1,830 research outputs found
Temperature induced solubility transitions of various poly(2-oxazoline)s in ethanol-water solvent mixtures
The solution behavior of a series of poly(2-oxazoline)s with different side chains, namely methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, pentyl, hexyl, heptyl, octyl, nonyl, phenyl and benzyl, are reported in ethanol-water solvent mixtures based on turbidimetry investigations. The LCST transitions of poly(2-oxazoline) s with propyl side chains and the UCST transitions of the poly(2-oxazoline) s with more hydrophobic side chains are discussed in relation to the ethanol-water solvent composition and structure. The poly(2-alkyl-2-oxazoline) s with side chains longer than propyl only dissolved during the first heating run, which is discussed and correlated to the melting transition of the polymers
Faster k-Medoids Clustering: Improving the PAM, CLARA, and CLARANS Algorithms
Clustering non-Euclidean data is difficult, and one of the most used
algorithms besides hierarchical clustering is the popular algorithm
Partitioning Around Medoids (PAM), also simply referred to as k-medoids. In
Euclidean geometry the mean-as used in k-means-is a good estimator for the
cluster center, but this does not hold for arbitrary dissimilarities. PAM uses
the medoid instead, the object with the smallest dissimilarity to all others in
the cluster. This notion of centrality can be used with any (dis-)similarity,
and thus is of high relevance to many domains such as biology that require the
use of Jaccard, Gower, or more complex distances.
A key issue with PAM is its high run time cost. We propose modifications to
the PAM algorithm to achieve an O(k)-fold speedup in the second SWAP phase of
the algorithm, but will still find the same results as the original PAM
algorithm. If we slightly relax the choice of swaps performed (at comparable
quality), we can further accelerate the algorithm by performing up to k swaps
in each iteration. With the substantially faster SWAP, we can now also explore
alternative strategies for choosing the initial medoids. We also show how the
CLARA and CLARANS algorithms benefit from these modifications. It can easily be
combined with earlier approaches to use PAM and CLARA on big data (some of
which use PAM as a subroutine, hence can immediately benefit from these
improvements), where the performance with high k becomes increasingly
important.
In experiments on real data with k=100, we observed a 200-fold speedup
compared to the original PAM SWAP algorithm, making PAM applicable to larger
data sets as long as we can afford to compute a distance matrix, and in
particular to higher k (at k=2, the new SWAP was only 1.5 times faster, as the
speedup is expected to increase with k)
Thermoresponsive poly(2-oxazoline) block copolymers exhibiting two cloud points: complex multistep assembly behavior
Aqueous solutions of poly(2-oxazoline) block copolymers consisting of a 2-ethyl-2-oxazoline block and a block consisting of a random copolymer of 2-ethyl-2-oxazoline and 2-n-propyl-2-oxazoline (PEtOx-block-P(EtOx-stat-PropOx)) have been studied by dynamic light scattering (DLS), static light scattering (SLS), and turbidimetry. Even at temperatures significantly below the lower critical solution temperature (LCST), polymer unimers are found to coexist with a few large aggregates with an open structure. When heated, the systems exhibit an intricate transmittance behavior whereby the samples becomes visually clear again after an initial cloud point and then exhibit a second cloud point at even higher temperatures. The DLS data indicate that the aggregates formed around the first cloud point restructure and fragment into smaller micelle-like structures ascribed to further dehydration of the more hydrophobic PPropOx containing block, causing the samples to become optically clear again. The observed fragmentation is confirmed by the SLS experiments. At even higher temperatures, both blocks become hydrophobic, causing the formation of large, compact aggregates, resulting in a second cloud point
Efficient cationic ring-opening polymerization of diverse cyclic imino ethers: unexpected copolymerization behavior
The recently developed fast microwave-assisted cationic ring-opening polymerization procedure for 2-oxazolines seems to be ideally suited for slower polymerizing cyclic imino ether monomers. In this study we report the effect of the cyclic imino ether structure on the polymerization rate under exactly the same microwave-assisted conditions revealing that indeed less reactive cyclic imino ethers, including 2-oxazines as well as 4- and 5-substituted 2-oxazolines, can be polymerized to at least 50% conversion for the slowest monomer, namely 5-methyl-2-butyl-2-oxazoline, within 10 h. In addition, the copolymerization behavior of 4-ethyl-2-butyl-2-oxazoline with 2-methyl-2-oxazoline and 2-phenyl-2-oxazoline unexpectedly revealed faster incorporation of the less reactive 4-ethy1-2-buty1-2-oxazoline monomer compared to 2-phenyl-2-oxazoline due to the increased bulk of the latter monomer amplifying the sterical hindrance for polymerization onto the 4-ethyl-2-butyl-2-oxazolinium propagating species
A reverse engineering approach to the suppression of citation biases reveals universal properties of citation distributions
The large amount of information contained in bibliographic databases has
recently boosted the use of citations, and other indicators based on citation
numbers, as tools for the quantitative assessment of scientific research.
