Furong Liu

Design and self-assembly of two-dimensional DNA crystals

Molecular self-assembly presents a ‘bottom-up’ approach to the fabrication of objects specified with nanometre precision. DNA molecular structures and intermolecular interactions are particularly amenable to the design and synthesis of complex molecular objects. We report the design and observation of two-dimensional crystalline forms of DNA that self-assemble from synthetic DNA double-crossover molecules. Intermolecular interactions between the structural units are programmed by the design of ‘sticky ends’ that associate according to Watson–Crick complementarity, enabling us to create specific periodic patterns on the nanometre scale. The patterned crystals have been visualized by atomic force microscopy.

New motifs in DNA nanotechnology

Recently, we have invested a great deal of effort to construct molecular building blocks from unusual DNA motifs. DNA is an extremely favorable construction medium. The sticky-ended association of DNA molecules occurs with high specificity, and it results in the formation of B-DNA, whose structure is well known. The use of stable-branched DNA molecules permits one to make stick-figures. We have used this strategy to construct a covalently closed DNA molecule whose helix axes have the connectivity of a cube, and a second molecule, whose helix axes have the connectivity of a truncated octahedron. In addition to branching topology, DNA also yields control of linking topology, because double helical half-turns of B-DNA or Z-DNA can be equated, respectively, with negative or positive crossings in topological objects. Consequently, we have been able to use DNA to make trefoil knots of both signs and figure of 8 knots. By making RNA knots, we have discovered the existence of an RNA topoisomerase. DNA-based topological control has also led to the construction of Borromean rings, which could be used in DNA-based computing applications. The key feature previously lacking in DNA construction has been a rigid molecule. We have discovered that DNA double crossover molecules can provide this capability. We have incorporated these components in DNA assemblies that use this rigidity to achieve control on the geometrical level, as well as on the topological level. Some of these involve double crossover molecules, and others involve double crossovers associated with geometrical figures, such as triangles and deltahedra.

Two dimensions and two states in DNA nanotechnology

Abstract The construction of periodic matter and nanomechanical devices are central goals of DNA nanotechnology. The minimal requirements for components of designed crystals are [1] programmable interactions, [2] predictable local intermolecular structures and [3] rigidity. The sticky-ended association of DNA molecules fulfills the first two criteria, because it is specific and diverse, and it results in the formation of B-DNA. Stable branched DNA molecules permit the formation of networks, but individual single branches are too flexible. Antiparallel DNA double crossover (DX) molecules can provide the necessary rigidity, so we use these components to tile the plane. It is possible to include DNA hairpins that act as topographic labels for this 2-D crystalline array, because they protrude from its plane. By altering sticky ends, it is possible to change the topographic features formed by these hairpins, and to detect these changes by means of AFM. We can modify arrays by restricting hairpins or by adding them to sticking ends protruding from the array. Although individual branched junctions are unsuitable for use as crystalline components, parallelograms of four 4-arm junction molecules are sufficiently rigid that they can be used to produce 2D arrays. The arrays contain cavities whose dimensions are readily tuned by changing the edges of their parallelogram components. We have used these arrays to measure directly the angle between the helices of the Holliday junction. The rigidity of the DX motif can also be exploited to produce a nanomechanical device predicated on the B-Z transition. Two DNA double crossover molecules have been joined by a segment of DNAcapable of undergoing the B-Z transition. In the B-conformation, the unconnected helices of the two molecules are on the same side of the connecting helix, whereas in the Z conformation they are on opposite sides, leading to movements of as much as 60Å. This effect is shown by fluorescence resonance energy transfer, because dyes attached to the unconnected helices have different separations in the two states.

Nicks, Nodes, and New Motifs for DNA Nanotechnology

The properties that make DNA such an effective molecule for its biological role as genetic material also make it a superb molecule for nanoconstruction. One key to using DNA for this purpose is to produce stable complex motifs, such as branched molecules. Combining branched species by sticky ended interactions, leads to N- connected stick figures whose edges consist of double helical DNA. Zero node removal or reciprocal crossover, leads to complex fused motifs, such as rigid multi-crossover molecules and paranemic crossover molecules. Multi-crossover molecules have been used to produce 2D arrays and a nanomechanical device. Algorithmic assembly and the use of complex complementarities for joining units are goals in progress that are likely to produce new capabilities for DNA nanotechnology.

Experiments in structural DNA nanotechnology: Arrays and devices

In recent years, the chemistry of DNA has expanded from biological systems to nanotechnology. The generalization of the biological processes of reciprocal exchange leads to stable branched motifs that can be used for the construction of DNA-based geometrical and topological objects, arrays and nanomechanical devices. The information in DNA is the basis of life, but it can also be used to control the physical states of a variety of systems, leading ultimately to nanorobotics; these devices include shape-changing, walking and translating machines. We expect ultimately to be able to use the dynamic information-based architectural properties of nucleic acids to be the basis for advanced materials with applications from nanoelectronics to biomedical devices on the nanometer scale.