News
Look out peptides, chemosensors are here iNo clear-cut rules exist IOI synttiesizing peptide-binding molecules, which makes the development of chemosensors for the detection of peptides quite a challenge. W. Clark Still and co-workers at C olumbia University, however, have recently made some progress in developing organic sensor molecules that preferentially bind to tripeptide sequences. They created small molecule peptidebinding receptors, which act as synthetic analogues of the antigen-binding sites of immunoglobulins, and equipixxl them with a sensitive fluorescence energytransfer signal-transduction system. When binding occurs, the change in the separation of a fluorophore and a
quencher causes a 300-500% increase in fluorescence (large enough to be observed with the naked eye). Increases in fluorescence that accompany binding can also be used to measure binding constants of the sensors with their cognate peptides. The peptide-binding receptor mole cules are based on synthetic, amidelinked oligomers of isophthalic acid and cyclic tra«s-1,2-diaminn eerivatives, which are known to provide high selectivity for peptidic substrates. A combinatorial library containing 3375 different N-acylated, side-chain protected tripeptides was screened to determine which tripeptide sequences the receptor molecules will selectively bind. Inspection of the library beads, each of which carried a single tripeptide sequence, was sufficient to detect when binding had occurred.
Chromatographic dispersion by NMR
tional Laboratory and co-workers at the University of Tubingen (Germany) study dispersion in packed chromatographic colDispersion in a packed chromatographic umns with pulsed-field gradient NMR. column can adversely affect the separation, The NMR experiment allows the simulparticularly as the scale of the separation taneous analysis of dispersion and flow veincreases. Georges Guiochon of the Unilocity in chromatographic columns. They versity of Tennessee and Oak Ridge Naplaced 4.4-mm i.d. columns packed with porous silica or monodispersed crosslinked polystyrene directly in the bore of an 89-mm bore 4.7-T superconducting magnet and parallel to the main magnetic field. The pulsed-field gradient could be selected to be either perpendicular or parallel to the column axis, so both the axial and transverse dispersion coefficients could be measured independently. The results are an average value of the axial and transverse dispersion coefficients. They used the data to measure the dependence of the local values of the height equivalent to a theoretical plate on fluid velocity. Data in a narrow velocity range could be fitted to nearly any simple model, but the data acquired over a wide velocity range needed more discriminating models. The results were best accounted for by the Giddings equation. In addition, by monitoring the observation time in the absence of flow, ,he eesearchers sould investigate the Experimental setup for pulsed field-gradient dependence of the molecular diffusivity of NMR in chromatographic columns, (a) 400-mm water in the packing material and thus dii.d. gradient system; (b) biocompatible frit; rectly measure packing tortuosity. (J. Am. (c) 15-mm i.d. rf coil; (d) 4.4-mm i.d. PEEK Chem. Soc. 1998 120 1494-505) column; and (e) porous packing. 304 A
Analytical Chemistry News & Features, May 1, 1998
Two chemosensors were designed and evaluated in organic solution. The first sensor was found to selectively bind to one tripeptide sequence in -1600; the other sensor had less selectivity and bound to one tripeptide sequence in -400. Interestingly, no tripeptide sequences were bound by both sensors. Because of the promising fluorosensing properties of the sensor molecules in solution, the authors incorporated the receptors into solid-state fluorescent chemosensors. Derivatives of the sensor molecules were coupled to synthesis beads, providing solid-state sensors that could be studied in real time by fluorescence microscoov Peptide concentrations as low as 1 uM could be detected with a microscope and a charire-coupled device camera (Science e998 279 ,81-53)
Single-shot analysis Most studies of laser plumes, such as those generated by laser-induced breakdown spectroscopy, involve partially analyzing multiple plumes and then stitching the data into what is assumed to be a complete characterization of a single event. Michael Myrick and coworkers at the University of South Carolina take a first stab at avoiding this assumption with a prototype system that provides singleshot spectroscopic imaging of a laser plume using a two-dimensional array of optical fibers. The fiber-optic detector consists of a 30 x 20 array of hexagonally packed, 250-um o.d., / / 2 optical fibers, which are drawn
Experimental setup showing (1) Nd:YAG laser, (2) mirror, (3) lens, (4) CCD camera, (5) laser plume, (6) fiber-optic array, (7) cones of light from the top and bottom fibers of the array, (8) lens, (9) spectrograph, (10) CCD camera, and (11) computer. (Adapted with permission. Copyright 1998 Society for Applied Spectroscopy.)
into a 600 x 1 distal array. An image at the two-dimensional proximal end of this detector is systematically reordered by row and column into a one-dimensional distal end. The distal end is then imaged by a spectrograph equipped with a holographic grating and a charge-coupled device (CCD) camera. Thus, the fiber-optic array acts as a dimension-reduction device and permits three-dimensional (spatial, spectral, and temporal) data to be recorded on a twodimensional detector without loss of spectroscopic resolution. The spatial and spectral information in the resulting light-
Let the gold fly
dispersed CCD images is extracted and deconvoluted by computer software. The authors demonstrate the system by collecting position-specific spectra that span a 190-nm wavelength range and offer subnanometer resolution. Temporal data require further development of the system, and the technique currently suffers from a tradeoff between spatial and spectral resolution; but the authors believe that—with proper modifications—they can optimize laser plumes used in many applications, such as laser surgery ctnd thin-film manufacturing. (Appl. Spectrosc. 1199 52 179-86)
of sulfur atoms). Interestingly, none of the peak spacings corresponded to the 253.5 Da of the alkane chains in the surface monolayer, indicating that all the alkanes had been cleaved from the surface. The
