Rapid chiral assembly of rigid DNA building blocks for molecular nanofabrication

Pyramidal and chiral groupings of gold nanocrystals assembled using DNA scaffolds

Nanostructures constructed from metal and semiconductor nanocrystals conjugated to and organized by DNA are an emerging class of materials with collective optical properties. We created discrete pyramids of DNA with gold nanocrystals at the tips. By taking small-angle X-ray scattering measurements from solutions of these pyramids, we confirmed that this pyramidal geometry creates structures which are more rigid in solution than linear DNA. We then took advantage of the tetrahedral symmetry to demonstrate construction of chiral nanostructures.

Scaffolded DNA origami of a DNA tetrahedron molecular container

We describe a strategy of scaffolded DNA origami to design and construct 3D molecular cages of tetrahedron geometry with inside volume closed by triangular faces. Each edge of the triangular face is approximately 54 nm in dimension. The estimated total external volume and the internal cavity of the triangular pyramid are about 1.8 x 10(-23) and 1.5 x 10(-23) m(3), respectively. Correct formation of the tetrahedron DNA cage was verified by gel electrophoresis, atomic force microscopy, transmission electron microscopy, and dynamic light scattering techniques.

Self-assembly of a nanoscale DNA box with a controllable lid

The unique structural motifs and self-recognition properties of DNA can be exploited to generate self-assembling DNA nanostructures of specific shapes using a 'bottom-up' approach. Several assembly strategies have been developed for building complex three-dimensional (3D) DNA nanostructures. Recently, the DNA 'origami' method was used to build two-dimensional addressable DNA structures of arbitrary shape that can be used as platforms to arrange nanomaterials with high precision and specificity. A long-term goal of this field has been to construct fully addressable 3D DNA nanostructures. Here we extend the DNA origami method into three dimensions by creating an addressable DNA box 42 x 36 x 36 nm(3) in size that can be opened in the presence of externally supplied DNA 'keys'. We thoroughly characterize the structure of this DNA box using cryogenic transmission electron microscopy, small-angle X-ray scattering and atomic force microscopy, and use fluorescence resonance energy transfer to optically monitor the opening of the lid. Controlled access to the interior compartment of this DNA nanocontainer could yield several interesting applications, for example as a logic sensor for multiple-sequence signals or for the controlled release of nanocargos.

Design and synthesis of novel hybrid metal complex-DNA conjugates: key building blocks for multimetallic linear DNA nanoarrays

We describe here a short and efficient synthetic route for incorporating terpyridine based metal complexes at the 3'-extremity of both single and bis-oligonucleotide (bis-ODN) stretches to form novel metal complex-ODN conjugates. All single stranded mono and bis-ODN tethered metal complexes and the respective duplex ODNs were characterized by circular dichroism spectroscopy and UV-Vis melting experiments. Duplexes formed by these hybrid metal complex-DNA conjugates showed around 4-5 degrees C stabilization with respect to the unmodified duplexes. Moreover hybridization of two complementary bis-ODN tethered metal complexes at different ratios in solution gave rise to a self-assembled nanometric linear network, which was characterized by non-denaturing gel electrophoresis and TEM studies. Thus, our simple synthetic strategy would contribute to develop multimetallic 2D-DNA nanoarrays where we can place different metal complexes at regular intervals along the ODN stretches.

The self-assembly of DNA Holliday junctions studied with a minimal model

In this paper, we explore the feasibility of using coarse-grained models to simulate the self-assembly of DNA nanostructures. We introduce a simple model of DNA where each nucleotide is represented by two interaction sites corresponding to the sugar-phosphate backbone and the base. Using this model, we are able to simulate the self-assembly of both DNA duplexes and Holliday junctions from single-stranded DNA. We find that assembly is most successful in the temperature window below the melting temperatures of the target structure and above the melting temperature of misbonded aggregates. Furthermore, in the case of the Holliday junction, we show how a hierarchical assembly mechanism reduces the possibility of becoming trapped in misbonded configurations. The model is also able to reproduce the relative melting temperatures of different structures accurately and allows strand displacement to occur.

