Biomedical Engineering Reference
In-Depth Information
Fig. 11.7 Schematic model
and AFM images of the DNA
origami. As represented, the
origami is made of two main
subunits, an external ring and
an internal disk constrained in
two opposite points to the
external ring. The flexibility
of the internal disk is
guaranteed by a
four-noncomplemented
nucleotide spacer (green
lines) (Reprinted with
permission from Ref. [ 51 ].
Copyright 2011 American
Chemical Society)
the internal disk and changes its conformation by moving the wing edges toward
each other. The motion induced by the probe-target hybridization in the origami
structure was clearly observed by AFM.
11.3.2.2
PX-JX 2 Rotors
PX DNA is a four-stranded molecule in which two parallel double helices are
joined by reciprocal exchange (crossing over) of strands at every point where
the strands come together. JX 2 is a topoisomer of PX DNA that contains two
adjacent sites where backbones juxtapose without crossing over. Seeman and
coworkers utilized two sets of set strands and fuel strands through a four-step
cycle to accomplish the PX-JX 2 -PX transition [ 52 ]. As shown in Fig. 11.8 a, the
PX structure compromises two long scaffold strands and two set strands (green).
The addition of fuel strands complementary to the two green set strands of the
PX device produces the unstructured intermediate. Then adding the pale-purple set
strands converts the intermediate to the JX 2 conformation. The transition back to PX
conformation through the intermediate is realized by adding purple fuel strands and
green set strands successively. This four-step cycle is exploited to move the DNA
half-hexagons that align along the same direction on PX structure to the opposite
directions on JX 2 structure. The morphology change is measured by atomic force
microscopy (AFM).
Seeman's research group has since developed several more complex molecular
devices based on the PX-JX 2 motif. They first enhanced the two-state PX-JX 2
device to three-state PX-JX 2 -BX device, which provides three different transitional
motions between the endpoints [ 53 ]. As shown in Fig. 11.8 b, three sets of set
strands and fuel strands are used to drive each transition through the same
intermediate conformation. The motions of this device are monitored by FRET,
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