Microscopic Analysis And Mechanism Study Of Interaction Between DNA And Metal Ions

Deoxyribonucleic acid(DNA)is among the most important biological macromolecules and is the main carrier of biological storage and transmission of information.Metal ions play an important role in DNA biosynthesis,conformation maintenance,function exertion and regulation.The interaction of metal ions and their complexes with DNA is also the key technical foundation to the development of anticancer drugs,the manufacture of DNA biosensors,and the formation of metal nanowires.Therefore,the study of the interaction between metal ions and DNA has always attracted wide attention in the fields of bioinorganic chemistry,supramolecular coordination chemistry,and DNA nanotechnology.In this thesis,we aim at the principle of the interaction of silver ions and DNA rich in cytosine(C),which can form C-Ag-C structure.The structure of C-Ag-C is characterized based on highresolution transmission electron microscopy,in-situ liquid cell transmission electron microscopy and electrochemical analysis,and mechanism of their interactions are found.The major research contents and results are as follows:(1)Direct characterization of Ag+-DNA complexes using transmission electron microscopy(TEM).In absence of reducing agents,the existence of crystalline particles is observed in TEM,and the particles are confirmed to be Ag by electron diffraction.However,when the Ag NO3 sample is directly characterized,no particles exist,indicating that DNA is an important factor in the reduction of Ag+ to Ag particles.By characterizing the arrangement of Ag particles,most of the particles form linear or curved chain-like arrangements.(2)Based on in-situ,liquid cell transmission electron microscopy(LCTEM),the Ag+ and DNA complexes are characterized in situ and their morphological changes are observed.In this thesis,two liquid cell packaging methods are used: carbon film liquid cells and silicon nitride liquid cells,and the observation results of the two liquid cells are compared.The precipitation,fusion and decomposition of Ag particles are observed using the carbon film liquid cells.The precipitation process of the particles is: solvated electrons eaq-and hydrogen radicals H· generated by the interaction between the electron beam and water reduce Ag+ to Ag0 atoms,and then Ag0 atoms nucleate and grow.The fusion of Ag particles can be expressed in two ways: When two particles of similar size are fused,they approach each other to form nanorods.When two particles with a large difference in size are fused,the small particles will move towards the large particles,and finally merge into the large particles to form larger spherical particles.Because the Ag particles are bound to DNA chain,while DNA will decompose under the irradiation of electron beam,two similar particles can break free and fuse only after DNA decomposition,the fusion process is much slower than previous studies.The decomposition of Ag particles is also affected by the DNA chain.Compared with the dispersed particles and the particles forming chains,it is found that the dispersed particles will shrink or even disappear after 10-20 s,while the interchain particles will be gradually decomposed after the DNA strands are irradiated into fragments,therefore the whole process is 2 ~3 min.The interaction of the electron beam with water in the silicon nitride liquid cell is more pronounced,and the particles are observed with lower resolution due to the thicker water layer.(3)Two prototypes of DNA electrochemical sensors,hairpin DNA sensor and graphene oxide adsorption-based DNA sensor,are constructed based on the binding principle of C-Ag-C.The hairpin DNA sensor self-assembles the single-stranded DNA with thiol modified at one end to the surface of the gold electrode.After treatment with different concentrations of Ag+,the singlestranded DNA is folded into a hairpin double-stranded due to the formation of C-Ag-C.Using the AC impedance method to test the impedance of the system under different conditions,it is found that at low Ag+ concentration,the impedance increase with the increase of the concentration.It indicates that the addition of a small amount of Ag+ causes the surface thiol ss DNA to form a dense DNA-membrane,hindering electron transfer.When an excess of Ag+ is added,it will be adsorbed on the DNA backbone through non-specific interaction,which will enhance the electron transfer.Besides,DNA sensor based on graphene oxide adsorption is constructed by coating graphene oxide(GO)on the treated gold electrode,in which GO can adsorb single-stranded DNA through π-π stacking.Because of the interaction of C-Ag-C,single-stranded DNA form a double-stranded structure and detach from GO.Differential pulse voltammetry is applied to detect the effect of different concentrations of Ag+ on the peak current of the system,and an Ag+ concentration sensor with a detection limit of 1 p M is realized.