Physicists in Japan have gained important new insights into how DNA might behave as an electrical conductor. Their discovery could help provide a better understanding of the role that conduction plays in how living cells detect and repair damaged DNA and could ultimately lead to strands of DNA being used in “molecular electronics” technologies of the future.
Biophysicists are keen to understand how electrons are conducted in DNA because conduction is thought to be an important mechanism by which enzymes recognize damaged DNA that, if not repaired, could lead to cancer. Some scientists also believe that conduction through DNA could protect the genomes of some organisms by transmitting the damage caused by oxidizing chemicals to certain locations on chromosomes where the damage causes the least harm.
Tiny electronic circuits
A better understanding of conduction could also lead to the engineering of new forms of DNA with properties more suited to electronic applications. DNA is an attractive building block for tiny electronic circuits because of its ability to assemble into complex interconnected patterns that would be required for assembling circuit components.
Not long after the double-stranded structure of DNA was revealed by Watson and Crick in 1953, scientists suspected that the molecule it might support electrical conduction. This is because the bases in the middle of the double helix stack in a way reminiscent of graphite – which is an excellent conductor. At about the same time, the physicist Leon Brillouin suggested that the DNA backbone – the long strands that support the bases and give DNA its structure – rather than the bases, might support conduction because of its periodic structure.
While the conductive properties of DNA have been studied using a wide range of techniques, most experiments have focused on understanding conduction in terms base stacking and have yielded conflicting results. Alternative or complementary conduction mechanisms – such as Brillouin’s backbone conduction – have been largely ignored.
Now, Tetsuhiro Sekiguchi of the Japan Atomic Energy Agency and Hiromi Ikeura-Sekiguchi at the AIST research centre are the first to measure how electrons move through the DNA backbone using a technique called resonant Auger spectroscopy ( (Phys. Rev Lett. 99 228102 ).
Spectator Auger decay
The team directed a beam of X-rays onto DNA to excite electrons from phosphorus atoms in the backbone of the molecule. If these electrons remain near to the site of their excitation, other electrons with a specific energy distribution indicative of “spectator Auger decay” will be emitted from the sample. However, if excited electrons are able to conduct along the backbone, the emitted electrons will have an energy distribution associated with “normal Auger decay.”
By comparing the relative intensities of Auger electrons with spectator and normal decays, the team could determine the time that it takes for an electron to move away from a phosphorous atom and take part in conduction – called the delocalization time.
What they found is that electrons in the backbone delocalize in less than one femtosecond (10-15) in wet DNA. These results imply that electron movement occurs a thousand times faster in the DNA backbone than in the bases stacked in the core.
This first observation of conduction along the backbone could help reconcile the seemingly contradictory results of the many base-stacking studies of conduction. Indeed, these latest results suggest that focusing on the interplay between electron transport through the backbone and the stacked bases could be crucial to understanding DNA conduction.
