Biosorption of americium-241 by Saccharomyces cerevisiae

The biosorption of radionuclide ²⁴¹Am from solution by Saccharomyces cerevisiae (S. cerevisiae), and the effects of experimental conditions on the adsorption were investigated. The preliminary results showed thatS. cerevisiae is a very efficient biosorbent. An average of more than 99% of the total ²⁴¹Am could be removed by S. cerevisiae of 2.1 g/l (dry weight) from ²⁴¹Am solutions of 17.54–4386.0 mg/l (2.22 MBq/l–555 MBq/l) with adsorption capacities of 7.45–1880.0 mg/g biomass (dry weight) (0.94 MBq/g–237.9 MBq/g). The adsorption equilibrium was achieved within 1 hour and the optimum pH ranged 1–3. No significant differences on ²⁴¹Am adsorption were observed at 10–45 °C, or in solutions containing Au³⁺ or Ag⁺, even 2000 times above ²⁴¹Am concentration. The relationship between concentrations and adsorption capacities of ²⁴¹Am indicated the biosorption process should be described by the Freundlich adsorption isotherm.     

Synthesis and optical resolution of a series of inherently chiral calix[4]crowns with cone and partial cone conformations

A series of inherently chiral calix[4]arenes with cone and partial cone conformations and with crown ether moieties of variable size have been readily synthesized. By taking advantage of the carboxy appendage on the lower rim, these were condensed with the chiral auxiliary (S)-BINOL to form diastereomers which, in most cases, could be separated by preparative TLC, or more desirably, by column chromatography on silica gel (diastereomeric excess >99 % based on HPLC analysis). Seven enantiopure antipodes of inherently chiral calix[4]crowns were obtained after hydrolysis. It has been found that both the size of the crown moiety and alkylation of the last phenolic hydroxy group (accompanied with or without a change in the conformation) affect the separation of the diastereomers.     

On the convergence of the affine-scaling algorithm

The affine-scaling algorithm, first proposed by Dikin, is presently enjoying great popularity as a potentially effective means of solving linear programs. An outstanding question about this algorithm concerns its convergence in the presence of degeneracy. In this paper, we give new convergence results for this algorithm that do not require any non-degeneracy assumption on the problem. In particular, we show that if the stepsize choice of either Dikin or Barnes or Vanderbei, et al. is used, then the algorithm generates iterates that converge at least linearly with a convergence ratio of , wheren is the number of variables and (0,1] is a certain stepsize ratio. For one particular stepsize choice which is an extension of that of Barnes, we show that the cost of the limit point is within O(/(1–)) of the optimal cost and, for sufficiently small (roughly, proportional to how close the cost of the nonoptimal vertices are to the optimal cost), is exactly optimal. We prove the latter result by using an unusual proof technique, that of analyzing the ergodic convergence of the corresponding dual vectors. For the special case of network flow problems with integer data, we show that it suffices to take = 1/(6mC), wherem is the number of constraints andC is the sum of the cost coefficients, to attain exact optimality.This research is partially supported by the U.S. Army Research Office, contract DAAL03-86-K-0171 (Center for Intelligent Control Systems), by the National Science Foundation, grant NSF-ECS-8519058, and by the Science and Engineering Research Board of McMaster University.     

Rapid determination of underivatized pyroglutamic acid, glutamic acid, glutamine and other relevant amino acids in fermentation media by LC-MS-MS

Determination of amino acids in a complex matrix without derivatization is advantageous, however, difficulties are found in both the detection and the separation of those compounds. In this study, a rapid and reliable LC-MS-MS method for the quantitation of underivatized amino acids in exocellular media was established. Injections were made directly after centrifugation of the samples, without further preparation. The separation of seven underivatized amino acids was achieved on a reversed-phase C18 column with pentadecafluorooctanoic acid as a volatile ion-pair reagent, and the specific detection of most amino acids was achieved by MS-MS of the specific transitions [M + H]+-->[M + H - 46]+. The calibration curves of all analytes were linear over the range of 1.0-1000 microg ml(-1) and the detection limits ranged from 0.1 to 5 ng ml(-1), with an injection volume of 20 microl. The inter-day and intra-day precisions ranged from 2.6 to 5.7% and 4.8 to 8.2%, respectively; the mean recoveries of the seven analytes were 81-104%, 91-107% and 93-101% respectively at the spiked level of 10, 40 and 200 microg ml(-1). A large number of fermentation samples were analysed using this method. The technique is simple, rapid, selective and sensitive, and shows potential for the high-throughput quantitation of amino acids from other biological matrices.     

基于混合PN／肖特基结构的新一代600V SiC肖特基二极管

第二代碳化硅(SiC)肖特基二极管——thinQ!2G是由一个肖特基结构和一个与它平行的独特的低电阻PN结组成,因此具备更加强大的浪涌电流处理能力,同时又继承了第一代碳化硅肖特基二极管所实现的良好的动态性能。