By Celeste Biever
Can DNA conduct electricity? Some physicists claim it is a
superconductor. Others believe it does not conduct electricity at all. And
biologists have agonised about how conductivity might affect its function.
A consensus is emerging. Although
the much-hyped molecule can transport electrons over a length of a few base pairs, allowing it to deflect
oxidative damage away from important sections (New Scientist print edition, 15
March), it fails to conduct over longer distances. That will dash long-held
hopes that the self-replicating molecule could be harnessed to make
self-assembling nanowires.
Researchers from the University of California, Los Angeles, have hammered the final nail in
the coffin with an exhaustive paper submitted to Phvsical Review Letters. Thev
show that conduction in a strand of DNA varies directly with humidity, implying
that it is the layer of polarised water molecules sheathina the DNA. rather
than the DNA itself, that is conducting overlong distances (see graphic).
'DNA has a water layer under practically any
conditions. We have systematically changed the number of water layers and shown that the conductivity arises
from water molecules, not
the electrons on the DNA," says principal investigator George
Gruner.
AC/DC
Before now, DNA has been shown to conduct an alternating
current, but not direct currents. The UCLA researchers point out that this is
exactly what you would expect if water is responsible for the conductivity.
Water is a polar molecule, across which electrons can shift
to produce an alternatina current. But they cannot travel freely from molecule
to molecule to produce direct current. The most natural explanation is that the
DNA is not conducting at all, says co-author Peter Armitage.
Other research published earlier in March by Phuan Ong, of
Princeton University in New Jersey, supports the idea that DNA is an insulator.
He removed the water and salt that clings to DNA, then tethered these
"clean" strands to gold electrodes. The molecule did not conduct
electricity.
Most of the
claims for DNA conductivity put forward over the past decade have since been
retracted, often after the original researchers realised that water, salts, or
even electrodes placed too close to the helix could produce the observed
effect. DNA's supposed superconducting properties, reported in Science in 2001,
are now thought to have been caused by a layer of rhenium atoms coating the
molecule.
Quantum tunnelling
So how can DNA conduct over short distances, but fail over
longer ones? The Molecule’s
short-range conductivity depends on electrons moving between base pairs that
run down the centre of the double helix.
To do this, they overcome the associated energy
barriers through a quantum mechanical effect known as tunnelling. And according
to theory, tunnelling becomes increasingly unlikely as the distances involved
grow longer.
All the same. some researchers cling to the idea that DNA
could be manipulated to carry a charge. Solid-state physicist Helene Bouchiat of
Paris-Sud University and chemist Bernd Giese of the University of Basel say DNA may be a
semi-conductor -a substance like silicon that becomes conductive if doped with
atoms containing extra electrons. If so, DNA could still be harnessed to make a
conducting wire, they say.
But the UCLA group has given up hope. "Is it also a
good candidate for a molecular wire? Our answer is an emphatic no," says
Armitage.
From New Scientist Online News 16:4228 March 03