DNA-based conductivity

Studies such as these have addressed fundamental issues regarding DNA-based interfacial electrochemical charge transfer. The conclusions carry over to the evolving area of single-molecule DNA-based conductivity as addressed by electrochemical in situ and DNA-target... 

Interfacial electrochemical ET between metallic electrodes and redox molecules through variable-length and variable-composition DNA-based molecules has disclosed important information about the molecular conduction mechanisms, based on monolayers of molecular thickness but averaged over two-dimensional macroscopic assemblies. Important conclusions are that the molecular contact can be a controlling factor and that the conductivity is hypersensitive to base pair order and stacking. The conductivity is effectively turned off when base pair mismatches or kinks invoked by external molecular structure-modifier binding (say cis-platinum ). This view carries over in part to DNA-based conductivity at the single-molecule level but here some other modification is needed. 

Other biomoleciiles imaged have included all DNA bases [44], polysaccharides [45] and proteins [46, 4lZ]. In many cases there is strong evidence that the imaging process is facilitated by the presence of iiltrathin (conducting) water films on the surface of the sample [48, 49 and 50],... 

Extended solid state n systems facilitate CT, particularly when doped [4-6]. The analogy between DNA and conductive solid state -stacks therefore establishes that a requisite condition for CT may exist in DNA. DNA contains an array of heterocyclic aromatic base pairs, stacked at a distance of 3.4 A, wrapped within a negatively charged sugar phosphate backbone [7] (Fig. 1). The interactions between the n electrons of the DNA base pairs provide the electronic coupling necessary for CT to occur. 

Because both charge transport and conduction are facilitated by electron motion between stacked base pairs, one expects that single-strand DNA should conduct far less well than double strand. Measurements both of rate constants and of conduction [62] indeed show that single-strand DNA is a far less capable transfer and transport agent for charge than is the duplex. 

An interesting experiment on a DNA-based network embedded in a cast film had already been done by Okahata et al. in 1998 [49]. In this pioneering experiment the DNA molecules were embedded (with side groups) in a polymer matrix that was stretched between electrodes (see Fig. 12). It was found that the conductivity parallel to the stretching direction (along the DNA) was 4.5 orders of magnitude larger than the perpendicular conductivity. 

The two approaches are not unrelated and a complementary analysis of both kinds of studies would finally shed light onto the detailed mechanisms for charge migration along DNA wires [51]. The kinetic theories are reviewed in other chapters of this book. Here, we focus on results obtained for the electronic structure of extended DNA base stacks, and describe their influence on the electrical conductivity of DNA-based nanostructures. 

To arrange AuNPs into a monolayer, Simon and coworkers [74] reported a simple protocol via the oligonucleotides complementary immobiUzation. 5 -Amino-modified oligonucleotide was immobilized on the substrate surface first, and then, 15 nm gold particles modified with thiolated DNA oligomers were coupled to the surface through DNA base pairing. The resulted nanoparticle monolayers were demonstrated to have a thermally activated conductivity. A similar approach has been used to insert a liposome into a DNA chip [75] (Fig. lib). 

Conduct all alkyladon reacdons in a properly functioning chemical fume hood. For safety precautions and decontaminadon procedures, see ref. 13. As the antisera to modified DNA bases often exhibit significant crossreacdvity to unmodified DNA, all slots must contain the same amount of DNA, e.g., 3 pg. Highly modified DNA should be diluted with unmodiBed DNA. 

For mutagenic activity, the risk of carcinogens and mutagens compounds which might be presented in the oil was also evaluated according to the international guidelines (OECD 471 and commission directive N° B13/14). Tests have been conducted at the CIT (International Center of Toxicity) Safety and Health Research Laboratories, 27005 Evreux, France. This test evaluates the potential of the Saro essential oil to induce reverse mutation in Salmonella typhimurium, knowing that the bacterial reverse test is able to identify substances that cause point mutations, by affections of DNA base-pairs (19, 20). Five strains of S. typhimurium TA 1535, TA 1537, TA 98 TA 100 and TA 102 were supplied for the study by B.N. Ames Laboratory (University of California, Berkeley or Oakland Research Institute, USA). 

High-resolution in situ STM as well as phase transition dynamics of nucleobases on Au(lll) and other low-index electrode surfaces supported by infrared spectroscopy have been reviewed recently by Nichols and coworkers [142] and Wandlowski and coworkers [143]. We refer to these reviews for details and note instead another aspect of single-molecule dynamics of DNA-based molecules. The observed electronic conductivity of oligonucleotides of variable length and variable base composition has opened almost a Pandora s box of novel DNA-based electronic properties. These include particularly photochemical and interfacial electrochemical ET. We refer to other recent reviews [144, 145] for this, still far from settled, issue but note the following STM-based studies that illuminate the conductivity issue at the single-molecule level (Figure 2.4). 

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