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Nanotube electrodes measure DNA conductivity DNA has the potential to be used in molecular electronic devices, but its electrical properties vary widely under different environmental conditions. Reliably measuring these properties can be complicated because DNA conductivity is sensitive to experimental design. Wonbong Choi and colleagues at Florida International University, Pohang University of Science and Technology (Korea), and Japan’s National Institute of Genetics have designed a new nanoelectronic platform to directly measure the conductivity of single DNA molecules. The scientists’ design consists of a pair of single-walled carbon nanotube (SWNT) electrodes positioned at the edges of a “nanotrench”. A DNA molecule is suspended across the nanotrench
by covalently attaching the ends to the two nanoelectrodes. The nanotrench creates a space between the DNA and the oxide surface, which is important because the oxide surface can interfere with the charge-transport properties through the DNA molecule. The conductivity of both single- and double-stranded DNA was measured. As expected, the double-stranded DNA was more conductive because of its regular stacking of nucleotide bases. The researchers also used their device to measure the temperature-dependent conductivity of DNA strands. As the temperature was increased from 25 to 40 °C, the current through the molecule rose. Above 40 °C, conductivity began to drop, which the scientists attribute to evaporation of water mol-
(a)
(b)
Schematic illustration of the electrical measurement system. (a) Single- or (b) doublestranded DNA is covalently attached to the SWNT electrodes and suspended above a nanotrench so as not to touch the silicon dioxide surface.
ecules from the DNA strand’s hydration shell. (Nano Lett. 2007, DOI 10.0121/nl0716451)
Flow cytometry: the eyes have it ©JKITAN 2007/SHUTTERSTOCK.COM
It’s been said that the eyes are the windows to the soul, but when C. P. Lin and colleagues at Harvard Medical School, Tufts University, Kyongpook National University (South Korea), and Boston University peer into eyes, they see something
cells flowing through the retinal blood vessels by scanning an excitation laser in a circle around the optic nerve head. They tried several methods for quantitating cells, including manual counting and two different software packages. The re-
much more quantitative. The scientists
searchers found that when either soft-
have developed a method to count cells
ware system was used, counts were ~15%
in vivo by scanning the blood vessels that
Measurements taken through the eyes provide more and better information than those taken elsewhere.
lower than that of manual method totals.
in vivo via a single artery in the ear. The
five large artery–vein pairs converge at
counting sensitivity by ~5×. The authors
blood volume that could be sampled was
the head of the optic nerve.
note that this improvement should lead
line the retina. Previously, cells have been counted
small, however, and the scientists were
The scientists injected anesthetized
Compared with the ear-scanning method, the retinal flow cytometer improved
to higher statistical confidence when
looking for an easy way to increase their
mice with fluorescently labeled lympho-
the method is used in future longitudinal
sample size. The retina is perfect because
cytes and tracked the number of these
studies. (Opt. Lett. 2007, 32, 3450–3452)
© 2008 American Chemical Societ y
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news
Functional analysis of membrane proteins with nanodiscs and SAMDI MS Because of their hydrophobicity and low
id disc encircled by a band of membrane
group tested the photoactivation of rho-
solubility, membrane proteins are diffi-
scaffold proteins (MSPs), and it was de-
dopsin, a membrane protein and G-pro-
cult to analyze. Most of the methods to
veloped as a way to solubilize membrane
tein-coupled receptor. When exposed to
solubilize membrane proteins end up dis-
proteins without interrupting their native
light, rhodopsin undergoes photoisomer-
rupting their functionality. Recently, Mi-
functionality. In this case, the MSP incor-
ization and can bind its coupled G-pro-
lan Mrksich of the University of Chicago
porated a hexahistidine tag. For the SAM-
tein, transducin.
and Stephen Sligar of the University of Il-
DI, the researchers prepared a SAM of
linois Urbana–Champaign joined forces
alkanethiolates on gold. The functional
ing assays with and without exposure to
to surmount this problem. They combined
groups on the SAM served two key pur-
light and concluded that rhodopsin re-
the nanodiscs from Sligar’s group with
poses—to prevent nonspecific adsorp-
tained its functionality while immoblized
the self-assembled monolayers (SAMs)
tion to the surface and to bind the hexa-
on the surface in the nanodisc. Final-
for MALDI (SAMDI) MS from Mrksich’s
histidine tag on the nanodiscs.
