Song Gao

Nucleic acid supercoiling as a means for ionic switching of DNA--nanoparticle networks.

Oligomeric nanoparticle networks, generated by the self‐assembly of bis‐biotinylated double‐stranded DNA fragments and streptavidin, have been studied by scanning force microscopy (SFM). SFM imaging revealed the presence within the networks of irregular thick DNA molecules, which were often associated with distinct, Y‐shaped structural elements. Closer analysis revealed that the Y structures are formed by condensation (thickening and shortening) of two DNA fragments, most likely through the supercoiling of two DNA molecules bound to adjacent binding sites of the streptavidin particle. The frequency of supercoiling was found to be dependent on the ionic strength applied during the immobilization of the oligomeric networks on mica surfaces. Potential applications of the structural changes as a means for constructing ion‐dependent molecular switches in nanomaterials are discussed.

High‐Quality Mapping of DNA–Protein Complexes by Dynamic Scanning Force Microscopy

Biomolecular complexes, nanocircles and aggregates of DNA and streptavidin (STV) are studied by dynamic scanning force microscopy. More structural details are observed with an improved dynamic mode assisted with a special feedback circuit (Q-control), as shown in the picture. Under otherwise identical conditions, these improvements indicate that the well cited models relating enlarged lateral size to the finite geometry of the SFM tip have to be modified for soft samples susceptible to tip-sample interactions.

Supramolecular DNA-streptavidin nanocircles with a covalently attached oligonucleotide moiety.

Abstract Covalent hybrid conjugates consisting of streptavidin (STV) and a 24-mer single-stranded DNA oligonucleotide have been used as a starting material for the synthesis of supramolecular nanocircles. For this, the covalent hybrid conjugates were oligomerized by cross-linking with 5′,5′-bis-biotinylated double-stranded DNA (dsDNA) fragments of various length. Heat denaturation of the resulting oligomeric conjugates and subsequent rapid cooling led to the formation of the nanocircles, in which the oligonucleotide-containing STV molecule is coupled with both ends of the circular bis-biotinylated dsDNA fragment. The circular structure of the bioconjugates was established by electrophoretic studies including Ferguson plot analysis as well as by scanning force microscopy (SFM) inspection. The formation process and the stability against degradation by ligand exchange with free D-biotin was compared for the nanocircles obtained from covalent oligonucleotide-STV hybrids and native STV. The former nanocircles revealed a decreased stability with respect to ring opening than the circles obtained from native STV. This suggested that the affinity of the covalent oligonu- cleotide-STV hybrid for binding biotinylated DNA is significantly decreased. Nevertheless, the single-stranded oligonucleotide moiety of the hybrid nanocircles can be used as a molecular handle for further functionalization. For instance, it was used for the selective DNA-directed immobilization at a surface, previously functionalized with complementary capture oligonucleotides. Moreover, we demonstrate that a pair of nanocircles, containing complementary oligonucleotide moieties, can be hybridized to form specific dimers, thereby generating a novel type of supramolecular DNA-protein nanostructures.

Investigation of the Covalently Attached Multilayer Architecture Based on Diazo‐Resins and Poly(4‐styrene sulfonate)

It is reported on the multilayer architecture containing a photo-reactive polyelectrolyte diazo-resins (DAR) as polycation and poly(4-styrene sulfonate) (PSS) as polyanion, held together by electrostatic interaction. Upon UV irradiation, the adjacent interfaces of the multilayer reacted to form a covalently crosslinking structure which greatly improved the stability of the films. These changes were confirmed by UV-VIS and FTIR spectroscopy. The thickness of the covalently attached films was characterized with X-ray diffraction, results from which showed that the thickness of the films can be adjusted in a nanometer scale by adding salts or diluting the solution of DAR. Atomic force microscopy (AFM) experiments showed that both the films before and after UV irradiation were closely compacted as well as smooth, with no obvious changes in morphology to be observed during the photoreaction. It was confirmed that this was an effective way to fabricate photosensitive and then stable layered nanoarchitecture.

Immobilization of gold nanoparticles on solid supports utilizing DNA hybridization

Abstract Self-organization of colloidal metal nanoparticles into micro- and nanostructured assemblies is currently of tremendous interest promising to find new size- and structure-dependent physical properties. Owing to its unique recognition capabilities and physicochemical stability, DNA can be used as a molecular linker for gold nanoparticles and is a promising construction material for their precise spatial positioning. Due to the enormous specificity of nucleic acid hybridization, the site-specific immobilization of DNA-functionalized gold colloids (1–40 nm) to solid supports, previously functionalized with a complementary DNA array, allows the fabrication of novel nanostructured surface architectures. Scanning force microscopy (SFM), used to characterize the intermediate steps of the DNA-directed immobilization (DDI) on a gold substrate, provides initial insight into the specificity and efficiency of this technique.

Nano-size stripes of self-assembled bolaform amphiphiles

Self-assembled nano-sized stripes are obtained spontaneously nby electrostatic adsorption of bolaform amphiphiles onto mica sheets. The nordered stripes are separated from each other by about 10 nm, and the nordered region can extend over macroscopic areas.