On existence of reporter strands in DNA-based graph structures

Through self-assembly of branched junction molecules many different DNA structures (graphs) can be assembled. We show that every multigraph can be assembled by DNA such that there is a single strand that traces each edge in the graph at least once. This strand corresponds to a boundary component of a two-dimensional orientable surface that has the given graph as a deformation retract. This boundary component traverses every edge at least once, and it defines a circular path in the graph that "preserves the graph structure" and traverses each edge.

Designed alpha-helical tectons for constructing multicomponent synthetic biological systems

One possible route to develop new synthetic-biological systems is to assemble discrete nanoscale objects from programmed peptide-based building blocks. We describe an algorithm to design such blocks based on the coiled-coil protein-folding motif. The success of the algorithm is demonstrated by the production of six peptides that form three target parallel, blunted-ended heterodimers in preference to any of the other promiscuous pairings and alternate configurations, for example, homodimers, sticky-ended assemblies, and antiparallel arrangements. The peptides were linked to promote the assembly of larger, defined nanoscale rods, thus demonstrating that targeted peptide-peptide interactions can be specified in complex mixtures.

Designer DNA nanoarchitectures

Naturally existing biological systems, from the simplest unicellular diatom to the most sophisticated organ such as the human brain, are functional self-assembled architectures. Scientists have long been dreaming about building artificial nanostructures that can mimic such elegance in nature. Structural DNA nanotechnology, which uses DNA as a blueprint and building material to organize matter with nanometer precision, represents an appealing solution to this challenge. On the basis of the knowledge of helical DNA structure and Watson-Crick base pairing rules, scientists have constructed a number of DNA nanoarchitectures with a large variety of geometries, topologies, and periodicities with considerably high yields. Modified by functional groups, those DNA nanostructures can serve as scaffolds to control the positioning of other molecular species, which opens opportunities to study intermolecular synergies, such as protein-protein interactions, as well as to build artificial multicomponent nanomachines. In this review, we summarize the principle of DNA self-assembly, describe the exciting progress of structural DNA nanotechnology in recent years, and discuss the current frontier.

Sensing of UO22+ and design of logic gates by the application of supramolecular constructs of ion-dependent DNAzymes

Supramolecular constructs composed of ion-dependent DNAzymes and their substrates were used to develop DNAzyme cascades that enabled the sensitive detection of UO22+ or the activation of logic gate operations. The supramolecular complex between the UO22+-dependent DNAzyme and its substrate leads, in the presence of UO22+, to the cleavage of the substrate and to the release of the HRP-mimicking DNAzyme that enables the optical analysis of UO22+ (detection limit 1 x 10-9 M). Similarly, supramolecular complexes between the Mg2+- and UO22+-dependent DNAzymes and tailored substrates enables the design of the "OR" and "AND" logic gates, using Mg2+ and UO22+ as inputs.

Fluorescent DNA nanotags based on a self-assembled DNA tetrahedron

Progress in fluorescence detection and imaging technologies depends on the availability of fluorescent labels with strong light absorption/emission characteristics. We have synthesized intercalator dye arrays on a compact 3-dimensional DNA-tetrahedron nanostructure. The template tolerates the structural distortions introduced by intercalation and allows concentration of multiple fluorophores within a small volume, resulting in brightly fluorescent nanotags with effective extinction coefficients in the order of 10(6) M(-1) cm(-1). Efficient energy transfer from intercalated donor dyes to covalently attached acceptor dyes in the nanotags allows the emission wavelength to be shifted to the red relative to the excitation light, providing wavelength tunability. The compact nature of the supramolecular DNA tetrahedron also provides a protective medium for the fluorophores, leading to improved photostability and enhanced resistance to nuclease digestion, relative to one- or two-dimensional nanotags described previously.

