THE discovery that a DNA molecule can conduct electricity may help in repair of damage caused in the molecule by exposure to the sun. This was tested by attaching a chemical group at one end of the DNA molecule, which is situated some distance away from the chemically damaged part. The experiment was conducted by Jacqueline Barton and her team at the California Institute of Technology, USA, who call the process chemistry at a distance'.

The chemical mechanism is not clear, but electrical conduction - the flow of electrons down the DNA molecule - is thought to be involved. The specific form of damagekhat was repaired is known to be caused by ultraviolet (uv) rays, and Barton and her team conjecture that this discovery might offer hints as to how to treat at least some of the harmful effects caused by overexposure to the sun.

Barton and her team carried out a number of experiments to show that DNA is an unusual organic molecule in that it can conduct electricity (proteins, in contrast, are resistors). Though the results of the experiments raised curiosity and interest, they did not convince everyone. Four years ago, a direct measurement of the conductivity Of DNA suggested that it is a better conductor than expected for an electrical resistor. However, this result could not be replicated by others.

The next experiment involved attaching a molecule containing an atom of ruthenium (which can donate electrons) to one end of a 15 base-pair- long DNA molecule. When irradiated by photons, the ruthenium could be induced to glow until it transferred an electron and 'de-excited' itself. When the experiment was repeated after attaching a chemical containing rhodium (an acceptor of electrons), there was no glow at all. The inference drawn by the scientists was that electrons travelled down so fast from ruthenium to rhodiurn that the glow was quenched before it could be measured.

As this inference depended on a negative result - on something not being found - it did not have many takers. More recently, Barton's team performed another experiment. Here a metal particle attached to one end of the DNA molecule functioned as an electron acceptor when subjected to excitation by photons, while guanine bases situated some distance away acted as electron donors. The efficiency of this process was so low that many doubted a mediatory role played by DNA. On an average, 10 million photons were needed per electron transfer reaction.

In the latest attempt to demonstrate that DNA can perform chemistry at a distance, Barton's team used DNA molecules which contained a thymine dimer (containing twice the number of atoms as ordinary thymine), such as might be caused by uv irradiation, and an electron-accepting rhodium particle at one end of the molecule. When the sample was exposed to visible light, the rhodium atoms got excited, and took an electron from the thymine dimer. This repaired the damage done to the DNA.

Crucially, the efficiency of electron transfer was the same at all distances. It did not decrease in direct proportion to the distance between the thymine dimer and the metal complex, as would have been the case if the intervening medium - the DNA - had been a resistor. The scientific community is sure to start searching for evidence of electrical conduction by DNA in living organisms, and also for the possible uses that the phenomenon can be put to.