729 research outputs found
Modeling vitreous silica bilayers
We computer model a free-standing vitreous silica bilayer which has recently
been synthesized and characterized experimentally in landmark work. Here we
model the bilayer using a computer assembly procedure that starts from a single
layer of amorphous graphene, generated using a bond switching algorithm from an
initially crystalline graphene structure. Next each bond is decorated with an
oxygen atom and the carbon atoms are relabeled as silicon. This monolayer can
be now thought of as a two dimensional network of corner sharing triangles.
Next each triangle is made into a tetrahedron, by raising the silicon atom
above each triangle and adding an additional singly coordinated oxygen atom at
the apex. The final step is to mirror reflect this layer to form a second layer
and then attach the two layers together to form the bilayer.
We show that this vitreous silica bilayer has the additional macroscopic
degrees of freedom to easily form a network of identical corner sharing
tetrahedra if there is a symmetry plane through the center of the bilayer going
through the layer of oxygen ions that join the upper and lower layers. This has
the consequence that the upper rings lie exactly above the lower rings, which
are tilted in general. The assumption of a network of perfect corner sharing
tetrahedra leads to a range of possible densities that we have previously
characterized in three dimensional zeolites as a flexibility window. Finally,
using a realistic potential, we have relaxed the bilayer to determine the
density, and other structural characteristics such as the Si-Si pair
distribution functions and the Si-O-Si bond angle distribution, which are
compared to the experimental results obtained by direct imaging
Modeling large scale species abundance with latent spatial processes
Modeling species abundance patterns using local environmental features is an
important, current problem in ecology. The Cape Floristic Region (CFR) in South
Africa is a global hot spot of diversity and endemism, and provides a rich
class of species abundance data for such modeling. Here, we propose a
multi-stage Bayesian hierarchical model for explaining species abundance over
this region. Our model is specified at areal level, where the CFR is divided
into roughly one minute grid cells; species abundance is observed at
some locations within some cells. The abundance values are ordinally
categorized. Environmental and soil-type factors, likely to influence the
abundance pattern, are included in the model. We formulate the empirical
abundance pattern as a degraded version of the potential pattern, with the
degradation effect accomplished in two stages. First, we adjust for land use
transformation and then we adjust for measurement error, hence
misclassification error, to yield the observed abundance classifications. An
important point in this analysis is that only of the grid cells have been
sampled and that, for sampled grid cells, the number of sampled locations
ranges from one to more than one hundred. Still, we are able to develop
potential and transformed abundance surfaces over the entire region. In the
hierarchical framework, categorical abundance classifications are induced by
continuous latent surfaces. The degradation model above is built on the latent
scale. On this scale, an areal level spatial regression model was used for
modeling the dependence of species abundance on the environmental factors.Comment: Published in at http://dx.doi.org/10.1214/10-AOAS335 the Annals of
Applied Statistics (http://www.imstat.org/aoas/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Modular and Versatile Trans-Encoded Genetic Switches
Current bacterial RNA switches suffer from lack of versatile inputs and are difficult to engineer. We present versatile and modular RNA switches that are trans-encoded and based on tRNA-mimicking structures (TMSs). These switches provide a high degree of freedom for reengineering and can thus be designed to accept a wide range of inputs, including RNA, small molecules, and proteins. This powerful approach enables control of the translation of protein expression from plasmid and genome DNA. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGa
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