379 research outputs found
Meteorological approaches to irrigation scheduling
CER70-71DFH53.National Irrigation Symposium papers presented on November 10-13, 1970 at the Nebraska Center for Continuing Education.Includes bibliographical references
Adapting meteorological approaches to irrigation scheduling to high rainfall areas
CER70-71DFH543.National Irrigation Symposium papers presented on November 10-13, 1970 at the Nebraska Center for Continuing Education.Includes bibliographical references
Evidence for sharper than expected transition between metastable and unstable states
In mean-field theory, i.e. infinite-range interactions, the transition
between metastable and unstable states of a thermodynamic system is sharp. The
metastable and the unstable states are separated by a spinodal curve. For
systems with short-range interaction the transition between metastable and
unstable states has been thought of as gradual. We show evidence, that one can
define a sharp border between the two regions. We have analysed the lifetimes
of states by considering the relaxation trajectories following a quench. The
average lifetimes, as a function of the quench depth into the two-phase region,
shows a very sharp drop defining a limit of stability for metastable states.
Using the limit of stability we define a line similar to a spinodal in the
two-phase region
Topological interactions between ring polymers: Implications for chromatin loops
Chromatin looping is a major epigenetic regulatory mechanism in higher
eukaryotes. Besides its role in transcriptional regulation, chromatin loops
have been proposed to play a pivotal role in the segregation of entire
chromosomes. The detailed topological and entropic forces between loops still
remain elusive. Here, we quantitatively determine the potential of mean force
between the centers of mass of two ring polymers, i.e. loops. We find that the
transition from a linear to a ring polymer induces a strong increase in the
entropic repulsion between these two polymers. On top, topological interactions
such as the non-catenation constraint further reduce the number of accessible
conformations of close-by ring polymers by about 50%, resulting in an
additional effective repulsion. Furthermore, the transition from linear to ring
polymers displays changes in the conformational and structural properties of
the system. In fact, ring polymers adopt a markedly more ordered and aligned
state than linear ones. The forces and accompanying changes in shape and
alignment between ring polymers suggest an important regulatory function of
such a topology in biopolymers. We conjecture that dynamic loop formation in
chromatin might act as a versatile control mechanism regulating and maintaining
different local states of compaction and order.Comment: 12 pages, 11 figures. The article has been accepted by The Journal Of
Chemical Physics. After it is published, it will be found at
http://jcp.aip.or
Approaching equilibrium and the distribution of clusters
We investigate the approach to stable and metastable equilibrium in Ising
models using a cluster representation. The distribution of nucleation times is
determined using the Metropolis algorithm and the corresponding
model using Langevin dynamics. We find that the nucleation rate is suppressed
at early times even after global variables such as the magnetization and energy
have apparently reached their time independent values. The mean number of
clusters whose size is comparable to the size of the nucleating droplet becomes
time independent at about the same time that the nucleation rate reaches its
constant value. We also find subtle structural differences between the
nucleating droplets formed before and after apparent metastable equilibrium has
been established.Comment: 22 pages, 16 figure
De novo design of a reversible phosphorylation-dependent switch for membrane targeting
Modules that switch protein-protein interactions on and off are essential to develop synthetic biology; for example, to construct orthogonal signaling pathways, to control artificial protein structures dynamically, and for protein localization in cells or protocells. In nature, the E. coli MinCDE system couples nucleotide-dependent switching of MinD dimerization to membrane targeting to trigger spatiotemporal pattern formation. Here we present a de novo peptide-based molecular switch that toggles reversibly between monomer and dimer in response to phosphorylation and dephosphorylation. In combination with other modules, we construct fusion proteins that couple switching to lipid-membrane targeting by: (i) tethering a 'cargo' molecule reversibly to a permanent membrane 'anchor'; and (ii) creating a 'membrane-avidity switch' that mimics the MinD system but operates by reversible phosphorylation. These minimal, de novo molecular switches have potential applications for introducing dynamic processes into designed and engineered proteins to augment functions in living cells and add functionality to protocells. The ability to dynamically control protein-protein interactions and localization of proteins is critical in synthetic biological systems. Here the authors develop a peptide-based molecular switch that regulates dimer formation and lipid membrane targeting via reversible phosphorylation.The authors thank the Biochemistry Core Facility of the Max Planck Institute of Biochemistry for LC-MS and CD spectroscopy services, Stefan Pettera and Stephan Uebel for assistance with peptide synthesis and analytical HPLC, and Katharina Nakel for assistance with cloning
Dynamic Critical Behavio(u)r of a Cluster Algorithm for the Ashkin--Teller Model
We study the dynamic critical behavior of a Swendsen--Wang--type algorithm
for the Ashkin--Teller model. We find that the Li--Sokal bound on the
autocorrelation time ()
holds along the self-dual curve of the symmetric Ashkin--Teller model, but this
bound is apparently not sharp. The ratio
appears to tend to infinity either as a logarithm or as a small power (0.05
\ltapprox p \ltapprox 0.12).Comment: 51062 bytes uuencoded gzip'ed (expands to 111127 bytes Postscript); 4
pages including all figures; contribution to Lattice '9
The instability of Alexander-McTague crystals and its implication for nucleation
We show that the argument of Alexander and McTague, that the bcc crystalline
structure is favored in those crystallization processes where the first order
character is not too pronounced, is not correct. We find that any solution that
satisfies the Alexander-McTague condition is not stable. We investigate the
implication of this result for nucleation near the pseudo- spinodal in
near-meanfield systems.Comment: 20 pages, 0 figures, submitted to Physical Review
Nucleation in Systems with Elastic Forces
Systems with long-range interactions when quenced into a metastable state
near the pseudo-spinodal exhibit nucleation processes that are quite different
from the classical nucleation seen near the coexistence curve. In systems with
long-range elastic forces the description of the nucleation process can be
quite subtle due to the presence of bulk/interface elastic compatibility
constraints. We analyze the nucleation process in a simple 2d model with
elastic forces and show that the nucleation process generates critical droplets
with a different structure than the stable phase. This has implications for
nucleation in many crystal-crystal transitions and the structure of the final
state
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