A heterocyclic analog of leucine-enkephalin

A series of di-, tri-, tetra- and pentapeptide analogs of leucine-enkephalin have been prepared in which the initial tyrosinylglycine fragment has been replaced by the 5,6-dihydro-5,5-dimethyl-2-oxo-3-phenyl-1(2H)-pyrazineacetic acid moiety.     

Optical rotation at the free surface of a nematic liquid crystal

When a small droplet of a nematic liquid crystal is placed on a horizontal glass plate in the presence of a magnetic field, the plane of polarization of light transmitted upward through the liquid crystal can be rotated. A defect usually forms preferring a direction perpendicular to the magnetic field and forming a diagonal of the droplet. This defect divides the free surface into regions giving rise to optical rotations that are clockwise and counterclockwise. It is suggested that the defect may be similar to a Helfrich splay-bend wall and the optical rotation in the regions near the defect may be explained by surface effects at the free surface of a liquid crystal.     

Chromatographic characterization of a phosphate-modified zirconia support for bio-chromatographic applications.

A phosphate-modified zirconia was investigated for its potential use as a high-performance inorganic cation-exchange support for the separation of proteins. This phosphate modification effectively blocks the sites responsible for the strong interactions of certain Lewis bases with the zirconia surface. It provides a more "bio-compatible" stationary phase, resulting in high recoveries for proteins and enzymes and retention of their enzymatic activity. The stability, loading capacity, selectivity, efficiency and separation mechanism on the phosphate-modified zirconia are reported. These studies have shown that phosphate-modified zirconia is a useful high-performance ion-exchange support for the separation of cationic proteins and for blocking the sites responsible for the high affinity of zirconia towards certain anions. This makes the phosphate modification interesting in its own right and as an intermediate stage for the development of other zirconia-based chromatographic supports.     

Manipulating the enone moiety of levoglucosenone: 1,3-Transposition reactions including ones leading to isolevoglucosenone

The manipulation of the enone moiety associated with the biomass-derived, homochiral and now abundant compound levoglucosenone (1) is described. While the trichloroacetimidates derived from the allylic alcohols 3 and 4 failed to engage in Overman-type rearrangements, certain ester derivatives reacted in the presence of Pd[0]-catalysts to give regio-isomeric mixtures of β,γ-unsaturated malonates or ketones, the structures of which were confirmed by single-crystal X-ray analyses. In other sequences involving 1,3-transposition reactions, an operationally simple means for converting compound 1 into isolevoglucosenone (2) is described.

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Column selectivity in reversed-phase liquid chromatography I. A general quantitative relationship

Retention factors k have been measured for 67 neutral, acidic and basic solutes of highly diverse molecular structure (size, shape, polarity, hydrogen bonding, pKa, etc.) on 10 different C18 columns (other conditions constant). These data have been combined with k values from a previous study (86 solutes, five different C8 and C18 columns) to develop a six-term equation for the correlation of retention as a function of solute and column. Values of k can be correlated with an accuracy of +/- 1-2% (1 standard deviation). This suggests that all significant contributions to column selectivity have been identified (and can be measured) for individual alkyl-silica columns which do not have an embedded polar group. That is, columns of the latter kind can be quantitatively characterized in terms of selectivity for use in the separation of any sample.     

Carboranethiol-modified gold surfaces. A study and comparison of modified cluster and flat surfaces

Four different carboranethiol derivatives were used to modify the surfaces of gold nanoparticles and flat gold films. The novel materials engendered from these modifications are extraordinarily stable species with surfaces that support self-assembled monolayers of 1-(HS)-1,2-C2B10H11, 1,2-(HS)2-1,2-C2B10H10, 1,12-(HS)2-1,12-C2B10H10, and 9,12-(HS)2-1,2-C2B10H10, respectively. Surprisingly, characterization of these materials revealed that a number of molecules of the carboranethiol derivatives are incorporated inside the nanoparticles. This structural feature was studied using a number of techniques, including X-ray photoelectron spectroscopy (XPS), UV-vis, and IR spectroscopies. Thermal desorption experiments show that carborane molecules detach and leave the nanoparticle surface mostly as 1,2-C2B10H10 isotopic clusters, leaving sulfur atoms bound to the gold surface. The surfaces of both the gold nanoparticles and the flat gold films are densely packed with carboranethiolate units. One carborane cluster molecule occupies an area of six to seven surface gold atoms of the nanoparticle and eight surface gold atoms of the flat film. XPS data showed that molecules of 1,12-(HS)2-1,12-C2B10H10 bind to the flat gold surface with only half of the thiol groups due to the steric demands of the icosahedral carborane skeleton. Electrochemical measurements indicate complete coverage of the modified gold surfaces with the carboranethiol molecules.     

Multipesticide residue method for fruits and vegetables: California Department of Food and Agriculture

Summary This procedure describes the CDFA method for multipesticide residue analysis of fresh fruits and vegetables. An aliquot of chopped sample is blended with acetonitrile. The resulting aqueous acetonitrile extract is filtered and cleaned up via reverse phase solid phase extraction apparatus. The pH of the filtrate is adjusted to neutral using phosphate buffer and the acetonitrile layer is separated from the aqueous layer by a salting out process. An aliquot of the acetonitrile layer is concentrated with a K-D evaporator by forming an azeotrope with n-hexane. The prepared sample is assayed for pesticide residues using GLC and HPLC. The performance of this method was evaluated by fortifying 6 representative fruits and vegetables with 7 chlorinated hydrocarbons, 7 organophosphate and 7 N-methylcarbamate pesticides at 0.1–0.2 ppm. No matrix interference was observed and the recovery of residues varied among different samples as well as different pesticides. Chlorothalonil results varied widely with irreproducible recoveries. In general, the method appears to be fast, rugged, and able to detect routinely at the 0.01 ppm level.