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Principle of Tensegrity and Mode of Assembly

A subtle balance of flexibility and stress is required to build good self-assembling tiles [158] but stable rigid 3° structural motifs are not an absolute requirement. The vertices of triangulated architectures can be flexible as triangulated structures should be able to resist deformation through tensegrity, a geometrical construction principle that combines stiff helical struts that push outward and flexible junctions that push inward (Fig. 10.10). By taking advantage of this principle, sta-

Fig. 10.10 Principle of tensegrity in DNA architectonics. The principle of tensegrity is illustrated in the construction of rigid objects like (a) a DNA triangle [154]; (b) a DNA tetrahedron [169] and (c) a DNA octahedron [102]. (a) (bottom): schematic representation of a DNA triangle that protrudes from a DNA

tile [154]; (b) (bottom): 3-D model and AFM image of a DNA tetrahedron (adapted with permission from reference [169]); (c) (bottom): low resolution 3-D structure model of a DNA octahedron obtained by cryo-EM and single image reconstruction (adapted with permission from Ref. [102]).

Fig. 10.10 Principle of tensegrity in DNA architectonics. The principle of tensegrity is illustrated in the construction of rigid objects like (a) a DNA triangle [154]; (b) a DNA tetrahedron [169] and (c) a DNA octahedron [102]. (a) (bottom): schematic representation of a DNA triangle that protrudes from a DNA

tile [154]; (b) (bottom): 3-D model and AFM image of a DNA tetrahedron (adapted with permission from reference [169]); (c) (bottom): low resolution 3-D structure model of a DNA octahedron obtained by cryo-EM and single image reconstruction (adapted with permission from Ref. [102]).

ble triangular DNA tiles able to assemble into extensive Kagome-like lattices [154, 155], a replicable octahedron cage [102] and rigid tetrahedron building blocks [169] have recently been constructed.

The structure of most DNA tiles imposes strong geometrical constraints over the positioning of their cohesive interfaces (Fig. 10.4). Typically, only a reduced number of different 4° supramolecular architectures can be generated from a particular design of tile. DNA cohesive interfaces are typically formed through complementary Watson-Crick base pairing between collinear tail connectors of adjacent tiles [159]. They can also occur through formation of paranemic crossovers between internal loops that are wrapped around one another and do not interpenetrate topologically [170] (Fig. 10.3). Variation in the number of tail connectors and their thermodynamic stability can be used to modulate the assembly process as a function of temperature, DNA molecules and salt concentration.

In the future, the use of triple helices [26], G-tetrads [27] and non Watson-Crick parallel strands [171] will probably expand the modes of assembly of DNA tiles. In fact, it has already been demonstrated that frayed 2-D and 3-D networks can potentially be generated with guanine-rich DNA oligonucleotides expected to form G quartets [172, 173]. Moreover, a continuous 3-D hexagonal lattice generated from a 13mer DNA oligonucleotide self-assembling through parallel-stranded base pairing was subsequently engineered to produce crystals with enlarged solvent channels [171].

Considering that DNA can fold into stable 3° aptamers and DNAzymes, it is clear that the full potential of DNA 3° structure for nanoconstruction has not been exploited yet. However, the real potential of DNA may lie more in the optimal use of its simple rules of assembly, based on the unique selectivity of Watson-Crick base pairing, rather than its 3° structure diversity, as exemplified by the recent development of scaffolded DNA origami [174] discussed in Section 10.7.1.4.

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