2,631 research outputs found
Enantiomerically pure β-phenylalanine analogues from α–β-phenylalanine mixtures in a single reactive extraction step
An efficient and selective method for the extraction of α-amino acids in preference over their β-isomers using PdCl2(PPh3)2 was discovered, which enables the separation of product mixtures obtained in the enantioselective enzymatic formation of β-amino acids.
Non-Markovian Persistence and Nonequilibrium Critical Dynamics
The persistence exponent \theta for the global order parameter, M(t), of a
system quenched from the disordered phase to its critical point describes the
probability, p(t) \sim t^{-\theta}, that M(t) does not change sign in the time
interval t following the quench. We calculate \theta to O(\epsilon^2) for model
A of critical dynamics (and to order \epsilon for model C) and show that at
this order M(t) is a non-Markov process. Consequently, \theta is a new
exponent. The calculation is performed by expanding around a Markov process,
using a simplified version of the perturbation theory recently introduced by
Majumdar and Sire [Phys. Rev. Lett. _77_, 1420 (1996); cond-mat/9604151].Comment: 4 pages, Revtex, no figures, requires multicol.st
Cluster Approximation for the Contact Process
The one-dimensional contact process is analyzed by a cluster approximation.
In this approach, the hierarchy of rate equations for the densities of finite
length empty intervals are truncated under the assumption that adjacent
intervals are not correlated. This assumption yields a first order phase
transition from an active state to the adsorbing state. Despite the apparent
failure of this approximation in describing the critical behavior, our approach
provides an accurate description of the steady state properties for a
significant range of desorption rates. Moreover, the resulting critical
exponents are closer to the simulation values in comparison with site
mean-field theory.Comment: 9 pages, Latex format, 2 postscript figure
Precision timing of PSR J1012+5307 and strong-field GR tests
We report on the high precision timing analysis of the pulsar-white dwarf
binary PSR J1012+5307. Using 15 years of multi-telescope data from the European
Pulsar Timing Array (EPTA) network, a significant measurement of the variation
of the orbital period is obtained. Using this ideal strong-field gravity
laboratory we derive theory independent limits for both the dipole radiation
and the variation of the gravitational constant.Comment: 3 pages, Proceedings of the 12th Marcel Grossmann Meeting on General
Relativity (MG 12
First order phase transition with a logarithmic singularity in a model with absorbing states
Recently, Lipowski [cond-mat/0002378] investigated a stochastic lattice model
which exhibits a discontinuous transition from an active phase into infinitely
many absorbing states. Since the transition is accompanied by an apparent
power-law singularity, it was conjectured that the model may combine features
of first- and second-order phase transitions. In the present work it is shown
that this singularity emerges as an artifact of the definition of the model in
terms of products. Instead of a power law, we find a logarithmic singularity at
the transition. Moreover, we generalize the model in such a way that the
second-order phase transition becomes accessible. As expected, this transition
belongs to the universality class of directed percolation.Comment: revtex, 4 pages, 5 eps figure
Improving membrane based multiplex immunoassays for semi-quantitative detection of multiple cytokines in a single sample
BACKGROUND: Inflammatory mediators can serve as biomarkers for the monitoring of the disease progression or prognosis in many conditions. In the present study we introduce an adaptation of a membrane-based technique in which the level of up to 40 cytokines and chemokines can be determined in both human and rodent blood in a semi-quantitative way. The planar assay was modified using the LI-COR (R) detection system (fluorescence based) rather than chemiluminescence and semi-quantitative outcomes were achieved by normalizing the outcomes using the automated exposure settings of the Odyssey readout device. The results were compared to the gold standard assay, namely ELISA. RESULTS: The improved planar assay allowed the detection of a considerably higher number of analytes (n = 30 and n = 5 for fluorescent and chemiluminescent detection, respectively). The improved planar method showed high sensitivity up to 17 pg/ml and a linear correlation of the normalized fluorescence intensity with the results from the ELISA (r = 0.91). CONCLUSIONS: The results show that the membrane-based technique is a semi-quantitative assay that correlates satisfactorily to the gold standard when enhanced by the use of fluorescence and subsequent semi-quantitative analysis. This promising technique can be used to investigate inflammatory profiles in multiple conditions, particularly in studies with constraints in sample sizes and/or budget
