For decades DNA has been regarded as a poten­tial build­ing block for mol­e­c­u­lar elec­tron­ics, but ran­dom sequences of DNA vary in their con­duc­tiv­ity — charge trans­fer through G–C (guanine–cytosine) pairs is much faster than through A–T (adenine–thymine) pairs. Charge can migrate along an A–T rich sequence by ‘hop­ping’ between G–C pairs, but this decreases its elec­tri­cal con­duc­tiv­ity. Japan­ese researchers have improved the elec­tri­cal con­duc­tiv­ity of DNA through sim­ple chem­i­cal mod­i­fi­ca­tions. Their work could pave the way for build­ing DNA cir­cuits and self-assembling, DNA-based electronics.

Recently Tet­suro Majima and col­leagues at Osaka Uni­ver­sity in Japan have found a way to tune the elec­tronic char­ac­ter­is­tics of the A-T pairs, mean­ing charge trans­port is no longer sequence depen­dent. They replaced one nitro­gen atom in ade­nine to C–H. This new deaza­ade­nine base is the same from the genetic point of view. On the other hand, its elec­tronic char­ac­ter­is­tics are quite sim­i­lar to gua­nine, so then elec­tri­cal con­duc­tiv­ity of DNA would increase dra­mat­i­cally.

Chem­i­cal struc­tures of ade­nine (A), 7-deazaadenine (Z), the pho­to­sen­si­tizer naph­thal­im­ide (NI) and the hole trap phe­noth­iazine (PTZ). © 2009, Nature Pub­lish­ing Group.

To prove that, the researchers made short sequences of A-T rich DNA mod­i­fied at one end with a pho­to­sen­si­tiser and at the other with a pos­i­tive charge ‘trap’ – phe­noth­iazine (PTZ). After inject­ing charge at the pho­to­sen­si­tive end using a laser, authors mea­sured charge trans­fer rate by observ­ing how quickly PTZ was oxi­dised to form a PTZ⁺ cation. Sim­ply swap­ping the ade­nine bases for deaza­adenines increased the rate of charge trans­fer by three orders of mag­ni­tude. In ran­dom DNA sequences, with mixed G-C and A-T pairs, charge trans­port was faster in those where ade­nine was replaced with deaza­ade­nine.

Schematic rep­re­sen­ta­tion of hole injec­tion by charge trans­fer between adenines, and long-range charge trans­fer through DNA in which the rate can be deter­mined from the for­ma­tion rate of PTZ⁺. The blue arrows rep­re­sent the increase in the HOMO level on replac­ing the N7 nitro­gen atom of A with a C–H group. The HOMO lev­els of the base pairs are cal­cu­lated the­o­ret­i­cally. © 2009, Nature Pub­lish­ing Group.

Kawai, K., Kodera, H., Osakada, Y., & Majima, T. (2009). Sequence-independent and rapid long-range charge trans­fer through DNA Nature Chem­istry DOI: 10.1038/nchem.171

15th April, 2009