We resequenced 876 short fragments in a sample of 96 individuals of Arabidopsis thaliana that included stock center accessions as well as a hierarchical sample from natural populations. Although A. thaliana is a selfing weed, the pattern of polymorphism in general agrees with what is expected for a widely distributed, sexually reproducing species. Linkage disequilibrium decays rapidly, within 50 kb. Variation is shared worldwide, although population structure and isolation by distance are evident. The data fail to fit standard neutral models in several ways. There is a genome-wide excess of rare alleles, at least partially due to selection. There is too much variation between genomic regions in the level of polymorphism. The local level of polymorphism is negatively correlated with gene density and positively correlated with segmental duplications. Because the data do not fit theoretical null distributions, attempts to infer natural selection from polymorphism data will require genome-wide surveys of polymorphism in order to identify anomalous regions. Despite this, our data support the utility of A. thaliana as a model for evolutionary functional genomics.
Prototropic equilibria in 4-thiouracil has been investigated by
high-level ab initio calculations. Six of the
most stable tautomers have been studied at various levels of theory
including the MP4(SDQ)/6-311G(2d,2p)//MP2/6-31G(d,p) level. At all levels of theory, the
2-keto-4-thio form is shown to be the most stable one
while the 4-thio-2-enolic form is predicted to be the second most
stable tautomer. Aqueous solvation is
shown only to destabilize the minor tautomers with an energy difference
between the two most stable tautomers
estimated to be 12.02 kcal/mol, while the corresponding difference in
the gas phase is 10.4 kcal/mol. At
MP4/6-311G(2d,2p)//MP2/6-31G(d,p) in the gas phase, the
following order of stability is observed; 4TU1 >
4TU2 > 4TU3 > 4TU4 > 4TU5 > 4TU6, while in an aqueous phase a
stability order of 4TU1 > 4TU2 >
4TU4 > 4TU3 > 4TU6 > 4TU5 is predicted. We corroborate our
theoretical findings with spectroscopic
studies of UV absorption and the luminescence spectra. Although
the UV absorption spectrum of the neutral
keto-thionic forms of 4-thiouracil in an aqueous medium is essentially
a single-component one indicating
that 4-thiouracil exists as a single tautomer in aqueous medium, the
luminescence spectrum of these compounds
in an ethanol solution was found to be a two-component one suggesting
that ethanol could play a key role in
stabilizing the minor tautomeric species in excited states. Using
combined experimental and theoretical results,
we conclude that in an aqueous medium, 4-thiouracil exists in the
2-keto-4-thio form and excludes the possible
existence of minor tautomers whereas in ethanol it could exist in two
tautomeric forms.
Ni/NiO powdered nanoparticles with average sizes 10-30 nm were prepared by a levitation-jet method involving the condensation of Ni metal vapour in a mixture of helium with various amounts of air or oxygen. The process was undertaken with the application of a DC electric field up to 6.5 kV cm À1 . The particles were characterized by X-Ray diffraction, transmission electron microscopy, BET adsorption and vibrating sample magnetometry. It was found that the intensity of the applied electric field and partial oxygen pressure correlated with the main structural and magnetic parameters of the nanoparticles, such as average particle size, residual ratio of nickel, coercivity and maximum magnetisation. The specific surface area of the particles correlated with the magnitude of the external electric field. Room-temperature hysteresis loops of weakly oxidized nanoparticles show ferromagnetic-like behaviour, whereas the strongly oxidized ones exhibit a low-field ferromagnetic feature superimposed to a paramagnetic signal, regardless of the particle size. Magnetic measurements allowed for the estimation of the residual metal Ni content in the powdered nanoparticles, which can be as low as 0.04 at.% depending on oxygen partial pressure and external electric field strength.
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