation or by surface oxidation prior to the hydrogen adsorption measurements. All the activated carbons have been analyzed by N₂ adsorption at 77 K using the thermodynamic isotherm presented in a previous study. The hydrogen adsorption capacity of the activated carbons has been well correlated to the micropore volume and the characteristic m₂ parameter of the thermodynamic isotherm accounting for the energy heterogeneity of the material. On the basis of the model presented here, we discuss how surface heterogeneity, in addition to the adsorption strength, might affect the ability of activated carbons and related materials to adsorb hydrogen.     

Coloring planar graphs in parallel

We present NC algorithms for vertex and edge coloring planar graphs. The vertex coloring algorithm 5 colors any planar graph, and the edge coloring algorithm Δ edge colors planar graphs with Δ ≥ 23 (and Δ + 1 edge colors planar graphs with Δ < 23), where Δ is the maximum degree in the graph.     

Secret Sharing Schemes with Detection of Cheaters for a General Access Structure

In a secret sharing scheme, some participants can lie about the value of their shares when reconstructing the secret in order to obtain some illicit benefit. We present in this paper two methods to modify any linear secret sharing scheme in order to obtain schemes that are unconditionally secure against that kind of attack. The schemes obtained by the first method are robust, that is, cheaters are detected with high probability even if they know the value of the secret. The second method provides secure schemes, in which cheaters that do not know the secret are detected with high probability. When applied to ideal linear secret sharing schemes, our methods provide robust and secure schemes whose relation between the probability of cheating and the information rate is almost optimal. Besides, those methods make it possible to construct robust and secure schemes for any access structure.     

Single Higgs-boson production at a photon-photon collider: General 2HDM versus MSSM

We revisit the production of a single Higgs boson from direct γγ-scattering at a photon collider. We compute the total cross-section σ(γγ→h) (for h=h⁰,H⁰,A⁰), and the strength of the effective gh⁰γγ coupling normalized to the Standard Model (SM), for both the general Two-Higgs-Doublet Model (2HDM) and the Minimal Supersymmetric Standard Model (MSSM). In both cases the predicted production rates for the CP-even (odd) states render up to 10⁴ (10³) events per 500 fb⁻¹ of integrated luminosity, in full consistency with all the theoretical and phenomenological constraints. Depending on the channel the maximum rates can be larger or smaller than the SM expectations, but in most of the parameter space they should be well measurable. We analyze how these departures depend on the dynamics underlying each of the models, supersymmetric and non-supersymmetric, and highlight the possible distinctive phenomenological signatures. We demonstrate that this process could be extremely useful to discern non-supersymmetric Higgs bosons from supersymmetric ones. Furthermore, in the MSSM case, we show that γγ-physics could decisively help to overcome the serious impasse afflicting Higgs boson physics at the infamous “LHC wedge”.