1 April 2011

Spin sensitivity of DNA surprises researchers

by Kate Melville

Weizmann Institute researchers investigating quantum interactions in biological molecules have shown that DNA is extremely sensitive to particle "spin." Their experiment shows that DNA can somehow discern and "filter" the electrons moving through it, a finding that could impact both medical science and electronics research.

Quantum phenomena are generally associated with extremely small systems, usually single atoms or molecules. Once a system exceeds a certain size, its quantum properties collapse and classical physics takes over. "Biological molecules are quite large, and they work at temperatures that are much warmer than the temperatures at which most quantum physics experiments are conducted. One would expect that the quantum phenomenon of spin, which exists in two opposing states, would be scrambled in these molecules - and thus irrelevant to their function," explained Ron Naaman, of the Institute's Chemical Physics Department.

But biological molecules have another property: they are chiral. In other words, they exist in either "left-" or "right-handed" forms that can't be superimposed on one another. Double-stranded DNA molecules are doubly chiral - both in the arrangement of the individual strands and in the direction of the helices' twist.

To investigate whether DNA might show some spin-selective properties, he fabricated self-assembling, single layers of DNA attached to a gold substrate. Naaman and his co-researchers then exposed the DNA to mixed groups of electrons with both directions of spin.

Intriguingly, he found the DNA reacted strongly with the electrons carrying one of those spins and hardly at all with the others. His findings, appearing in Science, show that the longer the molecule, the more efficient it was at choosing electrons with the desired spin. These findings imply that the ability to pick and choose electrons with a particular spin stems from the chiral nature of the DNA molecule, which somehow "sets the preference" for the spin of electrons moving through it.

Naaman says that DNA turns out to be a superb "spin filter," and the findings could have relevance for both biomedical research and the field of spintronics. If further studies, for example, bear out the finding that DNA only sustains damage from spins pointing in one direction, then exposure might be reduced and medical devices designed accordingly. On the other hand, DNA and other biological molecules could become a central feature of new types of spintronic devices.

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Source: Weizmann Institute of Science