Supramolecular domains in mixed peptide self-assembled monolayers on gold nanoparticles

Self-organization in mixed self-assembled monolayers of small molecules provides a route towards nanoparticles with complex molecular structures. Inspired by structural biology, a strategy based on chemical cross-linking is introduced to probe proximity between functional peptides embedded in a mixed self-assembled monolayer at the surface of a nanoparticle. The physical basis of the proximity measurement is a transition from intramolecular to intermolecular cross-linking as the functional peptides get closer. Experimental investigations of a binary peptide self-assembled monolayer show that this transition happens at an extremely low molar ratio of the functional versus matrix peptide. Molecular dynamics simulations of the peptide self-assembled monolayer are used to calculate the volume explored by the reactive groups. Comparison of the experimental results with a probabilistic model demonstrates that the peptides are not randomly distributed at the surface of the nanoparticle, but rather self-organize into supramolecular domains.

Self-assembly of DNA arrays into multilayer stacks

We describe the self-assembly of multilayer hexagonal DNA arrays through highly regular interlayer packing. Slow cooling of a mixture of three single-stranded DNA sequences with various Mg2+ concentrations leads to the self-assembly of diverse multilayer architectures. The self-assembled aggregates were deposited onto mica surfaces and examined with atomic force microscopy. The size of the two-dimensional arrays and subsequent stacking to form multilayer structures are highly dependent on Mg2+ concentration. DNA bilayers and multilayers of defined shape are favored in 2-5 mM Mg2+ with an average lateral size of 700 nm. Arrays are much larger (up to 20 microm across) in 10-15 mM Mg2+, although multiple layers still make up 20-60% of the observed structures. Domains within single layer architectures were identified using Moiré pattern analysis. Distinct structural phases within the multilayer assemblies include two layers translated by 17.5 nm and interlayer rotations of 20 degrees and 30 degrees. Three layer assemblies have cubic close packing and taller multilayer architectures of 2D DNA sheets were also identified.

Self-assembling DNA-caged particles: nanoblocks for hierarchical self-assembly

DNA is an ideal candidate to organize matter on the nanoscale, primarily due to the specificity and complexity of DNA based interactions. Recent advances in this direction include the self-assembly of colloidal crystals using DNA grafted particles. In this paper we theoretically study the self-assembly of DNA-caged particles. These nanoblocks combine DNA grafted particles with more complicated purely DNA based constructs. Geometrically the nanoblock is a sphere (DNA grafted particle) inscribed inside a polyhedron (DNA cage). The faces of the DNA cage are open, and the edges are made from double stranded DNA. The cage vertices are modified DNA junctions. We calculate the equilibriuim yield of self-assembled, tetrahedrally caged particles, and discuss their stability with respect to alternative structures. The experimental feasability of the method is discussed. To conclude we indicate the usefulness of DNA-caged particles as nanoblocks in a hierarchical self-assembly strategy.

Synthesis and Applications of Covalent Protein-DNA Conjugates

Synthetic protein-DNA conjugates are valuable tools with applications in fields including nanobiotechnology, bioanalytical chemistry, and molecular diagnostics, and various synthetic methods for their production have been developed during the past three decades. The present article reviews current methodologies for the synthesis of covalent protein-DNA conjugates with particular focus on the regiospecificity and stoichiometry of these reactions.

Fabrication of DNA nanowires by orthogonal self-assembly and DNA intercalation on a Au patterned Si/SiO2 surface

A novel Ru complex bearing both an acridine group and anchoring phosphonate groups was immobilized on a surface in order to capture double-stranded DNAs (dsDNAs) from solution. At low surface coverage, the atomic force microscopy (AFM) image revealed the "molecular dot" morphology with the height of the Ru complex ( approximately 2.5 nm) on a mica surface, indicating that four phosphonate anchor groups keep the Ru complex in an upright orientation on the surface. Using a dynamic molecular combing method, the DNA capture efficiency of the Ru complex on a mica surface was examined in terms of the effects of the number of molecular dots and surface hydrophobicity. The immobilized surface could capture DNAs; however, the optimal number of molecular dots on the surface as well as the optimal pull-up speed exist to obtain the extended dsDNAs on the surface. Applying this optimal condition to a Au-patterned Si/SiO 2 (Au/SiO 2) surface, the Au electrode was selectively covered with the Ru complex by orthogonal self-assembly of 4-mercaptbutylphosphonic acid (MBPA), followed by the formation of a Zr (4+)-phosphonate layer and the Ru complex. At the same time, the remaining SiO 2 surface was covered with octylphosphonic acid (OPA) by self-assembly. The selective immobilization of the Ru complex only on the Au electrode was identified by time-of-flight secondary-ion mass spectrometry (TOF-SIMS) imaging on the chemically modified Au/SiO 2 surface. The construction of DNA nanowires on the Au/SiO 2 patterned surface was accomplished by the molecular combing method of the selective immobilized Ru complex on Au electrodes. These interconnected nanowires between Au electrodes were used as a scaffold for the modification of Pd nanoparticles on the DNA. Furthermore, Cu metallization was achieved by electroless plating of Cu metal on a priming of Pd nanoparticles on the Pd-covered DNA nanowires. The resulting Cu nanowires showed a metallic behavior with relatively high resistance.

