# Synopsis: The DNA also collapses

Large electric fields, alternating at hundreds of hertz, cause DNA molecules to collapse, the opposite of previous observations that they stretch out at higher and lower frequencies.

Electric fields applied to DNA in solution can be used to manipulate this important biological molecule. In Physical Review Letters, researchers report that strong alternating fields cause the DNA polymer to collapse, in contrast to previous observations where alternating fields stretched it out.

Chunda Zhou of North Carolina State University and colleagues measured the configuration of double-stranded DNA in two different types of fabricated structures. Their “microchannel” device, $600$ nanometers deep and $10,000$ nanometers wide, gave the molecule enough room to adopt its free conformation. “Nanochannel” devices, long, narrow tubes with lateral dimensions of order $100$ nanometers, forced the molecule to align along the channel. For both structures, Zhou et al. found that alternating electric fields of several hundred volts per centimeter along the length of the channel collapsed the molecule to a small fraction of its initial size, which they monitored with microscope images of its fluorescence.

The group applied fields oscillating at hundreds of hertz. In contrast, many previous experiments showed the opposite effect, stretching of DNA, at either higher frequencies (typically hundreds of kilohertz) or lower frequencies (typically a few hertz or less). Zhou et al. suggest that intermediate frequency fields allow large segments of the molecule, but not its entire length, to become polarized. This polarization causes charge to pile up at bends or clumps, which then attract one another, leading to collapse. – Don Monroe

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