Purification and some properties of three serine carboxypeptidases from Aspergillus niger

Three enzymes exhibiting peptidyl-L-amino acid hydrolase and esterase activities have been purified by immobilized metal-ion affinity chromatography and ion-exchange chromatography. The three enzymes were entirely free of the acid protease activity that normally exists along with them in the crude culture filtrates of Aspergillus niger. Although all three exo-peptidases possessed nearly identical molecular weights (ca. 140,000), isoelectric points (ca. 5.0) and other properties, their affinities for the two substrates tested, carbobenzoxy-L-Glu-L-Tyr and benzoyl L-arginine ethyl ester, differed. All three peptidases were inhibited by phenylmethanesulphonyl fluoride, indicating that they are serine carboxypeptidases. They were also inhibited by tosyl phenylalanine chloromethyl ketone, suggesting the presence of a histidyl residue in their active sites. The differences in the number of accessible histidyl residues on the enzyme surfaces could explain the differences in their retentions on Cu2+-iminodiacetate-Sepharose 6B.     

Voltammetric sensing of sulfamethoxazole using a glassy carbon electrode modified with a graphitic carbon nitride and zinc oxide nanocomposite

A voltammetric sensor is described for the determination the antibiotic sulfamethoxazole (SMZ). It is based on the use of a glassy carbon electrode (GCE) modified with a nanocomposite prepared from graphitic carbon nitride and zinc oxide (g-C₃N₄/ZnO). The nanorod-like ZnO nanostructure were synthesized sonochemically. The g-C₃N₄/ZnO nanocomposite was then prepared by mixing g-C₃N₄ with ZnO, followed by ultrasonication. The morphology and structure of the nanocomposite were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy and transmission electron microscopy. Under the optimal conditions, the response of the electrode, typically measured between 0.8 and 0.9 V (vs. Ag/AgCl), increases linearly in the 20 nM to 1.1 mM SMZ concentration range, and the lower detection limit is 6.6 nM. This is better than that of many previously reported sensors for SMZ. The modified electrode is highly selective, well reproducible and maintains its activity for at least 4 weeks. It was applied to the determination of SMZ in spiked human blood serum samples in with satisfactory results.

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Graphical abstract Schematic presentation of the voltammetric sensor for sulfamethoxazole. It consists of a glassy carbon electrode modified with a nanocomposite prepared from graphitic carbon nitride (g-C₃N₄/ZnO) that was supported with zinc oxide nanorods.

     

Synthesis of phenanthroimidazole-based four coordinate organoboron compounds

The synthesis of N,C-coordinated organoboron compounds (B1-B4) based on phenanthroimidazole described. All the compounds were characterized by various spectroscopic techniques. Single crystal X-ray diffraction analysis of the compounds B1 and B3 revealed that the vicinity around the boron atom adopts tetrahedral geometry. Except compound B4, other organoboron compounds (B1-B3) does not show solvatochromism. All our newly synthesized organoboron compounds exhibited moderate to good solution state quantum yields (0.07–0.88) and moderate solid state quantum yields.     

Structures and electron attachment properties of halomethanes (CX(n)Y(m), X=H, F; Y=Cl, Br, I; n=0,4; m=4-n)

Theoretical studies of structures of neutral molecules and their anions as well as dissociative electron attachment properties are presented for the halomethanes of general formula CX(n) Y(m); X=H, F; Y=Cl, Br, I; n=0,4; m=4-n. The dissociative electron attachment seems to be the primary process resulting in toxicity of these species. The halomethane anions containing hydrogens are formed as radical-anion adducts. When H is replaced by F, these species become true sigma* radicals. The electron affinities are computed using a variety of computational techniques including the four-order M?ller-Plesset (MP4) technique that included 250 basis functions. It is challenging to compare the computed results with experiment due to dearth of experimental data and uncertainties in the existing experimental data. Thus in certain cases larger differences are found between the computed and experimental values.     

Spectrophotometric determination of sulfur dioxide in air, using thymol blue

A simple and sensitive spectrophotometric method was developed for the determination of trace amounts of sulfur dioxide. The method is based on the reaction of SO2 with a known excess of ICI as the oxidant. The unreacted ICI iodinates thymol blue under acidic conditions. The lambdamax of thymol blue is at 545 nm under acidic conditions, and on lodination lambdamax shifts to 430 nm. This shift results in a decrease in the absorbance at 545 nm. The amount of uniodinated thymol blue present depends on the concentration of unreacted ICI, which in turn depends on the SO2 concentration. The system obeys Beer's law in the range 0-30 microg SO2 in a final volume of 25 mL, having a molar absorptivity of 3.2 x 10(4) L/mol cm with a relative standard deviation (RSD) of 2% at 24 microg SO2 (n = 10). The uniodinated dye can be extracted into 5 mL isoamyl alcohol under acidic conditions for measurement of absorbance. The extraction method obeys Beer's law in the range 0-5 microg SO2, having a molar absorpitivity of 4.16 x 10(4) L/mol x cm with an RSD of 1.9% at 4 microg SO2 (n = 10). The method has been successfully applied to the determination of atmospheric SO2.     