ly, they added an inhibitor, which was
group to perform functional assays of a membrane protein. A nanodisc is a bilayer phospholip-
After confirming the immobilization
The team carried out transducin-bind-
seen to disrupt the rhodopsin–transducin
of the nanodiscs on the SAM via sur-
binding. (Angew. Chem., Int. Ed. 2007, 46,
face plasmon resonance and SAMDI, the
8796–8798)
Immunomagnetic diffractometry for biomarker detection In the quest for an to diffract the incident Diffrac improved biomarker delaser radiation. tion pa tter +2 +1 0 tection method, Cagri The researchers ob–1 –2 n Savran and colleagues at served that the FR–beads Serum Purdue University and attached specifically to containing FR the Mayo Clinic have the F–BSA on the surdesigned a technique face. The packing density called immunomagnetic of the bound FR–beads diffractometry. Their grew with increasing FR approach combines concentrations (700 fM immunomagnetic capto 11 nM). After creatImmunomagnetically captured FR-bound ture of an analyte on ing a calibration curve, magnetic microbeads functionalized beads, the researchers measured in situ assembly of an Immunomagnetic diffractometry FR assay. FR–beads form an in situ diffraction FR concentration in the optical diffraction gratgrating on microcontact-printed F–BSA patterns. Laser illumination yields a serum of cancer patients. ing, and measurement characteristic diffraction pattern that depends on the density of the attached The detection limit was of the diffraction. By beads. lower than that of sevvirtue of their size, the eral other biomarker asbeads enhanced the diffraction signal, ing to deposit alternating 15 μm lines says such as ELISAs. Immunomagnetic so no further amplification or labeling of folate-coupled bovine serum albumin diffractometry has the additional adwas needed. The target chosen for the (F–BSA) onto a gold surface. Magnetic vantages of speed, robustness, low cost, proof-of-principle study was the folate beads derivatized with the FR antibody and ease of miniaturization. The rereceptor (FR), a potential serum bio(FR–beads) captured FRs from serum; searchers say that the assay is applicable marker for cancer. they were subsequently bound to the F– to other biomarkers. (J. Am. Chem. Soc. The group used microcontact printBSA lines, effectively forming a grating 2007, DOI 10.1021/ja073094m) 514
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Pushing molecules back and forth through a nanopore MARC GERSHOW
Jene Golovchenko and plied a voltage of 120 Marc Gershow of HarmV for 2–32 ms (t delay) vard University have and then reversed the found a way to recaptranslocation directure individual DNA tion by switching the molecules with a solidvoltage to –120 mV state nanopore. The for 500 ms; they then method has helped the measured the time scientists study the berequired for the molhavior of the molecules ecule to reenter the as they approach and pore (t capture). They 5 nm translocate through found that t capture and the pore. A transmission electron microthe overall recapture In a typical nanograph of the SiN nanopore used in success rate depended pore experiment, an this study. strongly on t delay. electric field draws a The researchers molecule, such as a strand of DNA or a compared the observed behavior of the protein, through the pore in a single di- molecules with a theoretical model of rection. As the molecule moves through DNA under the control of an electric the pore, it blocks the flow of other force and found good agreement beions; some of the molecule’s characteris- tween the theoretical and experimental tics, such as length, can be determined data. The technique has applications by the duration of this blockage. Very beyond exploring molecular dynamlittle was known, however, about a mol- ics, including monitoring changes to a ecule’s behavior immediately before and molecule, such as DNA hybridization just after its passage through the pore. or shifts in conformation of a protein. Golovchenko and Gershow first ap(Nat. Nanotechnol. 2007, 2, 775–779)
Shared Instrumentation Grant program to accept proposals The National Institutes of Health (NIH) National Center for Research Resources has issued a call for proposals for its Shared Instrument Grant program. This program was designed to allow groups of at least three NIHfunded researchers to purchase or update expensive instrumentation. NIH expects to fund ~125 awards of $100,000–500,000 for a total of more than $40 million in the 2009 fiscal year. Eligible instruments include NMR spectrometers, X-ray diffractometers, mass spectrometers, and electron and confocal microscopes. Groups may submit applications from February 24 to March 24, 2008; funding can start as early as April 2009. For more details, see http://grants.nih.gov/grants/ guide/pa-files/PAR-08-036.html.
Gaetano Romaniello
The mystery of the pink paintings In 2003 after a dry, hot summer, the frescoes in the Crypt of the Original Sin in Matera, Italy, suddenly began to turn pink. To find out why, Paolo Visca and colleagues at the Third University of Rome and the Lazzaro Spallanzani National Institute for Infectious Diseases (both in Italy) recently analyzed bacteria growing on the frescoes. Classical methods of identifying bacte-
RNA sequencing determined that most of the microbes on the paintings were an unknown species related to Rubrobacter radiotolerans, but that did not fully explain the cause of the discoloration.
ria are based on growing the microbes in culture. Although the team members could clearly see evidence of a homogeneous microbial community on the frescoes via scanning electron microscopy, they found it nearly impossible to cultivate the bacteria. Genetic methods such as cloning and
Only after analyzing the samples with Raman spectroscopy could the researchers identify the presence of natural pigments called bacterioruberins. Visca and colleagues matched the pigments produced by the R. radiotolerans-related bacteria with pig-
Fresco in the Crypt of the Original Sin (left) with the discoloration and (right) after restoration. [(left) Provided by Paolo Visca; (right) Adapted with permission. Copyright 2007 la Repubblica.] ments present on the mural paintings. However, they could not pinpoint exactly which species was responsible because all bacterioruberin-producing species have identical Raman spectra. (Environ. Microbiol. 2007, 9, 2894–2902)
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