Phenomenological models for matrix-assisted laser desorption ion yields near the threshold fluence

Phenomenological models were proposed to explain the experimentally observed dependence of protein ion yields with laser fluence in matrix-assisted laser desorption. Assuming that the illuminating laser had a Gaussian intensity profile at the sample being examined, it was possible to fit the experimental points with a model that only assumes a fluence threshold for ion production. No additional dependence of protein ion yield on illuminating fluence above the threshold value was necessary to explain the data.     

Optimal addition of knots to cubature formulae for planar regions

Summary A method is described to add knots to a cubature formula of degree 2k–1 for an integral over a symmetric region, to obtain a cubature formula of degree 2k+1. This method is used to construct cubature formulae for the square, the circle, the hexagon and the entire plane.     

The biosynthesis of aristeromycin. Conversion of neplanocin A to aristeromycin by a novel enzymatic reduction

Partially purified cell-free extracts of the aristeromycin producer Streptomyces citricolor have been shown to catalyze the NADPH-dependent reduction of neplanocin A to aristeromycin. Stereochemical studies revealed that the reduction proceeds with anti geometry and involves transfer of the 4 pro-R hydrogen atom of NADPH to the 6′β position of aristeromycin.     

Enantiomeric impurity profiling in ephedrine samples by enantioselective capillary electrochromatography

This study reports on the development and preliminary validation of a capillary electrochromatographic (CEC) method for the enantioselective impurity profiling of D-ephedrine. As chiral selector a novel low-molecular-weight strong chiral cation exchanger, based on penicillamine sulfonic acid, immobilized on thiol-modified silica particles (3.5 microm) was employed. Under optimized conditions, the ephedrine enantiomers were separated on this chiral stationary phase (CSP) with an enantioselectivity of 1.11, an average efficiency of 321 550 plates per meter, and a resolution value of 4.77. A preliminary method validation was carried out to demonstrate the applicability of CEC for enantiomeric excess (ee) determination. Run-to-run repeatabilities (n = 5) reached relative standard deviation values (RSD) of 0.18 and 0.19% for the migration times of L- and D-enantiomer, respectively, 0.3% for the resolution, and about 0.9% for the peak efficiencies. An approach called self-internal standard method was utilized to measure a standard calibration curve. Excellent linearity with a correlation coefficient of R(2) = 0.9998 was found for samples with concentrations in the range between 0.03 and 5 mg.mL(-1) D-ephedrine spiked with L-ephedrine at a constant concentration of 0.2 mg.mL(-1). The high loadability of the investigated CSP and good peak sensitivity allowed us to determine less than 0.1% enantiomeric impurity with good accuracy. The limit of detection (LOD) for the L-enantiomer in a 3 mg.mL(-1) D-ephedrine solution was found to be 0.035% (S/N = 3) and the limit of quantitation (LOQ) 0.058% (S/N = 5). For L-ephedrine samples the strong cation-exchange (SCX)-type CSP with opposite configuration was utilized so that the enantiomeric impurity eluted before the main component peak yielding similar results in terms of separation and validation. Based on these results, the presented nonaqueous CEC methods are assessed as principally suitable for ee determination of ephedrine in terms of repeatability and method sensitivity.     

Limitations of the geometrodynamic clock

We examine sources of error in the geometrodynamic (or Marzke-Wheeler) clock, and choose parameters to minimize the total error. In theories with time-varying masses, there is an unavoidable minimum error. For a human-scale clock, the dominant error is from quantum uncertainty in the photon location.     

Homotopy theory, and change of base for groupoids and multiple groupoids

This survey article shows how the notion of change of base, used in some applications to homotopy theory of the fundamental groupoid, has surprising higher dimensional analogues, through the use of certain higher homotopy groupoids with values in forms of multiple groupoids.