Several biopolymers have well documented properties as organic electron conductors. These materials, exemplified by the cytochrome systems, have tetrapyrrole components (porphyrins) that are usually metal centered. The tetrapyrrole is a highly conjugated system that can interact with other tetrapyrroles in a face to face orientation with P bonding. Using model systems Collman and colleagues (J Amer Chem Soc 102:6027-6036 1980) has demonstrated that electron transfer is maximized when the face-to-face distances are maintained at 5-8 Angstroms. Electron transfer may be mediated both through P stacking and redox of the metal center. Phthlocyanins are biomimetics of porphyrins and these have been shown to exhibit modest electron conductivity when doped (Marks, Science 227:881-889 1985). Amperometric sensors have been constructed utilizing biotinylated polypyrroles (Cosnier et al. Analytical Chemistry 71: 3692-3697 1999) and proteins containing porphyrins (Mizutani, Sato et al. Electrochimica Acta 44: 3833-3838 1999). A challenge presented by this technology is the production of filaments of the heme or phthalocyanine entities in most efficient alignment for electron transfer. The development of biopolymer based molecular switches enable more rapid development of molecular transistors and integrated circuits.
Nucleic acids can function both as organic electron transfer materials and as templates for the deposition of electron conducting metals. The rate of electron transfer through organic conductors is approximately four orders of magnitude slower than through good metallic conductors. Double stranded deoxyribonucleic acid (dDNA) has now been demonstrated to function as an organic electron transfer material ("wire"). The electron transfer is effected through stacking and orientation of the bases (SO Kelley, JK Barton. Science 283: 375-381 1999; Wan, Fiebig et al. Proc Natl. Acad. Sci US, 96: 6-14-6019, 1999; Henderson, Jones et. al Proc. Natl. Acad Sci US, 96: 8353-8358 1999). DNA has also been shown to serve as a matrix for adsorption to gold or silver in the construction of nanowires and sensors (Elghanian, Storhoff et al. Science 277: 1078 - 1081 1997; Braun, Eichen et al. Nature 391: 775 1998). The nanowires are capable of electron conduction as metallic materials. The problems associated with this technology include the formation of uniform diameter and oriented polynucleotide fibers. Methods have yet to be developed for production of an ordered deposition of the "nanowires" (DNA or DNA gold complex with deposited metal)" on a support surface. An end product would be a nanoscale integrated circuit.
Nanotubes have been formed using organic polymers as templates (Rudolph AS, Ratna BR and Kahn B. Nature 352: 52-55 1991). The nanotubes have diameters of 1 nanometer or larger and have utility as molecular tweezers or surface probes (Gimzewski and Joachim Science 283 1683 1999). The molecular tweezers enable one to move single molecules on a solid surface for the construction of sensors and integrated molecular motor systems (Kim and Lieber Science 286: 2148-2150 1999; Baughman, Cui et al. Science 284: 1340-1344 1999). These structures may also be used to map the surface properties (i.e. uniform thickness, electrical conductivity, force required to separate two biomolecular complexes) of thin films (Gimzewski and Joachim Science 283 1683 1999). The nanoscale dimensions of the tubes, their physical strength and electronic conducting properties have utilities in a variety of industries including electronics, biomedicine, communications and QC in the manufacture of thin films. The technical issues to be addressed include mass production of uniformly thick tubules, the deposition of the tubules in an ordered manner and attachment of the tubes to larger electron conducting surfaces.