The possibility of developing molecular scale electronic devices has captured the imagination of the scientific community in recent times, and the self-assembly and molecular recognition properties of DNA make it an attractive candidate for a molecular wire. As the area is new, there are basic fundamentals in the application of this biopolymer that are unknown; indeed, an enormous debate is currently underway as to whether DNA can conduct or not [1, 2].
The currently accepted mechanisms for charge transfer and transport in double-stranded DNA involves the π-π interactions between adjacent base pairs . We have designed and synthesised modified nucleotides which, on incorporation into an oligomer, will alter the conduction properties of DNA in two ways:
1. Improving the point of contact between a double-stranded oligomer and a gold electrode.
The ability to assimilate a DNA strand to a metal contact is one of the most fascinating challenges of this novel area. To date [1, 2], the issue of metallic contact has been dealt with in two ways: (a) the nature of the connection remained unresolved or (b) a modified oligomer was coordinated to the gold surface to form a Au(I) thiolated species (R-S-Au) via a flexible (CH2)6S- linker at either the 3' or 5' terminal phosphate. We propose that elimination of the inherent flexibility found in the (CH2)6S- linker would specifically define the point of contact and maximise the proximity of the π-orbitals of the terminal base pair to the surface of the gold electrode.
2. Enhancing the π-stack
If, as current opinion suggests, the conduction pathway in double-stranded DNA involves π-π interactions between base pairs, the inclusion of modified nucleotides containing aromatic functionalities will enhance the π-stack and provide a more facile pathway for charge transport.
Presence of conduction: [a] Y. Okahata, T. Kobayashi, K. Tanaka and M. Shimomura, J. Am. Chem. Soc., 1998, 120, 6165; [b] H.-W. Fink and C. Schönenberger, Nature (London), 1999, 398, 407; [c] D. Porath, A. Bezryadin, S. de Vries and C. Dekker, Nature (London), 2000, 403, 635; [d] P. Tran, B. Alavi and G. Gruner, Phys. Rev. Lett., 2000, 85, 1564; [e] L. Cai, H. Tabata and T. Kawai, Appl. Phys. Lett., 2000, 77, 3105; [f] K.-H. Yoo et al., Phys. Rev. Lett., 2001, 87, article # 198102; [g] M. Hjort and S. Stafström, Phys. Rev. Lett., 2001, 87, article # 228101.
Absence of conduction: [a] E. Braun, Y. Eichen, U. Sivan and G. Ben-Yoseph, Nature (London), 1998, 391, 775; [b] P. J. de Pablo et al., Phys. Rev. Lett., 2000, 85, 4992; [c] A. J. Storm, J. van Noort, S. de Vries and C. Dekker, Appl. Phys. Lett, 2001, 79, 3881.
J. Jortner, M. Bixon, T. Langenbacher and M. E. Michel-Beyerle, Proc. Natl. Acad. Sci. USA, 1998, 95, 12759 and references therein.
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