In vivo cloning of artificial DNA nanostructures

Mimicking nature is both a key goal and a difficult challenge for the scientific enterprise. DNA, well known as the genetic-information carrier in nature, can be replicated efficiently in living cells. Today, despite the dramatic evolution of DNA nanotechnology, a versatile method that replicates artificial DNA nanostructures with complex secondary structures remains an appealing target. Previous success in replicating DNA nanostructures enzymatically in vitro suggests that a possible solution could be cloning these nanostructures by using viruses. Here, we report a system where a single-stranded DNA nanostructure (Holliday junction or paranemic cross-over DNA) is inserted into a phagemid, transformed into XL1-Blue cells and amplified in vivo in the presence of helper phages. High copy numbers of cloned nanostructures can be obtained readily by using standard molecular biology techniques. Correct replication is verified by a number of assays including nondenaturing PAGE, Ferguson analysis, endonuclease VII digestion, and hydroxyl radical autofootprinting. The simplicity, efficiency, and fidelity of nature are fully reflected in this system. UV-induced psoralen cross-linking is used to probe the secondary structure of the inserted junction in infected cells. Our data suggest the possible formation of the immobile four-arm junction in vivo.

Conformational flexibility facilitates self-assembly of complex DNA nanostructures

Molecular self-assembly is a promising approach to the preparation of nanostructures. DNA, in particular, shows great potential to be a superb molecular system. Synthetic DNA molecules have been programmed to assemble into a wide range of nanostructures. It is generally believed that rigidities of DNA nanomotifs (tiles) are essential for programmable self-assembly of well defined nanostructures. Recently, we have shown that adequate conformational flexibility could be exploited for assembling 3D objects, including tetrahedra, dodecahedra, and buckyballs, out of DNA three-point star motifs. In the current study, we have integrated tensegrity principle into this concept to assemble well defined, complex nanostructures in both 2D and 3D. A symmetric five-point-star motif (tile) has been designed to assemble into icosahedra or large nanocages depending on the concentration and flexibility of the DNA tiles. In both cases, the DNA tiles exhibit significant flexibilities and undergo substantial conformational changes, either symmetrically bending out of the plane or asymmetrically bending in the plane. In contrast to the complicated natures of the assembled structures, the approach presented here is simple and only requires three different component DNA strands. These results demonstrate that conformational flexibility could be explored to generate complex DNA nanostructures. The basic concept might be further extended to other biomacromolecular systems, such as RNA and proteins.

Metallic nanoparticles used to estimate the structural integrity of DNA motifs

Branched DNA motifs can be designed to assume a variety of shapes and structures. These structures can be characterized by numerous solution techniques; the structures also can be inferred from atomic force microscopy of two-dimensional periodic arrays that the motifs form via cohesive interactions. Examples of these motifs are the DNA parallelogram, the bulged-junction DNA triangle, and the three-dimensional-double crossover (3D-DX) DNA triangle. The ability of these motifs to withstand stresses without changing geometrical structure is clearly of interest if the motif is to be used in nanomechanical devices or to organize other large chemical species. Metallic nanoparticles can be attached to DNA motifs, and the arrangement of these particles can be established by transmission electron microscopy. We have attached 5 nm or 10 nm gold nanoparticles to every vertex of DNA parallelograms, to two or three vertices of 3D-DX DNA triangle motifs, and to every vertex of bulged-junction DNA triangles. We demonstrate by transmission electron microscopy that the DNA parallelogram motif and the bulged-junction DNA triangle are deformed by the presence of the gold nanoparticles, whereas the structure of the 3D-DX DNA triangle motif appears to be minimally distorted. This method provides a way to estimate the robustness and potential utility of the many new DNA motifs that are becoming available.

