Biomedical Engineering Reference
In-Depth Information
Fig. 9) [45, 46]. Although the schematic representations of motif 2 and 5 in
Fig. 9 may suggest the helices of a four-armed junction cross perpendicularly,
they actually possess an angle of about 60
◦
. AFM analyses showed that the
torsion angles between helices are relatively constant throughout the entire
lattice (Fig. 9) [45, 46]. By incorporating a protein (RuvA), a square-planar
configuration (motif 6 in Fig. 9) has been built [47].
Subsequent works found that these simple junction motifs often did
not yield desired regular superstructures because of their high conforma-
tional flexibility [48]. To solve this problem, a more rigid component called
a “crossover tile” was developed. The simplest crossover tile is the double
crossover (DX). DX tiles consist of two double-stranded helices, which inter-
change single strands at two crossover points [42]. Because of the decreased
Coulomb repulsion, the two antiparallel motifs DAE and DAO, where the mi-
nor groove of one helix lies in the major groove of the other helix, are more
stable (Fig. 8, motifs 3 and 4). Both the DAE and DAO have a rigidity compa-
rable to linear duplex DNA [49], which makes them ideal building blocks for
the construction of DNA-based materials with designable structures. Parane-
mic crossover (PX) molecules are composed of two parallel helices, and form
crossovers at every point possible. This renders the tiles more stable than the
comparable DX molecule [50, 51].
Based on these DX tiles, a number of other useful motifs were extended
in this way. Triple crossover (TX) tiles can be regarded as being built from
two DAO-DX tiles that share the central helix [52]. In the same way that DX
tiles extend to the TX, more complex crossover tiles were developed. DNA
Fig. 10
Complex DNA motifs.
a
DX triangle self-assembly to a pseudohexagonal lat-
tice [57].
b
DNA triangles ligated to produce a linear array [58].
c
DNA three-point star
motif assembly to the hexagonal arrays [59].
d
Hexagonal structure composed of six tri-
angular complexes, and extended to a pair of overlapping hexagonal tilings [60].
e
16
cross-tiles construct directly to one square [62].
f
Self-assembly of the cross motifs to 2D
lattice [61]. Reproduced with permission from cited references
