In vitro antibacterial and time-kill assessment of crude methanolic stem bark extract of Acacia mearnsii de wild against bacteria in shigellosis

Shigellosis is an important cause of worldwide morbidity and mortality among young children and old people for which treatment with antimicrobial agents is limited. Hence, the need for curative potentials obtainable from medicinal plants becomes inevitable. This study was carried out to assess the antibacterial potentials of crude methanolic extract of the stem bark of Acacia mearnsii against some selected bacteria of clinical importance in shigellosis. The bacteria were inhibited by the extract to produce concentration dependent inhibition zones. The extract exhibited a varied degree of antibacterial activity against all the tested isolates. The MIC values for Gram negative (0.0391-0.3125) mg/mL and those of Gram positive bacteria (0.0781-0.625) mg/mL indicated that the Gram negative bacteria were more inhibited by the extract than the Gram positive bacteria. Average log reduction in viable cell count in time-kill assay ranged between -2.456 Log?? to 2.230 Log?? cfu/mL after 4 h of interaction, and between -2.921 Log?? and 1.447 Log?? cfu/mL after 8 h interaction in 1× MIC and 2× MIC of the extract. The study provided scientific justification for the use of the crude methanolic extract from the stem bark of A. mearnsii in shigellosis. The degree of the antibacterial activity indicated that the crude extract is a potential source of bioactive compounds that could be useful for the development of new antimicrobial agents capable of decreasing the burden of drug resistance and cost of management of diseases of clinical and public health importance in South Africa.     

Synthesis and properties of isomerically pure anthrabisbenzothiophenes

The synthesis of three heptacyclic heteroacenes is described, namely anthra[2,3-b:7,6-b']bis[1]benzothiophenes (ABBTs). A stepwise sequence of aldol reactions provides regiochemical control, affording only the syn-isomer. The ABBTs are characterized by X-ray crystallography, UV-vis absorption, and emission spectroscopy, as well as cyclic voltammetry. Field effect transistors based on solution-cast thin films of ABBT derivatives exhibit charge-carrier mobilities of as high as 0.013 cm(2)/(V s).     

Cruciforms' polarized emission confirms disjoint molecular orbitals and excited states

Steady-state and time-resolved polarized spectroscopy studies reveal that electronic excitation to the third excited state of 1,4-distyryl-2,5-bis(arylethynyl)benzene cruciforms results in fluorescence emission that is shifted an angle of ca. 60°. This result is consistent with quantum chemical calculations of the lowest electronic excited states and their transition dipole moments. The shift originates from the disjointed nature of the occupied molecular orbitals being localized on the different branches of the cruciforms.     

Ionic-liquid-mediated active-site control of MoS2 for the electrocatalytic hydrogen evolution reaction

The layered crystal MoS(2) has been proposed as an alternative to noble metals as the electrocatalyst for the hydrogen evolution reaction (HER). However, the activity of this catalyst is limited by the number of available edge sites. It was previously shown that, by using an imidazolium ionic liquid as synthesis medium, nanometre-size crystal layers of MoS(2) can be prepared which exhibit a very high number of active edge sites as well as a de-layered morphology, both of which contribute to HER electrocatalytic activity. Herein, it is examined how to control these features synthetically by using a range of ionic liquids as synthesis media. Non-coordinating ILs with a planar heterocyclic cation produced MoS(2) with the de-layered morphology, which was subsequently shown to be highly advantageous for HER electrocatalytic activity. The results furthermore suggest that the crystallinity, and in turn the catalytic activity, of the MoS(2) layers can be improved by employing an IL with specific solvation properties. These results provide the basis for a synthetic strategy for increasing the HER electrocatalytic activity of MoS(2) by tuning its crystal properties, and thus improving its potential for use in hydrogen production technologies.     