Characterization of a thermostable methylaspartate ammonia lyase from Carboxydothermus hydrogenoformans
Methylaspartate ammonia lyase (MAL; EC 4.3.1.2) catalyzes the reversible addition of ammonia to mesaconate to give (2S,3S)-3-methylaspartate and (2S,3R)-3-methylaspartate as products. MAL is of considerable biocatalytic interest because of its potential use for the asymmetric synthesis of substituted aspartic acids, which are important building blocks for synthetic enzymes, peptides, chemicals, and pharmaceuticals. Here, we have cloned the gene encoding MAL from the thermophilic bacterium Carboxydothermus hydrogenoformans Z-2901. The enzyme (named Ch-MAL) was overproduced in Escherichia coli and purified to homogeneity by immobilized metal affinity chromatography. Ch-MAL is a dimer in solution, consisting of two identical subunits (∼49 kDa each), and requires Mg(2+) and K(+) ions for maximum activity. The optimum pH and temperature for the deamination of (2S,3S)-3-methylaspartic acid are 9.0 and 70°C (k(cat) = 78 s(−1) and K(m) = 16 mM). Heat inactivation assays showed that Ch-MAL is stable at 50°C for >4 h, which is the highest thermal stability observed among known MALs. Ch-MAL accepts fumarate, mesaconate, ethylfumarate, and propylfumarate as substrates in the ammonia addition reaction. The enzyme also processes methylamine, ethylamine, hydrazine, hydroxylamine, and methoxylamine as nucleophiles that can replace ammonia in the addition to mesaconate, resulting in the corresponding N-substituted methylaspartic acids with excellent diastereomeric excess (>98% de). This newly identified thermostable MAL appears to be a potentially attractive biocatalyst for the stereoselective synthesis of aspartic acid derivatives on large (industrial) scale. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s00253-011-3615-6) contains supplementary material, which is available to authorized users
Towards Classification of Phase Transitions in Reaction--Diffusion Models
Equilibrium phase transitions are associated with rearrangements of minima of
a (Lagrangian) potential. Treatment of non-equilibrium systems requires
doubling of degrees of freedom, which may be often interpreted as a transition
from the ``coordinate'' to the ``phase'' space representation. As a result, one
has to deal with the Hamiltonian formulation of the field theory instead of the
Lagrangian one. We suggest a classification scheme of phase transitions in
reaction-diffusion models based on the topology of the phase portraits of
corresponding Hamiltonians. In models with an absorbing state such a topology
is fully determined by intersecting curves of zero ``energy''. We identify four
families of topologically distinct classes of phase portraits stable upon RG
transformations.Comment: 14 pages, 9 figure
Theory of Branching and Annihilating Random Walks
A systematic theory for the diffusion--limited reaction processes and is developed. Fluctuations are taken into account via
the field--theoretic dynamical renormalization group. For even the mean
field rate equation, which predicts only an active phase, remains qualitatively
correct near dimensions; but below a nontrivial
transition to an inactive phase governed by power law behavior appears. For
odd there is a dynamic phase transition for any which is described
by the directed percolation universality class.Comment: 4 pages, revtex, no figures; final version with slight changes, now
accepted for publication in Phys. Rev. Let
Branching annihilating random walks with parity conservation on a square lattice
Using Monte Carlo simulations we have studied the transition from an "active"
steady state to an absorbing "inactive" state for two versions of the branching
annihilating random walks with parity conservation on a square lattice. In the
first model the randomly walking particles annihilate when they meet and the
branching process creates two additional particles; in the second case we
distinguish particles and antiparticles created and annihilated in pairs. Quite
distinct critical behavior is found in the two cases, raising the question of
what determines universality in this kind of systems.Comment: 4 pages, 4 EPS figures include
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