Assembly pathway analysis of DNA nanostructures and the construction of parallel motifs

We present a system for analyzing the assembly pathway of DNA nanostructures. This enables the identification, explanation, and avoidance of obstacles to proper structure formation. Potential problems include strand end-pinning and misfolding caused by the structural bias of nominally flexible junctions. We have used this system to guide the construction of parallel motifs that had previously, for unknown reasons, resisted assembly.

A full-automatic sequence design algorithm for branched DNA structures

Production of various structures by self-assembling single stranded DNA molecules is a widely used technology in the filed of DNA nanotechnology. Base sequences of single strands do predict the shape of the resulting nanostructure. Therefore, sequence design is crucial for the successful structure fabrication. This paper presents a sequence design algorithm based on mismatch minimization that can be applied to every desired DNA structure. With this algorithm, junctions, loops, single as well as double stranded regions, and very large structures up to several thousand base pairs can be handled. Thereby, the algorithm is fast for the most structures. Algorithm is Java-implemented. Its implementation is called Seed and is available publicly. As an example for a successful sequence generation, this paper presents the fabrication of DNA chain molecules consisting of double-crossover (DX) tiles as well.

From DNA nanotechnology to synthetic biology

Attempts to construct artificial systems from biological molecules such as DNA and RNA by self-assembly are compatible with the recent development of synthetic biology. Genetic mechanisms can be used to produce or control artificial structures made from DNA and RNA, and these structures can in turn be used as artificial gene regulatory elements, in vitro as well as in vivo. Artificial biochemical circuits can be incorporated into cell-like reaction compartments, which opens up the possibility to operate them permanently out of equilibrium. In small systems, stochastic effects become noticeable and will have to be accounted for in the design of future systems.

Assembly and structural analysis of a covalently closed nano-scale DNA cage

The inherent properties of DNA as a stable polymer with unique affinity for partner molecules determined by the specific Watson-Crick base pairing makes it an ideal component in self-assembling structures. This has been exploited for decades in the design of a variety of artificial substrates for investigations of DNA-interacting enzymes. More recently, strategies for synthesis of more complex two-dimensional (2D) and 3D DNA structures have emerged. However, the building of such structures is still in progress and more experiences from different research groups and different fields of expertise are necessary before complex DNA structures can be routinely designed for the use in basal science and/or biotechnology. Here we present the design, construction and structural analysis of a covalently closed and stable 3D DNA structure with the connectivity of an octahedron, as defined by the double-stranded DNA helices that assembles from eight oligonucleotides with a yield of approximately 30%. As demonstrated by Small Angle X-ray Scattering and cryo-Transmission Electron Microscopy analyses the eight-stranded DNA structure has a central cavity larger than the apertures in the surrounding DNA lattice and can be described as a nano-scale DNA cage, Hence, in theory it could hold proteins or other bio-molecules to enable their investigation in certain harmful environments or even allow their organization into higher order structures.

Reconfigurable, braced, three-dimensional DNA nanostructures

DNA nanotechnology makes use of the exquisite self-recognition of DNA in order to build on a molecular scale. Although static structures may find applications in structural biology and computer science, many applications in nanomedicine and nanorobotics require the additional capacity for controlled three-dimensional movement. DNA architectures can span three dimensions and DNA devices are capable of movement, but active control of well-defined three-dimensional structures has not been achieved. We demonstrate the operation of reconfigurable DNA tetrahedra whose shapes change precisely and reversibly in response to specific molecular signals. Shape changes are confirmed by gel electrophoresis and by bulk and single-molecule Förster resonance energy transfer measurements. DNA tetrahedra are natural building blocks for three-dimensional construction; they may be synthesized rapidly with high yield of a single stereoisomer, and their triangulated architecture conveys structural stability. The introduction of shape-changing structural modules opens new avenues for the manipulation of matter on the nanometre scale.