Assembly Of Nanocrystals With Ordered Structure Induced By Oligonucleotides

Most important biomolecules, such as DNA, protein etc, their sizes are 2 to 200 nm similar to nanocrystals. Because of the special structure and fundamental features, biomolecules display several surface and interface properties. Biomolecules, as a kind of important future building blocks, have been used for the formation of nanocrystal architectures of predesigned shapes, sizes and compositions. Nanocrystals have highly interesting optical, electronic, and magnetic properties due to the quantum confine effects and surface effects. The using of nanocrystals as a biotechnological tool is particularly attractive. DNA, as one of the most important biomolecules, is particularly suitable to serve as a construction material in nanosciences.On the point of physical chemistry and biochemistry, this dissertation is focused on studying the complex system of DNA and nanocrystals. First, the interaction between the DNA and CdS nanocrystals, which have different surface properties, were studied. It was found that the cooperativity between the DNA and CdS nanocrystals were different with the surface properties of the CdS nanocrystals changed. Based on the above results, we developed a method for mediate the architectures of the nanocrystals in the aggregates by the steric effect related to the length or the bending angle of the DNA linker. Furthermore, the protein-DNA interactions were introduced into the DNA-Au assembly system. This work puts forward the potentials of DNA-polypetide complexes in assembling nanoparticles into ordered nanostructures and the use of nanoparticle probes in following DNA-polypeptide interactions in biological events.There are mainly four parts in the dissertation:1) The interactions between the DNA and the surface Cd2+-rich, S2--rich, neutral CdS nanocrystals were studied. It is found that the binding of oligo-DNA to the surface S2--rich CdS nanocrystal shows negative cooperativity and the binding of oligo-DNA to the surface neutral CdS nanocrystal shows positive cooperativity. In case of the surface Cd2+-rich CdS nanocrystal, no cooperativity can be identified for the binding of oligo-DNA.2) The interactions at the interfaces of phosphate backbone sulfur modified oligo-DNA (PM oligo-DNA) and Au nanocrystals were studied. It was found that the PM oligo-DNA strands tended to be loaded approximately paralled on the particle surface, different with the most studied terminal modified oligo-DNA (TM oligo-DNA), in which the thiol group is modified at DNA's termini (5'or 3'). In the assemblies directed by TM oligo-DNA, the nanocrystals incline to be arranged in a closed-packed fashion and the distance between the nanocrystals is primarily dominated by the length of the DNA duplex. Due to the approximately parallel orientation of PM oligo-DNA on the particle surface, the amount of the helixes allowed to accompanying one nanocrystal in a two-dimensional (2D) plane is determined by their length. Architectures of the nanocrystals in the aggregates can be mediated by such a steric effect related to the length of the PM oligo-DNA linkers employed. When the helixes containing different length were employed, the same (or different) sized Au nanocrystals can be arranged in closed-packed, square, trigonal manners respectively.3) The architecture of the assembly of the Au nanocrystals could be mediated by the steric effect related to the bending angle of the linker DNA. It was found that whether for the TM oligo-DNA or PM oligo-DNA, the architecture of the assembly could be mediated by changing the bending angle of the linker DNA. For TM oligo-DNA loaded almost perpendicularly on the particle surface, the architecture is converted from closed-packed to non-closed-packed fashion with increased bending angle of the linker molecules as a result of increased steric effect. For PM oligo-DNA loaded approximately parallel on the particle surface, the architecture experiences a conversion from non-closed-packed to closed-packed fashion due to the decreased steric effect with the increased bending angle of the linker molecules. Such a work provides an alternate way to control the DNA directed assembly of colloidal nanocrystals with desired architectures.4) Assembly of the Au nanocrystals can be induced by the protein-DNA interactions. The DNA modified Au nanocrystals were found to aggregate after the addition of PLL. At low pH values, the lysine residues of the PLL were protonated, there was strong electrostatic interaction between the backbone phosphate groups of the DNA molecular and the positively charged lysine residues, which resulting the aggregation of the nanocrystals. With an increase of the pH value, the deprotonated lysine residues allow the formation ofα-helix andβ-sheet structures. The nanocrystals aggregated primarily due to the hydrogen bonds between the bases of the DNA and theα-helix of the PLL at this time. Such a work puts forward the potentials of DNA-polypetide complexes in assembling nanoparticles into ordered nanostructures and the use of nanoparticle probes in following DNA-polypeptide interactions in biological events.