DNA conductivity

Dekker and collaborators have published two numbers for DNA conduction in Ref. [65] they conclude that on the 100-nm-length scale DNA is an insulator, while on the 10-nm-length scale (see [72]) poly(GC) is suggested to be semiconducting. 

In another attempt to resolve the puzzle around the DNA conduction properties, de Pablo et al. [58] apphed a different technique to measure single A-DNA molecules on the surface in ambient conditions. They deposited many DNA molecules on mica, covered some of them partly with gold, and contacted the other end of one of the molecules (>70 nm from the electrode) with a metal AFM tip (see Fig. 7). No current was observed in this measurement. Furthermore, they covered 1,000 parallel molecules on both ends with metal electrodes ( 2 /zm apart) and again no current was observed. Yet another negative result, pubhshed in 2002, was obtained in a similar experiment by Zhang et al. [33] who stretched many single DNA molecules in parallel between metal electrodes and measured no current upon voltage application. Both results [33, 58] were consistent with the Storm et al. experiment [60]. 

NMR studies depicting natural products bound to DNA, conducted by the Patel laboratory, provided a detailed understanding as to how the carbohydrate motifs engaged double-stranded DNA and delivered selectivity.30 Structures of the DNA complexes of esperamicin At (Figure 3.2a)31 and calicheamicin X (Figure 3.2b)32 identified multiple interactions between the carbohydrate motifs and the DNA backbone. 

RNA, and proteins when compressed between platinum electrodes. Subsequent studies have established that proteins and ss DNA or RNA do not function as molecular wires however, the conductivity of ds DNA remains the subject of debate. Recently there have been several studies of DNA conductivity in fibers, single crystals, aligned films, and monolayer assemblies. Far from resolving the controversy, they seem to have intensified it. 

The double-strand structure of an oligonucleotide is shown schematically in Fig. 6-1. Anticipating discussion in later Sections, the molecule is shown in a upright orientation attached to an atomically planar metallic electrode surface (Au(lll), cf below) by chemisorption via a hexamethylenethiol group. Fig. 6-1 shows the four nucleobases presently in focus. We discuss first concepts and formalism of electron and hole transport of DNA-based molecules in homogeneous solution and at electrochemical interfaces. We then focus on DNA-based molecules in electrochemical nanogaps and STM in electrochemical environments in situ STM). Some case examples illustrate accordance and limitations of current theoretical views of DNA-conductivity. This adds to the comprehensive overview of interfacial electrochemical ET of DNA-based molecules by O Kelly and Hill in Chapter 5. 

As noted, merits and limitations of such views carry over to approaches to single-molecule DNA-conductivity in natural biological environments, to which we proceed next. 

We discuss two approaches to single-molecule DNA-conductivity, both referring to aqueous biological media. One approach has focused on isolated individual molecules, the other one on individual molecules but now inside an ordered two-dimensional 30-60 nm laterally extended assembly, with up to at least a hundred molecules inside a domain. The systems display partly concurring conductivity properties, particularly weak distance and base composition dependence, but the origin of these patterns is different in the two cases. 

Direct measurements of the electrical conductivity of DNA molecules allow comparisons with more conventional materials through which charge transport has been well characterized. Early studies of DNA conductivity utilized bulk or dry DNA samples and provided conflicting assessments of the extent of electron mobility [2,7,8], However, recent studies of single molecules or well-deflned DNA films have revealed significant levels of conductivity [26,27],... 

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