Biomedical Engineering Reference
In-Depth Information
It should be pointed out that in the catenane systems described above repeated
switching between the two states does not need to occur through a full rotation. In
fact, because of the intrinsic symmetry of the system, both the movement from the
state 0 to the state 1 and that from the state 1 to the state 0 can take place, with equal
probabilities, along a clockwise or anticlockwise direction. A full (360°) rotation
movement, which would be much more interesting from a mechanical viewpoint,
can only occur in ratchet-type systems, i.e., in the presence of asymmetry elements
which can be structural or functional in nature (Ballardini et al. 2001a, b ; Balzani
et al. 2003 ). This idea was implemented with a carefully designed catenane by rely-
ing on a sequence of photochemical, chemical, and thermally activated processes
(Hernández et al. 2004 ). NMR spectroscopy was employed to characterize the sys-
tem. A clever, albeit complex, way to obtain a unidirectional full rotation in a cat-
enane made of three interlocked macrocycles was also devised (Leigh et al. 2003 ) .
Other examples of molecular motors based on catenanes can be found in the litera-
ture (Balzani et al. 2000a, b, 2003 ; 2001b ; Abraham et al. 2004 ; Wang et al. 2004 ) .
3.4
Systems Based on DNA
The DNA molecule is not only the repository of genetic heritage, but also a very
interesting engineering material for nanotechnology (Niemeyer 1997, 2000 ) .
Moreover, it can be used for computation (Adleman 1994 ) . Tweezers and other
types of nanomachines based on conformational changes in DNA molecules have
been described in the past few years (Mao et al. 1999 ; Yurke et al. 2000 ; Li and Tan
2002 ; Yan et al. 2002 ). DNA-based motors exhibiting autonomous behavior have
also been reported (Chen and Mao 2004 ; Chen et al. 2004 ; Yin et al. 2004 ) . In some
of these systems and in the biped device described in the next section, DNA strands
play the dual role of structural components and fuel.
3.4.1
A DNA Biped Walking Device
This complex device (Fig. 10 ) consists of two components: a “track” comprising
three “stations” and two leg components, connected by fl exible linkers, walking on
the track (Sherman and Seeman 2004 ). Each station of the track and each leg of the
biped terminate with single-stranded DNA portions, called footholds (A, B, and C)
and feet (1 and 2), respectively, that are available to pair with complementary strands
of DNA. The sequences of the feet and footholds have been carefully selected to
minimize complementarities between them. A foot attaches to a foothold when a set
strand S complementary to both is added to the solution. Each of these linking
strands has an 8-base overhand or “toehold” which is not complementary to any of
the feet or footholds. The toehold allows the set strand to be removed by pairing
with a successively added unset strand U and the system is designed so that after the
unset procedure all the unset strands and the set strands with which they are paired
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