Polyanionic DNA

Explain clearly why polyanionic DNA molecules adsorb onto electrode surfaces at potentials more positive than Epzc, and suggest a protocol for desorbing them back to the solution. 

These results indicate that the surface-anchored DNA blocks the electrochemical reaction of [Fe(CN)6]" with the underlying Au electrode, due to the electrostatic repulsion between the polyanionic DNA and the anionic redox couple ions. 

Nucleic acids, DNA and RNA, are attractive biopolymers that can be used for biomedical applications [175,176], nanostructure fabrication [177,178], computing [179,180], and materials for electron-conduction [181,182]. Immobilization of DNA and RNA in well-defined nanostructures would be one of the most unique subjects in current nanotechnology. Unfortunately, a silica surface cannot usually adsorb duplex DNA in aqueous solution due to the electrostatic repulsion between the silica surface and polyanionic DNA. However, Fujiwara et al. recently found that duplex DNA in protonated phosphoric acid form can adsorb on mesoporous silicates, even in low-salt aqueous solution [183]. The DNA adsorption behavior depended much on the pore size of the mesoporous silica. Plausible models of DNA accommodation in mesopore silica channels are depicted in Figure 4.20. Inclusion of duplex DNA in mesoporous silicates with larger pores, around 3.8 nm diameter, would be accompanied by the formation of four water monolayers on the silica surface of the mesoporous inner channel (Figure 4.20A), where sufficient quantities of Si—OH groups remained after solvent extraction of the template (not by calcination). 

Because of the nature of electroporation, virtually any molecule can be introduced into cells. For transfer of DNA, the electroporation forces are important. An electrophoretic effect of the field causes the polyanion DNA to travel toward the positive electrode. Fluorescence studies have shown that DNA enters the cell through the pole facing the negative electrode, where the membrane is more destabilized and where the field will drive the DNA towards the center of the cell (245). Membrane resealing occurs after pore formation. Whereas pore formation happens in the microsecond time frame, membrane resealing happens over a range of minutes with variations depending on electrical parameters and temperature (246). 

Medically, anions are of great importance in many disease pathways. Cystic fibrosis, a genetic illness affecting a significant proportion of society, is caused by misregulation of chloride channels (17). There is, therefore, a real need for selective halide detection, as established methods of chloride analysis are unsuitable for biological applications (18). Cancer is caused by the uncontrolled replication of polyanionic DNA. Anion-binding proteins have also been implicated in the mechanism of Alzheimer s disease (19). 

Figure 8.6.4 Complementarity is also retained in electroneutral DNA duplexes in which one strand is a polycationic, the other the usual polyanionic DNA.

It can be concluded that CH6 as well as other aptamers bind cytochrome c rather unspeciflcally by ionic interactions of the polyanionic DNA and a protein containing a cationic cluster, at least when the aptamers are immobilized, as in SPR experiments. 

Block copolymers composed of a cationic segment and a hydrophilic segment spontaneously associate with polyanionic DNA to form block copolymer micelles. The distinct feature of the structure is that the core of the polyion complex between DNA and the polycation is coated by a layer of the hydrophilic polymer. The characteristic core-shell structure endows the complex a high colloidal stability and reduced interaction with blood components [12]. 

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