A NOVEL METHOD FOR FABRICATING COMPOSITE MOSAIC MEMBRANE WITH UNIQUE NF SELECTIVITY

A novel method of fabricating composite mosaic membranes was studied on the basis of interracial polymerization （IP） by coating a thin selective layer onto the surface of a micro-porous hollow-fiber membrane, in which, 2,5-diaminobenzene sulfonic acid was used as one monomer of the IP reaction, and a mixture of trimesoyl chloride （TMCI） and 4-（chloromethyl） benzoyl chloride as the other monomer. Through the IP reaction a thin selective layer with negatively charged groups could be first formed on the polyethersulfone （PES） support membrane. Then trimethylamine solution was introduced to modify the IP layer through a quaternization reaction. Thus the selective layer of this composite membrane contained both negatively charged and positively charged groups to perform the mosaic functionality. Characterization of the composite mosaic membranes was carried out through permeation experiments using different inorganic salts and dyes. The experimental results showed that the membranes could permeate both mono- and bi-valent inorganic salts, but reject larger organic molecules. Such a mosaic membrane is potentially useful for the separation of salts from water-soluble organics, especially in dye and textile industries.     

Clathration-induced asymmetric transformation of cefadroxil

[structure: see text] The cephalosporin antibiotic Cefadroxil can be epimerized at the alpha-carbon of its amino acid side chain using pyridoxal as the mediator. By clathration with 2,7-dihydroxynaphthalene, the desired diastereomer can be selectively withdrawn from the equilibrating mixture of epimers. In this way, an asymmetric transformation of Cefadroxil can be accomplished. This opens the possibility of the production of Cefadroxil starting from racemic p-hydroxyphenylglycine, in contrast to the current industrial synthesis that employs the D-amino acid in enantiopure form.     

On a class of martingale inequalities

Let Z = {Z₀, Z₁, Z₂,…} be a martingale, with difference sequence X₀ = Z₀, X_i = Z_i ? Z_{i ? 1}, i ≥ 1. The principal purpose of this paper is to prove that the best constant in the inequality λP(sup_i |X_i| ≥ λ) ≤ C sup_iE |Z_i|, for λ > 0, is C = (log 2)^?1. If Z is finite of length n, it is proved that the best constant is $\text{C}{}_{\mathrm{n}}\text{= [n(2}{}^{\mathrm{<mtext>1</mtext><mtext>n</mtext>}}\text{? 1)]}{}^{\mathrm{?1}}$. The analogous best constant C_n(z) when Z₀ ≡ z is also determined. For these finite cases, examples of martingales attaining equality are constructed. The results follow from an explicit determination of the quantity G_n(z, E) = sup_zP(max_i=1,…,n |X_i| ≥ 1), the supremum being taken over all martingales Z with Z₀ ≡ z and E|Z_n| = E. The expression for G_n(z,E) is derived by induction, using methods from the theory of moments.     

The circular bacteriocins gassericin A and circularin A

Gassericin A, a bacteriocin produced by Lactobacillus gasseri LA39, shows antibacterial activity against a number of Gram-positive food-borne pathogenic bacteria. Circularin A produced by Clostridium beijerinckii ATCC25752 is active against C. tyrobutyricum, a known cheese-spoilage bacterium. Both bacteriocins were purified to homogeneity from culture supernatants by reverse-phase chromatography and the subsequently determined amino acid sequences were used to clone the bacteriocin structural genes. Mature gassericin A and circularin A are class V circular bacteriocins comprised of 58 and 69 amino acid residues, respectively. Both bacteriocins are resistant to several peptidases and proteases, as are other cyclic bacteriocins. Heterologous expression of gassericin A in Escherichia coli was used to produce a non-cyclic mature peptide, which was shown to have a specific activity 173-fold lower than the circular molecule. The minimal region for production and secretion of active circularin A is comprised of five genes, as was deduced by heterologous gene expression in Enterococcus faecalis. Gassericin A and circularin A have limited mutual similarity in their primary sequences. Unlike most bacteriocins, including gassericin A, circularin A has a three-amino-acid-leader sequence.     

Macroporous polymeric materials for bioanalytical microchips

Macroporous copolymers derived from 2-cyanoethyl methacrylate were prepared using low-molecular-weight and polymeric porogens. The influence of the polymerization conditions on the pore characteristics and surface morphology of the macroporous materials was examined. The possibility of efficiently using the synthesized polymeric matrices for protein assay was evaluated with a model biocomplementary couple as example.     

Switchable mirrors of chiral liquid crystal gels

Chiral nematic liquid crystal compositions containing mono- and di-acrylates in mixtures with non-reactive liquid crystal molecules were produced. Defect-free planar macroscopic alignment of the molecules was induced between uniaxially rubbed substrates. Polymerization of the acrylate groups was initiated photochemically leading to the formation of a new class of chiral gels consisting of a lightly cross-linked network containing non-reactive molecules. In this way, the chiral pitch and, hence, the reflection colour became permanently fixed. Under the influence of an electric field the gels could be switched to the homeotropic state and reverted to the planar defect-free state upon removal of the electric field. Depending on the composition of the gel, either the position of the reflection band remained the same while its magnitude and/or width decreased, or its position shifted to smaller wavelengths with increasing electric field. Some of the gels showed sharp switching characteristics with a hysteresis making them suitable for passive matrix addressing with a high degree of multiplexability. The gels also showed fast switching times as they reverted to the defect-free optical state upon removal of the electric field. The phase structure of the gels was studied using dielectric spectroscopy which showed that the gels have an inhomogeneous structure.     

Hollow fiber ultrafiltration: The concept of partial backwashing

In dead-end hollow fiber filtration with humic acid solutions, we observed that the humic material accumulates in the final part of the fiber. We proved the hypothesis that backwashing of the last part of the fiber is sufficient to operate the filtration process in a sustainable manner. This strategy works very well for feed solutions that result in either local accumulation of the material at the end of the fiber, or that form a well-defined concentration polarization layer that does not lead to deposition. When deposition throughout the whole module occurs, the proposed cleaning strategy is unsuccessful. This overall deposition occurs when a force balance over the particle, results in net transport to the membrane surface over the entire length of the module. We simulated such conditions by the addition of calcium to the feed solutions. The latter results in large aggregates carried towards the whole membrane area by convective water flow, and an increased interaction between the aggregates and the membrane.