Trapping DNA Through Thermophoresis

Trapping DNA through thermophoresis might have a bearing on the origins of life, as a new experiment shows. The DNA molecules in our bodies are protected behind a nuclear membrane and a cellular membrane, but on the early Earth nascent life forms might have consisted of DNA floating in a free aqueous environment. How would such fragile entities keep from diluting themselves to death?

One answer might be thermophoresis, a process (known for almost 150 years) by which heat can repel polymers. Generally the longer the molecule the greater the thermal repulsion will be, just as molecules or particles will be separated in a centrifuge according to mass.

An experiment conducted by Dieter Braun and Albert Libchaber at Rockefeller monitors fluorescent-tagged DNA molecules as they are harried by a laser-generated heat spot. As expected the DNA was repelled, carried along by a convective flow away from the heat.

But surprisingly the DNA then came back; the convection, scrutinized more carefully, was seen to be a circular cell pattern. The DNA had become trapped in a small zone (20 microns across and with a DNA concentration enhanced by a factor of 1000) centered around the heat spot.

Braun (212-327-8160, braund@rockefeller.edu) says this is the first quantitative experimental evidence, on a microscopic level, that biological molecules (DNA was used rather than RNA because RNA can quickly degrade in the presence of proteins in the solution) can be trapped in this way.

Demonstrating a mechanism for confining early metabolic and replicative life forms in a far-from-equilibrium environment such as localized heat sources (e.g., hydrothermal vents) immersed in a cold ocean, should be of interest to biologists who ponder the advent of life. (Braun and Libchaber, Physical Review Letters, 28 October 2002; see also research website; independent thermophoresis expert: Werner Kohler in Bayreuth, Germany, werner.koehler@uni-bayreuth.de)