Biomedical Engineering Reference
In-Depth Information
a
b
c
D1
CF
F
A
B
+
+
D2
d
+
Fig. 6.4 Conformation changes of a DNA actuator from the relaxed state ( a ) into the straightened
state ( b ). A branch migration process ( c ) returns the machine in the original relaxed state ( d )
Fig. 6.5 Closed state of the
actuator in Fig. 6.4
the resulting straightened configuration being of about 37 pN. The initial, relaxed
state is recovered by addition of a complementary strand to F, denoted CF, which
removes F by hybridization after toehold binding followed by branch migration.
Breaking of base pair bonds due to thermal fluctuations occurs in a time interval of
about 10s. The nanoactuator can be repeatedly cycled between the relaxed and
straightened configurations, the fluorescence intensity that signals the straightened
state decreasing in time due to the decrease of the concentration of actuators in
solution caused by F and CF strands addition.
The nanoactuator in Fig. 6.4 can be developed into a more complex, three-state
machine, acquiring a new stable closed form, besides the relaxed and straightened
conformations ( Simmel and Yurke 2002 ). This rigid closed form is represented in
Fig. 6.5 , the state of the device changing between the three alternatives depending on
the fuel strand added in the solution. Compared to the relaxed state, the fluorescence
decreases by half in the closed state and increases with 40% in the straightened state
since the average distance between the dyes is 5.1 nm in the relaxed state, 2.3 nm
in the closed state, and 13 nm in the straightened state. The transitions between
different states occur in tens of seconds.
An autonomous DNA nanomotor that does not require fuel strands to change its
configuration, but RNA-cleaving DNA enzymes, has been demonstrated in Chen
et al. ( 2004 ). It extracts chemical energy from the covalent bonds of a DNA/RNA
chimera substrate to which it binds and converts it into mechanical motion, as
do natural protein motors. The sequence of processes that lead from the closed to
the opened state is represented schematically in Fig. 6.6 .
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