Dielectrophoresis induced clustering regimes of viable yeast cells

We experimentally study the transient clustering behavior of viable yeast cells in a dilute suspension suddenly subjected to a nonuniform alternating current (AC) electric field of a microelectrode device. The frequency of the applied electric field is varied to identify two distinct regimes of positive dielectrophoresis. In both regimes, the yeast cells eventually cluster at electrodes' edges, but their transient behavior as well as their final arrangement is quite different. Specifically, when the frequency is much smaller than the cross-over frequency, the nearby yeast cells quickly rearrange in well-defined chains which then move toward the electrodes' edges and remain aligned as elongated chains at their final location. However, when the frequency is close to the cross-over frequency, cells move individually toward the regions of collection and simply agglomerate along the electrodes' edges. Our analysis shows that in the first regime both the dielectrophoretic (DEP) force and the mutual DEP force, which arises due to the electrostatic particle-particle interactions, are important. In the second regime, on the other hand, the DEP force dominates.     

A silaproline‐containing dipeptide

The silaproline‐containing dipeptide N‐(3,3‐di­methyl‐1‐pivaloyl‐1‐aza‐3‐sila‐5‐cyclo­pentyl­carbonyl)‐l ‐alanine iso­propyl­amide, C₁₇H₃₃N₃O₃Si, has two independent molecules in the asymmetric unit and each adopts a β‐II folded conformation, where the amide on the terminal C interacts intramolecularly with the pivaloyl O atom. The five‐membered silaproline ring is C^β‐puckered, an infrequent conformation for the homol­ogous proline ring.     

Correlating thermochemical data for gas-phase ion chemistry

This article presents a brief review of correlation schemes which relate ionization energies, ionic heats of formation, proton affinities and heterolytic bond strengths with some simple property of the ion, most commonly its size. The schemes reviewed here have mainly been developed for homologous series and simple atom or group substitutions at charge-bearing sites. The utility of such schemes is illustrated by appropriate examples.     

1,3,5-Triazepane-2,6-diones as structurally diverse and conformationally constrained dipeptide mimetics: identification of malaria liver stage inhibitors from a small pilot library

The development of the 1,3,5-triazepane-2,6-dione system as a novel, conformationally restricted, and readily accessible class of dipeptidomimetics is reported. The synthesis of the densely functionalized 1,3,5-triazepane-2,6-dione skeleton was achieved in only four steps from a variety of simple linear dipeptide precursors. To extend the practical value of 1,3,5-triazepane-2,6-diones, a general polymer-assisted solution-phase synthesis approach amenable to library production in a multiparallel format was developed. The conformational preferences of the 1,3,5-triazepane-2,6-dione skeleton were investigated in detail by NMR spectroscopy and X-ray diffraction. The ring exhibits a characteristic folded conformation which was compared to that of related dipeptide-derived scaffolds including the more planar 2,5-diketopiperazine (DKP). Molecular and structural diversity was increased further through post-cyclization appending operations at urea nitrogens. Preliminary biological screens of a small collection of 1,3,5-triazepane-2,6-diones revealed inhibitors of the underexplored malaria liver stage and suggest strong potential for this dipeptide-derived scaffold to interfere with and to modulate biological pathways.     

Electro-hydrodynamic micro-fluidic mixer

Fluid mixing in microchannels is needed for many applications ranging from bio-arrays to micro-reactors, but is typically difficult to achieve. A simple geometry micro-mixer is proposed based on the electro-hydrodynamic (EHD) force present when the fluids to be mixed have different electrical properties and are subjected to an electric field. The electrodes are arranged so that the electric field is perpendicular to the interface between the two fluids, creating a transversal secondary flow. The technique is demonstrated experimentally using the flow of two liquids with identical viscosity and density, but different electrical properties. The volume flow rate and average velocity are 0.26 microl s(-1) and 4.2 mm s(-1), respectively, corresponding to a Reynolds number Re= 0.0174. The effect of a continuous (DC) electric field and two alternating (AC)- sinusoidal and square - electric fields is explored. At the appropriate parameter values, very good mixing takes place in less than 0.1 s, over a very short distance (within a fraction of the width 250 microm of the electrodes).     

Reactivity, chemoselectivity, and diastereoselectivity of the oxyfunctionalization of chiral allylic alcohols and derivatives in microemulsions: comparison of the chemical oxidation by the hydrogen peroxide/sodium molybdate system with the photooxygenation

The chiral allylic alcohols 1a-d and their acetate (1e) and silyl ether (1f) derivatives have been oxidized by the H2O2/MoO4(2)- system, a convenient and efficient chemical source of singlet oxygen. This chemical peroxidation (formation of the allylic hydroperoxides 2) has been conducted in various media, which include aqueous solutions, organic solvents, and microemulsions. The reactivity, chemoselectivity, and diastereoselectivity of this chemical oxidation are compared to those of the sensitized photooxygenation, with the emphasis on preparative applications in microemulsion media. While a similar threo diastereoselectivity is observed for both modes of peroxidation, the chemoselectivity differs significantly, since in the chemical oxidation with the H2O2/MoO4(2)- system the undesirable epoxidation by the intermediary peroxomolybdate competes efficiently with the desirable peroxidation by the in situ generated singlet oxygen. A proper choice of the type of microemulsion and the reaction conditions furnishes a high chemoselectivity (up to 97%) in favor of threo-diastereoselective (up to 92%) peroxidation.     

Development of novel Lewis acid catalyzed cycloisomerizations: synthesis of bicyclo[3.1.0]hexenes and cyclopentenones

The Lewis acid catalyzed cyclization of hexatrienes and pentadienals to bicyclo[3.1.0]hexenes and cyclopentenones, respectively, was investigated. The application of the former reaction to the total synthesis of photodeoxytridachione, a molluscan polypropionate, is described.     

Diastereoselective hydroxylation of 6-substituted piperidin-2-ones. An efficient synthesis of (2S,5R)-5-hydroxylysine and related alpha-amino acids

The synthesis of (2S,5R)-5-hydroxy-6-oxo-1,2-piperidinedicarboxylates (5) and related (3S,6R)-3-hydroxy-6-alkyl-2-oxo-1-piperidinecarboxylates has been developed. The approach is based on the asymmetric hydroxylation of enolates generated from the corresponding N-protected-6-substituted piperidin-2-ones. The utility of 5a as a precursor in the synthesis of (2S,5R)-5-hydroxylysine (1), an amino acid unique to collagen and collagen-like proteins, has also been demonstrated. (2S)-6-oxo-1,2-piperidinedicarboxylates (6) required for hydroxylation studies were prepared in 38-74% yield, starting from conveniently protected aspartic acid as inexpensive chiral adduct. Hydroxylation of 6 to 5 proceeds in high yield and excellent diastereoselectivity by treatment of their Li-enolate with (+)-camphorsulfonyloxaziridine at -78 degrees C. Ring opening of di-tert-butyl (2S,5R)-6-oxo-1,2-piperidinedicarboxylate ((5R)-5a) under reductive conditions afforded the corresponding 1,2-diol (17) in 91%, which was further transformed to (2S,5R)-5-hydroxylysine in four steps (84%). 17 is also a versatile intermediate in the preparation of tert-butyl (2S,5R)-2-[(tert-butoxycarbonyl)amino]-5-hydroxy-6-iodohexanoate (3) and tert-butyl (2S)-2-[(tert-butoxycarbonyl)amino]-4-[(2R)-oxiranyl]butanoate (4), two amino acid derivatives used in the total synthesis of the bone collagen cross-link (+)-pyridinoline (2a).