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groups, the absolute configuration can be determined. That is useful for the reaction offrraws-2-butenewith the silicon surface because it leads to chiral RR or SS structures, whereas m-2-butene produces a nonchiral RS or SR product. (Nature 1998, 392,909-11)
Biased diffusion of single molecules Diffusion studies of single molecules in solution using confocal fluorescence microscopy have shown thatfluorescentlylabeled macromolecules exhibit large deviations in the frequency of fluorescent bursts compared with the predictions of Poisson statistics. These deviations are thought to be the result of diffusion biasing caused by the optical trapping effect. D. Klenerman and coworkers at the University of Cambridge (U.K.) investigated the role of the optical trapping effect on the biased diffusion of single Rhodamine 6G (R6G) dye molecules. In addition, the diffusion of a number of single molecules is investigated over a large mass range (479-68,064 amu). Short time-scale deviations of the photon burst frequency are observed and found to increase with increasing size of the solute species. During a 15-ms interval, ,he ffuorescent burst for R6G occurs at a frequency that is ~1.7x greater than predicted by Poisson statistics. The deviations depend on power in a manner that is consistent with optical trapping theory, and thus they are attributed to a weak optical trapping effect. The optical trapping effect is dependent on the polarizability of the molecule. It has been predicted that molecules with resonantly large polarizabilities may be strongly affected by optical trapping. In this study, small molecules are found to exhibit optical biasing only when excited at their resonance frequencies. (J. Phys. Chem. B 1998,102, 3160-67)
Confocal microscope coupled with an argon ion laser excitation source and a photomultiplier detector.
Caught in the trap Individual microparticles, whether single cells or microdroplets, can be difficult to manipulate and analyze. Laser trapping makes the job easier. Visible light has most often been used for the trapping, but that spectral region can be unfriendly to organic and biological samples, causing photo or thermal damage. If Raman spectroscopy is used as the monitoring tool, visible light has the added disadvantage of creating a strong fluorescence background signal. Katsuhiro Ajito of Nippon Telegraph and Telephone Qapan) )ypassed those problems with a Raman microprobe and laser trancing system that combines a near-IR laser holographic notch filters and a charge-coupled device detector The same laser was used to trap the droplet and excite Raman scattering Ajito demonstrated his system with the analysis of a toluene microdroplet in a hemispherical water droplet. The toluene droplets ranged from 10 to 30 um in diameter. Because the toluene droplets gradually aggregated at the center of the water droplet surface, the laser trapping was carried out with a 1-um-diameter laser beam at the water surface away from the area where the
toluene congregated. The toluene droplet was trapped in the vertical direction almost immediately and was completely trapped in the lateral direction within 20 s. In addition, ,he toluene microdroplet was stable in the water over several minutes. After the laser was turned off, the toluene microdroplet rapidly moved and was defocused as it moved along the surface of the water droplet. (Appl. Spectrosc. .998,52,339-42)
Image of a near-IR focused laser spot and a single toluene microdroplet in water immediately after irradiation was started. (Adapted with permission. Copyright 1998 Society for Applied Spectroscopy.)
Stable peptide retains its structure Numerous methods have been used to probe the conformations of gas-phase ions of large proteins formed by electrospray ionization (ESI), including collision cross-section measurements, ion-molecule reactions, and hydrogen/deuterium (H/D)-exchange. How- Structure of natural RES-701-1. ever, it is unclear whether the three-dimensional structures of proteins in thetic analogue of the peptide. Significant the gas phase have any resemblance to their differences are shown between ESIconformation in the solution phase. Joseph generated singly (M + H)+ and doubly (M + A Loo and co-workers at Parke-Davis report 2H)2+ protonated ions of the natural and on a situation in which the conformation of a synthetic peptides by collisionally activated polypeptide based on gas-phase MS/MS dissociation (CAD). MS/MS measuremeasurements is consistent with the known ments show that the two peptides have difsolution-phase structure ferent relative abundances of the two product ions. Differences in the structures of The researchers examine the structure the two peptides are confirmed in the soluof RES-701-1, a hexadecapeptide isolated from Streptomyces, as well as that of a syn- tion phase by H/D-exchange experiments.
