Dynamics and bistability in a reduced model of the lac operon

It is known that the lac operon regulatory pathway is capable of showing bistable behavior. This is an important complex feature, arising from the nonlinearity of the involved mechanisms, which is essential to understand the dynamic behavior of this molecular regulatory system. To find which of the mechanisms involved in the regulation of the lac operon is the origin of bistability, we take a previously published model which accounts for the dynamics of mRNA, lactose, allolactose, permease and beta-galactosidase involvement and simplify it by ignoring permease dynamics (assuming a constant permease concentration). To test the behavior of the reduced model, three existing sets of data on beta-galactosidase levels as a function of time are simulated and we obtain a reasonable agreement between the data and the model predictions. The steady states of the reduced model were numerically and analytically analyzed and it was shown that it may indeed display bistability, depending on the extracellular lactose concentration and growth rate.     

Dynamic behavior in mathematical models of the tryptophan operon

This paper surveys the general theory of operon regulation as first formulated by Goodwin and Griffith, and then goes on to consider in detail models of regulation of tryptophan production by Bliss, Sinha, and Santillan and Mackey, and the interrelationships between them. We further give a linear stability analysis of the Santillan and Mackey model for wild type E. coli as well as three different mutant strains that have been previously studied in the literature. This stability analysis indicates that the tryptophan production systems should be stable, which is in accord with our numerical results. (c) 2001 American Institute of Physics.     

One-pot synthesis of new 6-(alkylamine)dibenzo[c,e][1,2]oxaphosphinine-6-oxides

Several new 6-(alkylamine)-6H-dibenz[c,e][1,2]oxaphosphinine-6-oxides were prepared through a one-pot reaction, starting with 2-phenylphenol, phosphorus trichloride, and a Zn catalyst, to form 6-chloro-6H-dibenz[c,e][1,2]oxaphosphine. The alkylamine derivatives were subsequently prepared through a nucleophilic substitution reaction involving aliphatic amines and H₂O₂ oxidation under soft conditions. This method has the advantages that it is a one-pot synthesis, does not require an inert atmosphere, and involves in situ catalyst formation.     

Structural evolution of zeolitic imidazolate framework-8

We report the structural evolution of zeolitic imidazolate framework-8 (ZIF-8) as a function of time at room temperature. We have identified the different stages of ZIF-8 formation (nucleation, crystallization, growth, and stationary periods) and elucidated its kinetics of transformation. We hypothesize that the observed semicrystalline-to-crystalline transformation may take place via solution- and solid-mediated mechanisms, as suggested by the observed phase transformation evolution and Avrami's kinetics, respectively. A fundamental understanding of ZIF-8 structural evolution as demonstrated in this study should facilitate the preparation of functional metal-organic framework phases with controlled crystal size and extent of crystallinity.     

Compact cancer biomarkers discovery using a swarm intelligence feature selection algorithm

Biomarker discovery is a typical application from functional genomics. Due to the large number of genes studied simultaneously in microarray data, feature selection is a key step. Swarm intelligence has emerged as a solution for the feature selection problem. However, swarm intelligence settings for feature selection fail to select small features subsets. We have proposed a swarm intelligence feature selection algorithm based on the initialization and update of only a subset of particles in the swarm. In this study, we tested our algorithm in 11 microarray datasets for brain, leukemia, lung, prostate, and others. We show that the proposed swarm intelligence algorithm successfully increase the classification accuracy and decrease the number of selected features compared to other swarm intelligence methods.     

Alumina-supported SAPO-34 membranes for CO2/CH4 separation

SAPO-34 membranes were prepared by in situ crystallization on alpha-Al2O3 porous supports. The crystal size of the seeds was effectively controlled in the 0.7 to 8.5 micron range by employing different structure-directing agents. Seeds smaller than 1 micron produced membranes with CO2/CH4 separation selectivities higher than 170 and unprecedented CO2 permeances as high as 2.0 x 10(-6) mol/m2.s.Pa at 295 K and a feed pressure of 224 kPa. The membranes effectively separated CO2/CH4 mixtures up to 1.7 MPa.     

Characterizing the Hilbert transform by the Bedrosian theorem

It is proved that a bounded linear translation invariant operator on L²(Rd) satisfies the Bedrosian theorem if and only if it is a linear combination of the compositions of the partial Hilbert transforms and the identity operator. This observation justifies a definition of multidimensional analytic signals in the papers [T. Bulow, G. Sommer, Hypercomplex signals—a novel extension of the analytic signal to the multidimensional case, IEEE Trans. Signal Process. 49 (2001) 2844-2852] and [S.L. Hahn, Multidimensional complex signals with single-orthant spectra, Proc. IEEE 80 (1992) 1287-1300].     

Single‐Step Sulfo‐Selenization Method to Synthesize Cu2ZnSn(SySe1−y)4 Absorbers from Metallic Stack Precursors

Pentenary Cu₂ZnSn(S_ySe_1?y)₄ (kesterite) photovoltaic absorbers are synthesized by a one‐step annealing process from copper‐poor and zinc‐rich precursor metallic stacks prepared by direct‐current magnetron sputtering deposition. Depending on the chalcogen source—mixtures of sulfur and selenium powders, or selenium disulfide—as well as the annealing temperature and pressure, this simple methodology permits the tuning of the absorber composition from sulfur‐rich to selenium‐rich in one single annealing process. The impact of the thermal treatment variables on chalcogenide incorporation is investigated. The effect of the S/(S+Se) compositional ratio on the structural and morphological properties of the as‐grown films, and the optoelectronic parameters of solar cells fabricated using these absorber films is studied. Using this single‐step sulfo‐selenization method, pentenary kesterite‐based devices with conversion efficiencies up to 4.4 % are obtained.     

A Semi-Analytical Solution for Countercurrent Spontaneous Imbibition in Water-Wet Fractured Reservoirs

Transport in Porous Media - Countercurrent spontaneous imbibition (SI) is an important flow mechanism for oil recovery in fractured reservoirs during waterflooding. SI plays a key role in the...