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ANALYTICAL CURRENTS Imaging nanotubes using electron diffraction Examining the structure of crystals or nanostructures is typically conducted by X-ray diffraction or electron microscopy. However, J. M. Zuo and colleagues at the University of Illinois at Urbana– Champaign and Motorola Labs have developed a new method that creates images from electron diffraction energies. The technique, called nanoarea electron diffraction (NAED), is used by the researchers to look at carbon nanotubes and may also fill a niche where nonperiodic structures cause difficulty with crystallography and lens aberrations limit microscope resolutions. In NAED, a nanometer-sized coherent electron beam strikes the object’s surface and the diffraction pattern of electron in-
tensities is measured. However, recording the amplitude or intensity of the diffraction pattern is not enough to form an image; phase information is also needed. To do this, the researchers used a previously developed iterative process that operates independently of the starting phases. By using the process, a 50-nmlong double-walled nanotube was imaged with a 1-Å resolution. Zuo and colleagues indicate that this technique may be extensively used for other carbon-based molecules, including biological macromolecules. (Science 2003, 300, 1419–1421)
Figure Not Available for Use on the Web
Seeking the metabolome To better understand how cells function, re-
chester (both in the U.K.) have developed a
bolites varied depending on the life cycle
searchers are taking a holistic approach
high-throughput strategy called “metabolic
stage and genetic background of the yeast
with genomics and proteomics. Recently,
footprinting” to probe the metabolome of
culture, which they say helps researchers
they have added metabolomics, in which
budding yeast. Instead of examining intra-
classify unknown mutants into functional
low-molecular-weight metabolites are cata-
cellular molecules, the researchers focused
categories.
loged as a way to study the physiological
on metabolites found in the surrounding
processes within cells, to the growing list
liquid culture medium. Once the medium
ing from the exponential growth phase to
of “-omics” disciplines.
was separated from the yeast cells, it was
the stationary phase were more complex
loaded into a mass spectrometer. Kell and
than those from earlier stages, which sug-
co-workers found that the pattern of meta-
gests that more metabolites were being
Douglas Kell and colleagues at the University of Wales and the University of Man-
Metabolic footprints of yeast transition-
excreted or secreted. Kell and colleagues then looked at metabolic profiles for a collection of known single-deletion mutants using various data analysis techniques, which allowed them to identify groups of strains with similar
Figure Not Available for Use on the Web
footprints. Mutants with defects in similar metabolic pathways clustered together on data plots, as expected, which suggests that these methods may be useful for identifying and classifying unknown mutants. (Nat. Biotechnol. 2003, 21, 692–696)
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ANALYTICAL CURRENTS STM selectively moves or desorbs molecules J. I. Pascual and colleagues at the Fritz-Haber-Institut der Max-Planck-Gesellschaft (Germany), Institut de Ciéncia de Materials de Barcelona-CSIC (Spain), Université Paul Sabatier (France), and the Brookhaven National Laboratory selec-
Separating multiply charged peptide ions by MS The MS peaks of multiply charged tryptic peptide ions are often drowned out by singly charged impurities. I. V. Chernushevich and colleagues at MDS Sciex and Samuel Lunenfeld Research Institute (both in Canada) have devised a new “multiple charge separation” (MCS) technique to solve this problem using a quadrupole TOF mass spectrometer.
