Knowledge of soil–plant transfer of naturally occurring radionuclides can be essential to assure an adequate radiological protection. Available data are mainly for anthropogenic radionuclides and biased for temperate climates. Wheat plantlets were grown using soil collected in Mediterranean regions and transfer factors, TF, for 234,238U, 226Ra, 210Po and stable elements (K, Na, Ca and Mg) were determined. U, Ra and Po were mainly located in roots. Calcium presented the highest TF values, whereas for radionuclides were much lower. Uranium TFs were correlated with total and exchangeable potassium concentration in soil. Calcium and radium TFs were correlated with total calcium concentration in soil.
Hybrid type I PKS/NRPS biosynthetic pathways typically proceed in a collinear manner wherein one molecular building block is enzymatically incorporated in a sequence that corresponds to gene arrangement. In this work, genome mining combined with the use of a fluorogenic azide‐based click probe led to the discovery and characterization of vatiamides A–F, three structurally diverse alkynylated lipopeptides, and their brominated analogues, from the cyanobacterium Moorea producens ASI16Jul14‐2. These derive from a unique combinatorial non‐collinear PKS/NRPS system encoded by a 90 kb gene cluster in which an upstream PKS cassette interacts with three separate cognate NRPS partners. This is facilitated by a series of promiscuous intermodule PKS‐NRPS docking motifs possessing identical amino acid sequences. This interaction confers a new type of combinatorial capacity for creating molecular diversity in microbial systems. 相似文献
The ionic liquid 1‐n‐butyl‐3‐methylimidazolium heptachlorodiferrate (BMI.Fe2Cl7) is efficiently used as catalyst in the cationic emulsion polymerization of styrene. The effect of different reaction temperatures, surfactant, and ionic liquid concentrations on polymer properties as molecular weight distribution and particle size is evaluated. High weight average molecular weights, above 1000 kDa, are achieved at 70% of conversion in 100 nm polystyrene particles formed mainly by micellar nucleation. Particle sizes and molecular weights increase with the decrease of the amount of surfactant. Even at low concentrations, BMI.Fe2Cl7/styrene molar ratio equal to 1/1000, the ionic liquid proves to be efficient for the emulsion polymerization of styrene, and lower ionic liquid concentrations lead to the formation of longer polymer chains. 相似文献
We review the main arguments against antigravity, a different acceleration of antimatter relative to matter in a gravitational field, discussing and challenging Morrison’s, Good’s and Schiff’s arguments. Following Price, we show that, very surprisingly, the usual expression of the Equivalence Principle is violated by General Relativity when particles of negative mass are supposed to exist, which may provide a fundamental explanation of MOND phenomenology, obviating the need for Dark Matter. Motivated by the observation of repulsive gravity under the form of Dark Energy, and by the fact that our universe looks very similar to a coasting (neither decelerating nor accelerating) universe, we study the Dirac-Milne cosmology, a symmetric matter-antimatter cosmology where antiparticles have the same gravitational properties as holes in a semiconductor. Noting the similarities with our universe (age, SN1a luminosity distance, nucleosynthesis, CMB angular scale), we focus our attention on structure formation mechanisms, finding strong similarities with our universe. Additional tests of the Dirac-Milne cosmology are briefly reviewed, and we finally note that a crucial test of the Dirac-Milne cosmology will be soon realized at CERN next to the ELENA antiproton decelerator, possibly as early as fall 2018, with the AEgIS, ALPHA-g and Gbar antihydrogen gravity experiments. 相似文献
We consider a second-degree algebraic curve describing a general conic constraint imposed on the motion of a massive spinless particle. The problem is trivial at classical level but becomes involved and interesting concerning its quantum counterpart with subtleties in its symplectic structure and symmetries. We start with a second-class version of the general conic constrained particle, which encompasses previous versions of circular and elliptical paths discussed in the literature. By applying the symplectic FJBW iteration program, we proceed on to show how a gauge invariant version for the model can be achieved from the originally second-class system. We pursue the complete constraint analysis in phase space and perform the Faddeev-Jackiw symplectic quantization following the Barcelos-Wotzasek iteration program to unravel the essential aspects of the constraint structure. While in the standard Dirac-Bergmann approach there are four second-class constraints, in the FJBW they reduce to two. By using the symplectic potential obtained in the last step of the FJBW iteration process, we construct a gauge invariant model exhibiting explicitly its BRST symmetry. We obtain the quantum BRST charge and write the Green functions generator for the gauge invariant version. Our results reproduce and neatly generalize the known BRST symmetry of the rigid rotor, clearly showing that this last one constitutes a particular case of a broader class of theories. 相似文献
The gas‐phase elimination kinetics of tetrahydropyranyl phenoxy ethers: 2‐phenoxytetrahydro‐2H‐pyran, 2‐(4‐methoxyphenoxy)tetrahydro‐2H‐pyran, and 2‐(4‐tert‐butylphenoxy)tetrahydro‐2H‐pyran were determined in a static system, with the vessels deactivated with allyl bromide, and in the presence of the free radical inhibitor toluene. The working temperature and pressure were 330 to 390°C and 25 to 89 Torr, respectively. The reactions yielded DHP and the corresponding 4‐substituted phenol. The eliminations are homogeneous, unimolecular, and satisfy a first‐order rate law. The Arrhenius equations for decompositions were found as follows:
The analysis of kinetic and thermodynamic parameters for thermal elimination of 2‐(4‐substituted‐phenoxy)tetrahydro‐2H‐pyranes suggests that the reaction proceeds via 4‐member cyclic transition state. The results obtained confirm a slight increase of rate constant with increasing electron donating ability groups in the phenoxy ring. The pyran hydrogen abstraction by the oxygen of the phenoxy group appears to be the determinant factor in the reaction rate. 相似文献
Benzyl is a resonantly stabilized radical that commonly occurs as an intermediate in the combustion of aromatic compounds. The bimolecular reaction of benzyl with HO2 is important in the oxidation of toluene, especially at low to moderate temperatures, where unimolecular decomposition of the benzyl radical is slow. We show that the addition of HO2 to the methylene site in benzyl produces a vibrationally excited benzylhydroperoxide adduct, with over 60 kcal mol−1 (251 kJ mol−1) of excess energy above the ground state. RRKM simulations are performed on the benzyl + HO2 reaction, using thermochemical and kinetic parameters obtained from ab initio calculations, with variational transition state theory (VTST) for treatment of barrierless radical + radical reaction kinetics. Our results reveal that the benzyl + HO2 reaction proceeds predominantly to the benzoxyl radical + OH at temperatures of around 800 K and above, with the production of stabilized benzylhydroperoxide molecules dominating at lower temperatures. The heat of formation of the benzyl radical is calculated as 52.5 kcal mol−1 (219.7 kJ mol−1) at the G3B3 level of theory, in relative agreement with other recent determinations of this value. 相似文献