Total synthesis of (+)-crocacin A

Total synthesis of the potent antifungal and cytotoxic agent (+)-crocacin A is described. The crucial (Z)-5,6-enoic amide moiety in this molecule was built by stereoselective partial reduction of a skipped diyne precursor. The diene, thus obtained, was transformed into a silyl epoxide that was regioselectively opened with an azide ion to furnish an α-azido-β-hydroxyalkylsilane intermediate. Peterson elimination of this β-hydroxysilane component in the final step resulted in the formation of the (Z)-8,9-enamide moiety of the molecule leading to a successful completion of its total synthesis.     

The effect of frequency change on discrimination of pulse bursts in an electrocutaneous tactual vocoder

Tactual vocoders (artificial hearing systems) transduce the acoustic energy of speech into patterns of stimulation that are presented to the skin. In an electrocutaneous tactual vocoder, energy within an acoustic-frequency band is generally represented at a particular skin locus by the rate or frequency of brief electrical pulse bursts. At present little is known about tactual sensitivity for changes in the frequency of brief, bipolar electrical pulses used in several current electrocutaneous vocoder designs. Accordingly, discrimination of frequency change of electrocutaneous bipolar signals was evaluated for standard frequencies of 48, 100, 148, 200, and 248 Hz at signal durations of 50, 100, and 250 ms. Bipolar pulses (height = 10 mA, width = 13 microseconds) were presented on a single electrode placed slightly above and 8 cm to either the left or right of the navel. In a same-different task, three practiced subjects judged pairs of stimuli separated by an interstimulus interval (ISI) of 300 ms. For standard frequencies of 48 and 100 Hz, psychometric functions were similar for all subjects and all signal durations. For these frequencies, delta f/f was constant at approximately 0.3. By contrast, for standard stimuli greater than 100 Hz, the Weber fraction was found to increase dramatically as a function of both standard frequency and signal duration. In a second, similar experiment the Weber fraction for a 248-Hz standard increased as ISI was decreased below 300 ms. By contrast, ISI had little effect on sensitivity for standard stimuli of 48 and 100 Hz. Overall, these results suggest guidelines for possible intensity coding schemes for future electrocutaneous vocoder designs.     

Pyranocarbazoles from Murraya koenigii (L.) Spreng. as antimicrobial agents

The bioassay guided fractionation of methanolic extract of Murraya koenigii (L.) Spreng. leaves resulted in the isolation of seven pyranocarbazoles. These were evaluated against four bacterial strains and ten Candida sp. including two matched pair of fluconazole sensitive/resistant clinical isolates. Out of seven, three i.e. Koenine (mk279), Koenigine (mk309) and Mahanine (mk347) exhibited significant antibacterial activity MIC90 3.12–12.5 μg/mL against bacterial strains Streptococcus aureus and Klebsiella pneumonia compared with standard drug Kanamycin MIC90 12.5 μg/mL. However, only mk309 was found active against variety of Candida species MIC90 12.5–100 μg/mL. It was observed that hydroxylation at C-6 and C-7 positions in the studied pyranocarbazoles activate the bioactivity. Simultaneously, decrease in Log P value compares with ?H and ?O?CH3 substituted derivatives. The study is focused on selective antifungal and antibacterial activity of pyranocarbazoles on bacterial strains S. aureus, K. pneumonia and variety of Candida species with structure activity relationship observations.     

From Intramolecular (Circular) in an Isolated Molecule to Intermolecular Hole Delocalization in a Two‐Dimensional Solid‐State Assembly: The Case of Pillarene

To achieve long‐range charge transport/separation and, in turn, bolster the efficiency of modern photovoltaic devices, new molecular scaffolds are needed that can self‐assemble in two‐dimensional (2D) arrays while maintaining both intra‐ and intermolecular electronic coupling. In an isolated molecule of pillarene, a single hole delocalizes intramolecularly via hopping amongst the circularly arrayed hydroquinone ether rings. The crystallization of pillarene cation radical produces a 2D self‐assembly with three intermolecular dimeric (sandwich‐like) contacts. Surprisingly, each pillarene in the crystal lattice bears a fractional formal charge of +1.5. This unusual stoichiometry of oxidized pillarene in crystals arises from effective charge distribution within the 2D array via an interplay of intra‐ and intermolecular electronic couplings. This important finding is expected to help advance the rational design of efficient solid‐state materials for long‐range charge transfer.     

FHBC,a Hexa‐peri‐hexabenzocoronene–Fluorene Hybrid: A Platform for Highly Soluble,Easily Functionalizable HBCs with an Expanded Graphitic Core

Materials based upon hexa‐peri‐hexabenzocoronenes (HBCs) show significant promise in a variety of photovoltaic applications. There remains the need, however, for a soluble, versatile, HBC‐based platform, which can be tailored by incorporation of electroactive groups or groups that can prompt self‐assembly. The synthesis of a HBC–fluorene hybrid is presented that contains an expanded graphitic core that is highly soluble, resists aggregation, and can be readily functionalized at its vertices. This new HBC platform can be tailored to incorporate six electroactive groups at its vertices, as exemplified by a facile synthesis of a representative hexaaryl derivative of FHBC. Synthesis of new FHBC derivatives, containing electroactive functional groups that can allow controlled self‐assembly, may serve as potential long‐range charge‐transfer materials for photovoltaic applications.     

Crystal and molecular structure ofo-aminobenzoato (o-(p-dimethylaminobenzalidine)benzoato)(o-ammoniobenzoate)dimethyltin(IV), an example of a mixed carboxylate complex of tin

The structure of the title compound, [Me₂Sn(O₂CC₆H₄NH₂-o)(O₂CC₆H₄(N=C(H)C₆H₄NMe₂-p)-o)(^–O₂CC₆H₄NH₃-o)], has been determined at room temperature. The structure features a distorted trigonal bipyramidal coordination geometry for Sn, defined by three O atoms derived from chemically different, monodentate carboxylate ligands and two methyl groups. Crystals are monoclinic, space groupP2₁/n witha=14.718(5),b=10.981(4),c=20.321(3) Å, =110.74(2)°,Z=4; refinement on 2480 reflections gaveR=0.046.     

Synthesis and structural studies of peptides containing a glucose-derived furanoid sugar amino acid

Conformational analysis of peptides containing a glucose-derived furanoid sugar amino acid (Gaa) by detailed NMR and constrained MD studies revealed that peptides with repeating Gaa-Leu-Val units had conformational signatures very similar to those of linear homooligomers of Gaa.     

Bacterial transport in porous media: New aspects of the mathematical model

Transport of bacteria is an important aspect from scientific, industrial and environmental point of view. In this work, a one-dimensional mathematical model based on linear equilibrium adsorption of bacteria has been developed to predict bacterial transport through porous media. This model is more realistic than existing models because of its coupling both physicochemical and biological phenomena. Two important biological phenomena, the growth and decay of bacterial cells and chemotactic/chemotaxis of bacteria along with physicochemical properties have been adequately incorporated which are quite new aspects in our model. In agreement with experimental study by [D.K. Powelson, R.J. Simpson, C.P. Gerba, J. Environ. Qual. 19 (1990) 396], model simulations indicated that enhancement of breakthrough occurs due to increase in flow velocity, cell concentration, substrate concentration, respectively. It has also been found that chemo tactic has a significant effect on bacterial transport, especially under conditions of considerable substrate gradient and at low pore velocity. The importance of threshold concentration of captured cells (σ₀) on bacterial transport has also been identified which is also a new aspect in our model.