Tuning the photocatalytic activity of CdS nanocrystals through intermolecular interactions in ionic-liquid solvent systems

Synthetic solvent systems for the fine-tuned preparation of CdS nanocrystallites, active in visible-light photocatalytic hydrogen production, were studied. To control crystallite size and spectral properties, the CdS crystals were synthesised by using different solvent systems, containing a series of tetrabutylammonium amino carboxylate ionic liquids as the crystal-growth control agents. Six samples of CdS, all with similar physical and spectral properties, exhibited greatly varying photocatalytic activity, with the most active sample outperforming the least active one by almost 60%. To rationalise this effect, the intermolecular interactions of the synthesis solvent system with the growing CdS nanocrystallites were characterised by using the Reichart betaine dye and the E(T)(N) polarity scale. A correlation was observed between the E(T)(N) values of the solvent system and the photocatalytic activity of the CdS nanocrystallite, suggesting that the hydrogen-bond-donating ability and/or dipolarity/polarisability interactions of the solvent system led to the preferential formation of active surfaces/surface sites on the CdS crystals.     

Phenylethanoid glycosides isolated from Paraboea paniculata

Phytochemical studies of the leaves and rhizomes of Paraboea pa niculata (Gesneriaceae) are reported for the first time. Three phenylethanoid glycosides were isolated and characterized as 3,4-dihydroxyphenethyl-(3"-O-beta-D-apiofuranosyl)-beta-D-glucopyranoside, calceoralarioside E, and acteoside. These isolates exhibited weak cytotoxic activity against the K-562 cell line with a 50% of cell killing rate of 40.18 microM, 27.05 microM, and 27.24 microM, respectively. In the DPPH free radical scavenging assay, their IC50 values were determined as 75.89 microM, 25.00 microM, and 26.04 microM, respectively.     

Unexpected Course of the Reaction of 2‐Unsubstituted 1H‐Imidazole 3‐Oxides with Ethyl Acrylate

The attempted ethenylation at C(2) of 2‐unsubstituted 1H‐imidazole N‐oxides with ethyl acrylate (=prop‐2‐enoate) in the presence of Pd(OAc)₂ does not occur. In contrast to the other aromatic N‐oxides, the [2+3] cycloaddition of imidazole N‐oxides predominates, and 3‐hydroxyacrylates, isomeric with the cycloadducts, are key products for the subsequent reaction. The final products were identified as dehydrated 2+1 adducts of 1H‐imidazole N‐oxide and ethyl acrylate. The role of the catalyst is limited to the dehydration of the intermediate 3‐hydroxypropanoates to give 1H‐imidazol‐2‐yl‐substituted acrylates.     

Novel Synthesis of 2‐Alkylquinolizinium‐1‐olates and Their 1,3‐Dipolar Cycloaddition Reactions with Acetylenes

Several 2‐alkylquinolizinium‐1‐olates 9 , i.e., heterobetaines, were prepared from ketone 11 , the latter being readily available either from pyridine‐2‐carbaldehyde via a Grignard reaction, followed by oxidation with MnO₂, or from 2‐picolinic acid (=pyridine‐2‐carboxylic acid) via the corresponding Weinreb amide and subsequent Grignard reaction. Mesoionic heterobetaines such as quinolizinium derivatives have the potential to undergo cycloaddition reactions with double and triple bonds, e.g., 1,3‐dipolar cycloadditions or Diels? Alder reactions. We here report on the scope and limitations of cycloaddition reactions of 2‐alkylquinolizinium‐1‐olates 9 with electron‐poor acetylene derivatives. As main products of the reaction, 5‐oxopyrrolo[2,1,5‐de]quinolizines (=‘[2.3.3]cyclazin‐5‐ones’) 19 were formed via a regioselective [2+3] cycloaddition, and cyclohexadienone derivatives, formed via a Diels? Alder reaction, were obtained as side products. The structures of 2‐benzylquinolizinium‐1‐olate ( 9a ) and two ‘[2.3.3]cyclazin‐5‐ones’ 19i and 19l were established by X‐ray crystallography.     

Accelerating the convergence of value iteration by using partial transition functions

This work proposes an algorithm that makes use of partial information to improve the convergence properties of the value iteration algorithm in terms of the overall computational complexity. The algorithm iterates on a series of increasingly refined approximate models that converges to the true model according to an optimal linear rate, which coincides with the convergence rate of the original value iteration algorithm. The paper investigates the properties of the proposed algorithm and features a series of switchover queue examples which illustrates the efficacy of the method.