ABSTRACT
The question as to whether or not DNA can transport electric current is of great importance because, if DNA can indeed transport a charge carrier, such a mechanism may prove useful in understanding a signaling pathway for biological functions (Boal et al. 2005; Boon et al. 2003; Rajski et al. 2000; Kneuer et al. 2000; Shukla et al. 2004; Drummond et al. 2003; Yavin et al. 2005) as well as relevant biocompatible molecular or nanoelectronic applications (Ito and Fukusaki 2004; Wettig et al. 2003; Bhalla et al. 2003; Odenthal and Gooding 2007; Boon et al. 2002; Zwolak and Di Ventra 2002; Tabata et al. 2003). Indeed, the question was raised immediately after Watson and Crick’s discovery of the helical structure of DNA (Watson and Crick 1953). Eley and Spivey (Eley and Spivey 1962) theoretically predicted that the previously determined longitudinal distance of 3.4 Å (Watson and Crick 1953) between neighboring basal planes is close enough to make it possible for the Pz orbitals of ring carbon and nitrogen atoms to overlap along the long axis of the DNA helix; this can also guide charge transport, as in the case of graphite, which has a layer separation of 3.35 Å (Delhaes 2000). In the experimental setup established to demonstrate their idea, they sandwiched calf thymus DNA between two metal electrodes and measured direct current (DC) resistivity while pressing the two electrodes toward opposite directions. They successfully measured a DC resistivity of 5 × 1011 Ω·cm
5.1 Electrical Properties of DNA ....................................................................... 121 5.1.1 Charge Transport in Dry DNA ......................................................... 121 5.1.2 Electrical Conductivity of DNA-A Summary ............................... 130
5.2 Magnetic Properties of DNA ........................................................................ 134 5.2.1 Historical Recount ............................................................................ 134 5.2.2 DNA Magnetism ............................................................................... 135 5.2.3 Discotic Liquid Crystals as DNA-Mimicking Compounds.............. 152
5.3 Concluding Remarks .................................................................................... 153 References .............................................................................................................. 154
(Eley and Spivey 1962). This initial report provided suf—cient evidence to stimulate subsequent studies by many researchers. In other following studies, however, contradictory experimental results began to appear. As a consequence, the question as to whether or not DNA can transport charge carriers has gradually become increasingly murky (Endres et al. 2004; Taniguchi and Kawai 2006; Ventra and Zwolak 2004; Kwon et al. 2009; Shinwari et al. 2010; Kanvah et al. 2010; Genereux and Barton 2010; Fink and Schönenberger 1999; Priyadarshy et al. 1996; de Pablo et al. 2000; Zhang et al. 2002; Cai et al. 2000; Hwang et al. 2002; Maiya and Ramasarma 2001; Porath et al. 2000). Nonetheless, the early results of DNA researchers have provided an abundance of physical insights into possible charge transport mechanisms. The —rst candidate is an electron or a hole °owing through the π-stacks of the DNA helical structure (Voityuk et al. 2000; Takada et al. 2004; Wan et al. 1999; Henderson et al. 1999; Hennig et al. 2004; Wang and Chakraborty 2006; Barnett et al. 2001; Feng et al. 2006; Klotsa et al. 2005; Fink 2001; O’Neill and Barton 2004; Kats and Lebedev 2002; Apalkov and Chakraborty 2005; Gutierrez et al. 2005; Wei et al. 2005; Albuquerque et al. 2005; Matsuo et al. 2005; Otsuka et al. 2002). The second one is an ionic conduction of counter ions including Na+, K+, etc., which coordinate in a one-to-one manner with the PO4− anion of the DNA helix backbone but can also move around from here to there substantially under physiological conditions (Neubert et al. 1985; Okahata et al. 1998; Cai et al. 2000; Otsuka et al. 2002; Taniguchi et al. 2003; Ha et al. 2002; Terawaki et al. 2005). The next factor to be considered is the participation of water molecules in conduction (Kleine-Ostmann et al. 2006; Otsuka et al. 2002; Taniguchi et al. 2003; Ha et al. 2002; Brovchenko et al. 2007; Lee and Jin 2010). The —nal issue is the possibility that the experimentally determined DC voltage could have been generated by piezoelectricity under strong mechanical compression conditions (Fukada and Ando 1972; Zimmerman 1976; Fukada 1995; Hayakawa and Wada 1973).
