News Catalyst screening The advent of methods for producing large numbers of catalysts has increased the need for the efficient screening of catalysts. IR thermography, colorimetric assays, and fluorescent substrates were used for this in the past. Now, Scott J. Miller and Gregory T. Copeland at Boston College describe a fluorescing molecular sensor that can be used in both microwell-plate and singlebead-single-catalyst assays. Using an acid sensor based on aminomethylanthracene—a pH-sensitive fluorophore that fluoresces when protonated— the sensor responds to the accumulation of acetic acid in acyl transfer reactions. Because die amount of acetic acid produced is proportional to the number of catalytic turnovers, detecting this product provides a way to monitor the reactions and evaluate the efficiencies of the catalysts. Copeland and Miller tested the sensor in a 96-well microplate format by monitoring the performance of seven catalysts at three concentrations in parallel acyl transfer experiments. As expected, the "super-
acylation" catalysts PPY and DMAP were the most active, and NMI and two peptidemodified alkylimidazoles were much less active. In addition, the assay was able to distinguish between one peptide, which has a D-Pro-Aib backbone, and the other, which has an L-Pro-Aib backbone. The researchers also tested a singlebead-single-catalyst protocol by attaching the sensor to Wang resin that was functionalized with the potential catalysts BOC7t(Me)His-D-Pro-Aib-Phe (2-Resin) and BOCJJT (Me) His-L-Pro-Aib-Phe (3-Resin). A stoically deactivated histidine derivative (4-Resin) and a control lacking a histidine moiety (5-Resin) were also prepared. 2-Resin fluoresced most strongly, followed by 3-Resin and 4-Resin. 5-Resin was essentially nonfluorescent. Because analogous results were obtained when all four resins were mixed in a single reaction vessel, the authors suggest that this approach could be used to screen single-bead-single-catalyst libraries using fluorescence microscopy or fluorescence-activated bead sorting (/. Am Chem Soc 1999 121 4306-07)
Fluorescence micrographs showing beads functtonallzed wiih (a) 2-Resin and 5-Resin, (b) 2-Resin and 3-Resin, and (c) all four resins. Beads with 2-Resin fluoresced most strongly and appear lightest.
Multibeam circular dichroism detector
proach is to use left- and right-circularly polarized beams that intersect precisely in the Double-beam circular dichroism (CD) detec- sample cell. In addition, split-type flow cells, tors have previously been developed to allow which use two photodiodes to offset noise in the light source, have been developed, but CD to be combined with HPLC. One apdifferences between the diodes can make it difficult to maintain a baseline. Now, Atsushi Yamamoto and colleagues at the Toyama Institute of Health, Kanazawa University, and Shimadzu Corporation (all in Japan) have developed a third type Schematic diagram of the multibeam HPLC-CD detector. The of device, which can detect a prism and the quartz plate are outside the dotted line because CD wave in the wavelengtti they are not included in conventtonal photodiode array axial direction. detectors. .Adapted with permission. Copyright 1999 Royal Society of Chemistry.. This detector uses a po438 A
Analytical Chemistry News & Features, July 1, 1999
Blue and green LECs Polymer light-emitting diodes (PLEDs) have potential for use in emissive flat panel displays. Low oper ating voltages and high efficiencies are realized with red-orange and green PLEDs; however, the same does not hold true for blue PLEDs. On the other hand, electroluminescence (EL) occurs for all colors at low onset voltages with polymer lightemitting electrochemical cells (LECs). G. Leising and co-workers at Technische Universitat Graz (Austria), University of California-Santa Barbara and Max-Planck-Institut fur Polymerforschung (Germany) describe the preparation and analysis of blue and green LECs based on blends of a ladder-tvpe polvfoaraphenylene) polymer (#z-LPPP) LECs based on m-WPP require much lower voltage (2.2-2.7 V in the reverse direction) for the onset of EL than PLEDs (-12 V) based on m-LPPP. Blue light emission from LECs based on m-WPP was detected at 2.2 V in the reverse direction, one of the lowest onset voltages for blue light emission ever reported for an EL device. In addition to low onset voltages, w-LPPP-based LECs exhibited low response time (
