Web Release Date: July 11,
Designing Nanosensors Based on Charged Derivatives of Gramicidin A
and
Contribution from Department of Chemical Engineering and Department of Biomedical Engineering, University of Michigan, 1101 Beal Avenue, Ann Arbor, Michigan 48109-2099, and Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, MC 0358, La Jolla, California 92093-0358
Received February 22, 2007
Abstract:
Detection of chemical processes on a single molecule scale is the ultimate goal of sensitive
analytical assays. We recently reported the possibility to detect chemical modifications on individual
molecules by monitoring a change in the single ion channel conductance of derivatives of gramicidin A
(gA) upon reaction with analytes in solution. These peptide-based nanosensors detect reaction-induced
changes in the charge of gA derivatives that were engineered to carry specific functional groups near their
C-terminus.1 Here, we discuss five key design parameters to optimize the performance of such
chemomodulated ion channel sensors. In order to realize an effective sensor that measures changes in
charge of groups attached to the C-terminus of a gA pore, the following conditions should be fulfilled: (1)
the change in charge should occur as close to the entrance of the pore as possible; (2) the charge before
and after reaction should be well-defined within the operational pH range; (3) the ionic strength of the
recording buffer should be as low as possible while maintaining a detectable flow of ions through the pore;
(4) the applied transmembrane voltage should be as high as possible while maintaining a stable membrane;
(5) the lipids in the supporting membrane should either be zwitterionic or charged differently than the
derivative of gA. We show that under the condition of high applied transmembrane potential (>100 mV)
and low ionic strength of the recording buffer (
0.10 M), a change in charge at the entrance of the pore
is the dominant requirement to distinguish between two differently charged derivatives of gA; the conductance
of the heterodimeric gA pore reported here does not depend on a difference in charge at the exit of the
pore. We provide a simple explanation for this asymmetric characteristic based on charge-induced local
changes in the concentration of cations near the lipid bilayer membrane. Charge-based ion channel sensors
offer tremendous potential for ultrasensitive functional detection since a single chemical modification of
each individual sensing element can lead to readily detectable changes in channel conductance.
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