tively induce inelastic tunneling electrons via scanning tunneling microscopy (STM) to move and desorb individual NH3 molecules on a copper surface. They say their method shows that STM can probe single-molecule events at very low yields, which could lead to the discovery of reaction pathways that are inaccessible using conventional approaches. Selectively exciting molecular vibrations, typically using laser pulses, can directly influence the speed and outcome
Figure Not Available for Use on the Web
of chemical reactions. Pascual and his group applied this method to NH3 molecules adsorbed on a clean Cu(100) surface. They controlled breaking the NH3–Cu(100) bond by increasing the energy of the tunneling electrons. When moving the molecules using their technique, the researchers observed two bond-breaking mechanisms, each activated by the excitation of a different internal vibration. At a tunneling current of ~0.5 nA, the NH3 molecules accumulated enough energy to overcome the ~600-meV desorption barrier. (Nature 2003, 423, 525–528)
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They describe MCS as a “trap and slowly release” approach in which ions generated by nanoelectrospray are trapped in a quadrupole ion guide and released by slowly lowering the voltage at the exit lens. Ions leave the ion guide in order of increasing charge state and enter a TOF instrument for analysis. Special software allows the researchers to record only those ions released after the first set of ions, which are predominantly singly charged, have escaped. The researchers used the MCS technique to analyze a femtomolar bovine serum albumin tryptic digest and found that they could suppress singly charged ions by 500-fold, but they also lost about half of the multiply charged ions that they wanted to analyze. To test the sensitivity of the MCS technique, the background noise was artificially increased by diluting the sample in tap water and methanol. Nevertheless, most of the singly charged ions were suppressed and highly charged species were still observed. When the researchers ran a mixture of peptides from three different proteins through the quadrupole TOF using MCS, they could unambiguously identify only one of the proteins. MS/MS performed on some of the ions chosen on the basis of the MCS spectrum allowed the researchers to identify all three proteins. However, the method still needs further development. (Rapid Commun. Mass Spectrom. 2003, 17, 1416– 1424)
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AFM and Langmuir–Blodgett measure molecular weight
versity of North Carolina–Chapel Hill, Uni-
molecular density, the researchers
versität Ulm (Germany), and Carnegie
determined the absolute value of
Mellon University combine the Langmuir–
the number average molecular
Blodgett (LB) method and atomic force mi-
weight. They assumed that the
croscopy (AFM) to visualize molecular and
brush molecules had uniform struc-
colloidal species. They say their AFM–LB
tures along the backbone, and
method can accurately determine number
therefore the molecular weight
average molecular weight and molecular
distribution was virtually identical
weight distribution without prior informa-
to the length distribution. The researchers say their re-
tion about the chemical composition and Although accurate molecular weight dis-
1.5
0.0 105
gel permeation chromatography (GPC) using a multi-angle laser
ward, especially for linear chain homopoly-
light-scattering detector (MALLS)
mers, researchers can run into problems
and differential viscometer and
when studying large molecules with com-
static light scattering (SLS). In one
plex architecture or chemical composition.
experiment, the number average
Sheiko’s team demonstrated their technique
molecular weights obtained using
on cylindrical brush polymer molecules. LB
SLS and MALLS-GPC were 1.1 6
10 and 0.8 10 , respectively, and
the monolayer, and AFM gave accurate
Sheiko’s team obtained 0.8 106 ±
measurements of the molecular density,
0.11 using their AFM–LB approach.
allowing the researchers to examine and
(J. Am. Chem. Soc. 2003, 125, 6725–
count ~3000 molecules for every sample.
6728)
108
1
3
0.25 0.20 0.15 0.10 0.05 0.00 0
6
experiments provided the mass density of
107 106 Molar mass (D)
(b)
sults were in good agreement with
tribution measurements can be straightfor-
1.0
0.5
Length fraction, W (log L)
architecture of macromolecules.
(a)
W (log M)
From the ratio of mass density to
Sergei Sheiko and colleagues at the Uni-
2 log L (nm)
(a) MALLS-GPC diagram of molecular weight distribution of a cylindrical brush polymer sample. (b) Length distribution measured by AFM for 3060 molecules.
Speedy chiral separations Whether the goal is high-throughput screening or being first to market, in the world of pharmaceuticals, speed is everything. Joseph Schlenoff and Hassan Rmaile pick up on this theme by showing, for the first time, that the polyelectrolyte multilayer (PEMU) approach can separate chiral compounds with good flux selectivities, which is a parameter that takes into account differences in optical isomer concentrations and their diffusion rates. This technique could offer faster and easier chiral separations than rival methods such as HPLC and super-
critical fluid chromatography. PEMUs are formed by sequentially stacking charged polymers to create a uniform, continuous film. In this study, different polypeptides formed the film layers, which were coated on a rotating disk electrode used to measure flux. Electroactive analytes, such as L- and D-ascorbic acid and D- and L-DOPA, were investigated to demonstrate PEMU’s promise. The film worked as hoped. Two chiral multilayers were better than one at selectively separating one analyte enantiomer from another, and revers-
ing the chirality of the layers also inverted the chiral selectivity. Moreover, when a layer selective for the D-form was combined with one that favors the L-form, they canceled each other out, and there was no preference. The key to the PEMU approach is that it combines selectivity with speed. Thus, the equilibrium concentrations inside the researchers’ best-performing multilayer showed a mere 4% difference between Land D-ascorbic acid. However, the flux selectivity was almost 29%. (J. Am. Chem. Soc. 2003, 125, 6602–6603)
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