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ANALYTICAL CURRENTS Building a better trap (b)
(a)
By genetically and chemically engineering a protein, Eiry Kobatake and colleagues from the Tokyo Institute of Technology (Japan) and Imperial College (U.K.) have constructed the first reagentless optical sensing system with a self-adhering capability. This unique protein was able to detect glutamine while bound to an unmodified surface. The researchers worked with the glutamine-binding protein QBP, which has an “open” structure with two domains that spring close when glutamine binds to the active site. By using site-directed mutagenesis, one of QBP’s amino acids, residing in a region that changes significantly when glutamine binds, was changed to a cysteine. That amino acid, in turn, was bound to one of three thiol-reactive fluo-
Glnbinding site
(c) Glu Asn
QBP
Gln
(Gly4-Ser)n spacer h
E12
A detector with a bite. (a) The QBP protein modified with a fluorophore and E12, which is a hydrophobic spacer. (b) The protein bound to a surface. (c) The trap closes when glutamine is bound, and the change in fluorescence is measured.
rophores. The result was an environmentsensitive fluorescent protein that can optically determine glutamine binding. The protein was also fused with a glycine–serine (E12) spacer. The spacer, which spontaneously adheres to a hydrophobic surface, was used to anchor
electrode and hover above it until the volt-
vices can easily become clogged by sus-
age is switched to a different electrode. In
pended particles, proteins, and cells. To pre-
ac mode, the electric field creates a di-
vent this, Orlin Velev and colleagues at North
electrophoretic force, which attracts the
Carolina State University developed a mi-
droplets on the basis of their polarizability.
crochip system that contains no solid walls.
In dc mode, the droplets move by coulom-
Instead, microliter- and nanoliter-sized
bic repulsion or attraction.
(b)
The chips could be valuable for studying microscale chemical transport and mixing.
In the new liquid/liquid microchip de-
In addition to driving liquid droplets, they
vice, tiny droplets of water or dodecane
could be used to manipulate suspended
float on a dense layer of fluorinated oil. To
solid particles that form by precipitation
manipulate the droplets, the researchers
when two droplets mix. Other complex ap-
applied either ac or dc electric fields
plications, such as confining living cells or
through an array of electrodes below the
genetic material into single droplets, could
oil surface. The droplets migrate to one
also be possible. (Nature 2003, 426, 515–516)
© 2004 AMERICAN CHEMICAL SOCIETY
the QBP detector to an unmodified microplate surface. The modified system measured glutamine in the range of 0.1–50 µM. The procedure can be extended to other types of biomolecules. (J. Am. Chem. Soc. 2003, 125, 16,228– 16,234)
(a)
The miniature channels in microfluidic de-
face under the control of an electric field.
h
Microplate surface
Free-floating microdroplets
droplets glide along a fluorinated oily sur-
h’
h’
Fluorophore
(a) Nanoliter-sized droplets containing latex (white) and gold nanoparticles (black) are driven by an electric field. (b) The droplets mix when they touch near the junction. (Adapted with permission. Copyright 2003 Macmillan Publishing Ltd.)
F E B R U A R Y 1 , 2 0 0 4 / A N A LY T I C A L C H E M I S T R Y
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