2,895 research outputs found
Homochirality in an early peptide world
A recently proposed model of non-autocatalytic reactions in dipeptide
reactions leading to spontaneous symmetry breaking and homochirality is
examined. The model is governed by activation, polymerization, epimerization
and depolymerization of amino acids. Symmetry breaking is primarily a
consequence of the fact that the rates of reactions involving homodimers and
heterodimers are different, i.e., stereoselective, and on the fact that
epimerization can only occur on the N-terminal residue and not on the Cterminal
residue. This corresponds to an auto-inductive cyclic process that works only
in one sense. It is argued that epimerization mimics both autocatalytic
behavior as well as mutual antagonism - both of which were known to be crucial
for producing full homochirality.Comment: 19 pages, 7 figures, 3 boxe
Suzaku observations of X-ray excess emission in the cluster of galaxies A3112
We analysed the Suzaku XIS1 data of the A3112 cluster of galaxies in order to
examine the X-ray excess emission in this cluster reported earlier with the
XMM-Newton and Chandra satellites. The best-fit temperature of the intracluster
gas depends strongly on the choice of the energy band used for the spectral
analysis. This proves the existence of excess emission component in addition to
the single-temperature MEKAL in A3112. We showed that this effect is not an
artifact due to uncertainties of the background modeling, instrument
calibration or the amount of Galactic absorption. Neither does the PSF scatter
of the emission from the cool core nor the projection of the cool gas in the
cluster outskirts produce the effect. Finally we modeled the excess emission
either by using an additional MEKAL or powerlaw component. Due to the small
differencies between thermal and non-thermal model we can not rule out the
non-thermal origin of the excess emission based on the goodness of the fit.
Assuming that it has a thermal origin, we further examined the Differential
Emission Measure (DEM) models. We utilised two different DEM models, a Gaussian
differential emission measure distribution (GDEM) and WDEM model, where the
emission measure of a number of thermal components is distributed as a
truncated power law. The best-fit XIS1 MEKAL temperature for the 0.4-7.0 keV
band is 4.7+-0.1 keV, consistent with that obtained using GDEM and WDEM models.Comment: 8 pages, 10 figures, accepted to A&
Stability of the Period-Doubled Core of the 90-degree Partial in Silicon
In a recent Letter [N. Lehto and S. Oberg, Phys. Rev. Lett. 80, 5568 (1998)],
Lehto and Oberg investigated the effects of strain fields on the core structure
of the 90-degree partial dislocation in silicon, especially the influence of
the choice of supercell periodic boundary conditions in theoretical
simulations. We show that their results for the relative stability between the
two structures are in disagreement with cell-size converged tight-binding total
energy (TBTE) calculations, which suggest the DP core to be more stable,
regardless of the choice of boundary condition. Moreover, we argue that this
disagreement is due to their use of a Keating potential.Comment: 1 page. Submitted to Comments section of PRL. Also available at
http://www.physics.rutgers.edu/~dhv/preprints/rn_dcom/index.htm
Temperature affected the formation of arbuscular mycorrhizas and ectomycorrhizas in Populus angustifolia seedlings more than a mild drought
Arbuscular mycorrhizal (AM) plants and fungi associate with lower soil organic matter, higher pH, lower phosphorus and higher nitrogen than ectomycorrhizal (EM) ones. However, soil conditions correlate with climatic factors, and we suggest that temperature and humidity have also direct roles in the success of mycorrhiza types. The hypothesis here is that EM perform better at low temperatures than AM, and AM resist drought better than EM. Narrowleaf cottonwood (Populus angustifolia E. James) forms both AM and EM. We grew seedlings in soil at 14, 20 and 26 °C in factorial combinations with adequate watering and a cyclic mild drought for 4 and 7 weeks. As hypothesized, the percent of EM root tips was largest at 14 °C, while the proportional root length with AM was largest at the two higher temperatures. However, unlike expectations, drought increased EM formation slightly, while the AM colonization was lower in the dry treatment. Plant growth was reduced more by low temperature than drought. Root branching was more prominent at low temperature and root length and mass growth at higher temperatures. Soil nutrient availability did not provide a direct explanation to the results, as both soluble soil N and P were the same in 14 and 20 °C, while the change in mycorrhiza colonization took place between these temperatures. Differences in root morphology (root branching vs length) may affect the proportions of the mycorrhiza types at different temperature regimes. The most likely explanation to the differential colonization is that temperature affects AM and EM fungi in a different way. In nature, temperature and humidity regimes are tightly correlated, and temperature as such may be a stronger determinant for the success of mycorrhiza types than has been previously considered. The poorer performance of AM in low-temperature and drought conditions may reflect stress avoidance rather than stress tolerance by AM fungi.Peer reviewe
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