Schiff bases of nickel(II), copper(II) porphyrins and dibenzo-18-crown-6 interspersed bis -metal porphyrins. Protonation studies

Schiff-base (SB) derivatives of Ni(II) and Cu(II) porphyrins endowed with various amine functions (R−NH₂),n-butylamine,p-anisidine andm-nitroaniline have been prepared from corresponding formyl porphyrins. Protonation studies of these SB derivatives reveal a marked red shift of the optical absorption bands in the visible region relative to the unprotonated imines. The magnitude of the observed red shifts in the protonated derivatives, (SBH⁺) are found to depend on the electron-withdrawing or electron-donating nature of the R group of the amines. The results of the optical absorption,¹H NMR, EPR, and cyclicovoltammetric studies are illustrative of the fact that protonation of the SB derivatives results in a localized positive charge, in the periphery of the porphyrin (p) system. The dibenzo-18-crown-6 interspersedbisporphyrin schiff bases have been prepared fromtrans 4,4′-diamino dibenzo-18-crown-6 and formyl porphyrins. The protonation of these SB derivatives is found to proceed in a concerted fashion. The cation complexation studies by the crown ether entity in thebisporphyrin systems have been investigated using optical absorption, magnetic resonance and electrochemical methods. The redox characteristics of the protonated dimeric SB porphyrins reveal that the first oxidation step involves a two-electron transfer reaction. This is important in view of their possible usage in multielectron transfer reactions of biological and catalytic interest.     

Small sample in-vivo neutron activation analysis using californium sources

This work describes an in vivo neutron activation analysis facility for small samples, such as rats or human hand, using two 100 g²⁵²Cf neutron sources. The irradiation area is a cylindrical space, of 12 cm diameter and about 15 cm length, with fairly uniform neutron flux distribution. Experimental data on the reproducibility, effects of volume and other conditions for in vivo measurements are given. Comparative atomic absorption data on calcium measurements on rats are reported. The facility is now used for animal experiments as well as human hand irradiations in clinical investigations involving calcium metabolism and bone diseases.     

MONITORING LIGHT-INDUCED CHANGES IN ISOLATED, INTACT EYE LENSES

Fluorescence spectral changes occurring upon irradiation with 300 nm light have been monitored in situ in isolated, intact, whole lenses from the eyes of several species. The findings corroborate observations on other individual constituent protein molecules in the solution state, and also reveal features attributable to the supramolecular protein assembly that exists in the whole lens. Irradiation of the lens with 300 nm light causes red shifts in the tryptophan emission spectrum, suggesting alterations in the protein packing in the lens. Intermolecular energy transfer from tryptophan to one of the photoproducts, presumably N-formylkynurenine (N-FK), occurs in the condensed-phase sample. The N-FK formed is photodegraded efficiently in the lens, indicating that the photodynamic effects of endogenous N-FK might not be as severe as has been thought. Species variation in the photoevents are evident, particularly in avian lenses that contain the variant δ-crystallin as the core protein. The photoinduced changes in the near-UV circular dichroism of δ-crystallin (which is α-helical, as opposed to the β-sheet structure of α-, β-, and -γ-crystallins), isolated from chicken lenses, are remarkably different from other crystallins. Irradiation of δ-crystallin leads to a drastic reduction of circular dichroism intensity in the 250–300 nm region, whereas no changes are seen in the peptide absorption band.     

Diastereoselective Mukaiyama aldol reaction of (N-alkyl) isatins with 2-(trimethylsilyloxy)furan by using lanthanum triflate

An efficient vinylogous Mukaiyama aldol reaction (VMAR) of 2-(trimethylsilyloxy)furan with various (N-alkyl)isatins is described in the presence of lanthanum(III) triflates (5 mol %). The reaction proceeds rapidly and affords the corresponding 3-hydroxy-(5-oxo-2,5-dihydrofuran-2-yl)indolin-2-one in high yields with good diastereoselectivities (threo:erythro ratio up to ≤95:5).