Double helixes of nano-ice

Researchers at the University of Nebraska-Lincoln (UNL) have used computer modeling to find double helixes of nano-ice molecules 'that resemble the structure of DNA and self-assemble under high pressure inside carbon nanotubes.' The scientists think their discovery may help other researchers studying the protein structures that cause Alzheimer's disease and mad cow disease.

Researchers at the University of Nebraska-Lincoln (UNL) have used computer modeling to find double helixes of nano-ice molecules 'that resemble the structure of DNA and self-assemble under high pressure inside carbon nanotubes.' Of course, these computer simulations need to be confirmed by real experiments. But the scientists think their discovery may help other researchers studying the protein structures that cause diseases such as Alzheimer's and mad cow disease.

These computer simulations have been done by Chemistry professor Xiao Cheng Zeng and two members of his research group, Jun Wang and Jaeil Bai.

For these computer simulations, they used a supercomputer called "Prairiefire," a 128-node Beowulf cluster equipped with 256 2.2 GHz Opteron (64-bit) processors from AMD, which reaches a speed of 888.5 GFlops in the High-Performance Linpack benchmark.

Before going further, below is a spectacular computer image of a nano-ice double helix obtained with Prairiefire. Oxygen atoms are shown in blue in the inner helix and in purple in the outer helix, while the hydrogen atoms are white (Credit: Xiao Cheng Zeng research group at UNL). Here is a link to a larger version.

Computer image of a nano-ice double helix

As you can guess, even the researchers were surprised by these self-assembling nanostructures looking like double helixes of DNA.

"We were shocked to see these molecules arrange themselves in this way," said Zeng. "We thought it would be like two tubes, one inside the other, but it didn't do that. It was helical, like DNA. I'm just speculating, but maybe the helix is a way for molecules to arrange themselves in a very compact, efficient way under high pressure."
"This ice formation can be viewed as a self-assembling process, and self-assembly is a way for molecules to bond together through weak hydrogen bonds. One example of a self-assembling material is protein. Proteins can self-assemble into structures like amyloid fibrils that can build up in the brain to cause Alzheimer's disease or prions that cause mad cow disease."

Before getting there, these computer simulations obviously need to be confirmed -- especially because some simulations gave very different results.

[The researchers used] simulated carbon nanotubes ranging in diameter from 1.35 to 1.9 nanometers [and] earth-like temperatures ranging from 117 degrees Fahrenheit to 9 degrees below zero F., but with pressures ranging from 10 to 40,000 atmospheres, with each series lasting no more than a few 10s of nanoseconds.
Most of the experiments produced the expected tubular structures, but in a simulation in a 1.35-nanometer tube at minus-9 degrees F. and 40,000 atmospheres, the ice transformed into a braid of double helix that resembles DNA in structure and in the weak bonds between the helixes.

For more information, this research work has been published in the Proceedings of the National Academy of Sciences under the title "Multiwalled ice helixes and ice nanotubes" (published online before print December 14, 2006). Here is a link to the abstract.

It has also been commented by New Scientist in "DNA-like ice 'seen' inside carbon nanotubes" (Tom Simonite, New Scientist, December 12, 2006). This short article contains three additional figures about this phenomenon.

Sources: University of Nebraska-Lincoln news release, December 11, 2006; and various websites

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