Citations counts are often interpreted as proxies for the scientific influence
of papers, journals, scholars, and institutions. However, a rigorous and
scientifically grounded methodology for a correct use of citation counts is
still missing. In particular, cross-disciplinary comparisons in terms of raw
citation counts systematically favors scientific disciplines with higher
citation and publication rates. Here we perform an exhaustive study of the
citation patterns of millions of papers, and derive a simple transformation of
citation counts able to suppress the disproportionate citation counts among
scientific domains. We find that the transformation is well described by a
power-law function, and that the parameter values of the transformation are
typical features of each scientific discipline. Universal properties of
citation patterns descend therefore from the fact that citation distributions
for papers in a specific field are all part of the same family of univariate
distributions.Comment: 9 pages, 6 figures. Supporting information files available at
http://filrad.homelinux.or
The MCM-Binding Protein ETG1 Aids Sister Chromatid Cohesion Required for Postreplicative Homologous Recombination Repair
The DNA replication process represents a source of DNA stress that causes potentially spontaneous genome damage. This effect might be strengthened by mutations in crucial replication factors, requiring the activation of DNA damage checkpoints to enable DNA repair before anaphase onset. Here, we demonstrate that depletion of the evolutionarily conserved minichromosome maintenance helicase-binding protein ETG1 of Arabidopsis thaliana resulted in a stringent late G2 cell cycle arrest. This arrest correlated with a partial loss of sister chromatid cohesion. The lack-of-cohesion phenotype was intensified in plants without functional CTF18, a replication fork factor needed for cohesion establishment. The synergistic effect of the etg1 and ctf18 mutants on sister chromatid cohesion strengthened the impact on plant growth of the replication stress caused by ETG1 deficiency because of inefficient DNA repair. We conclude that the ETG1 replication factor is required for efficient cohesion and that cohesion establishment is essential for proper development of plants suffering from endogenous DNA stress. Cohesion defects observed upon knockdown of its human counterpart suggest an equally important developmental role for the orthologous mammalian ETG1 protein
On Schubert's Problem of Characteristics
The Schubert varieties on a flag manifold G/P give rise to a cell
decomposition on G/P whose Kronecker duals, known as the Schubert classes on
G/P, form an additive base of the integral cohomology of G/P. The Schubert's
problem of characteristics asks to express a monomial in the Schubert classes
as a linear combination in the Schubert basis.
We present a unified formula expressing the characteristics of a flag
manifold G/P as polynomials in the Cartan numbers of the group G. As
application we develop a direct approach to our recent works on the Schubert
presentation of the cohomology rings of flag manifolds G/P.Comment: 27page
Two-loop Yang-Mills diagrams from superstring amplitudes
Starting from the superstring amplitude describing interactions among
D-branes with a constant world-volume field strength, we present a detailed
analysis of how the open string degeneration limits reproduce the corresponding
field theory Feynman diagrams. A key ingredient in the string construction is
represented by the twisted (Prym) super differentials, as their periods encode
the information about the background field. We provide an efficient method to
calculate perturbatively the determinant of the twisted period matrix in terms
of sets of super-moduli appropriate to the degeneration limits. Using this
result we show that there is a precise one-to-one correspondence between the
degeneration of different factors in the superstring amplitudes and
one-particle irreducible Feynman diagrams capturing the gauge theory effective
action at the two-loop level.Comment: 42 pages plus appendices, 10 figure
Mechanisms of Intragastric pH Sensing
Luminal amino acids and lack of luminal acidity as a result of acid neutralization by intragastric foodstuffs are powerful signals for acid secretion. Although the hormonal and neural pathways underlying this regulatory mechanism are well understood, the nature of the gastric luminal pH sensor has been enigmatic. In clinical studies, high pH, tryptic peptides, and luminal divalent metals (Ca2+ and Mg2+) increase gastrin release and acid production. The calcium-sensing receptor (CaSR), first described in the parathyroid gland but expressed on gastric G cells, is a logical candidate for the gastric acid sensor. Because CaSR ligands include amino acids and divalent metals, and because extracellular pH affects ligand binding in the pH range of the gastric content, its pH, metal, and nutrient-sensing functions are consistent with physiologic observations. The CaSR is thus an attractive candidate for the gastric luminal sensor that is part of the neuroendocrine negative regulatory loop for acid secretion
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