news Fluorescence lights up dynamic dendrites Moreover, these MS profiles were quite
somewhat larger area: ~25 µm. Multiple
In the past decade, near-field scanning optical microscopy (NSOM) combined with shear-force feedback control has proven to be an ideal method for molecular-scale imaging. Researchers in Holland recently discovered startling, submolecular dynamics after they acquired the first micrographs that combine topographical and fluorescence images of individual dendritic molecules. David N. Reinhoudt, Niek F. van Hulst, and colleagues at the University of Twente (The Netherlands) synthesized dendritic organopalladium spheres 100–150 nm in diameter, each containing a single fluorescent core. After spin-coating the (a) Topographical and (b) fluodendrites onto a glass substrate, the sample was rescence images of dendritic scanned with a modified NSOM probe. The spheres are superimposed to customized probe was produced by milling the give image (c). The false-color end of a 70-nm NSOM tip with a focused ion scale indicates the polarity of beam, leaving behind a small, shear-force senslight emitted by fluorescent ing protrusion, or supertip, at the edge of the cores inside their host denprobe. NSOM fluorescence and shear-force drites, ranging from green (vertiimages were collected simultaneously, and the cally polarized) to yellow (unpodata were realigned to account for the offset larized, or polarization at 45˚) to between the shear-force supertip and the red (horizontally polarized). NSOM aperture. The resulting images reveal the fluorescing cores associated with dendritic molecules. The fluorescence polarization is an indication of core orientation, as emitted light is polarized in the direction of the emission dipole. The polarity data clearly distinguish individual molecules, which have well-defined polarization, from clumps of molecules with unpolarized fluorescence, and result in the superposition of light from many, randomly oriented emitters. The researchers were surprised to find that the fluorescence polarity from most cores changed over the course of a scan, indicating that the emission dipoles rotated within their host molecules with periods ranging from milliseconds to seconds. (J. Phys. Chem. A 1999, 103, 11264–11270)
controls check for contamination.
Retention by theory
consistent from vesicle to vesicle. The new method works with samples that have volumes of >300 aL and MALDI matrix solutions that have picoliter volumes. The procedure begins by depositing multiple vesicles onto a glass microscope slide. The vesicles are rinsed, and a micropipette is used to transfer a single vesicle onto a glass coverslip. Solution is then rapidly removed, and MALDI matrix solution is added to create a 10- to 20-µm spot. After drying, the coverslip is attached to a standard MALDI plate. To fully analyze the spot, the excitation laser irradiates a
Among the peptides identified in these experiments were califins with their subunits and N-terminal peptides. The authors predict that by using ~1 pL of matrix solution and a laser focused to just ~5-µm diameter, the method could analyze vesicles as small as 200 nm in diameter. (Nature Biotechnol. 2000, 18, 172–175)
It is considered the “Mt. Everest” of separation science: Given just a structure and experimental conditions, accurately predict the retention characteristics of a solute molecule. J. Ilja Siepmann and colleagues at the University of Minnesota and Rohm and Haas take on the challenge and, using powerful molecular simulations and transferable force fields, obtain microscopic pictures of the partitioning of 10 alkanes between a helium vapor phase and a squalene liquid phase in gas-liquid chromatography. The analysis is made even more difficult because the alkanes include two sets of topological isomers, such as 2,5-dimethylhexane and 3,4-dimethylhexane, which are constructed from the same set of methyl, methylene, and methine “building blocks”. The molecular simulations use configurational-bias Monte Carlo simulations in the Gibbs ensemble. Kovats retention indices, a measure of the relative retention times, are calculated directly from the partition functions and agree well with experimental values. In addition, calculated Gibbs free energies of transfer for the normal alkanes conform to Martin’s equation, which is the basis of linear free-energy relationships used in many modeling packages. (J. Phys. Chem. B 1999, 103, 11191–11195)
M A R C H 1 , 2 0 0 0 / A N A LY T I C A L C H E M I S T R Y
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