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Antifreeze Proteins Observed at Work

Proteins desired for preserving organs and frozen food longer are now visible under the microscope
ice crystals



COURTESY OF IDO BRASLAVSKY
DENVER — Researchers have spotted powerful antifreeze proteins swarming ice crystals and stunting their growth. They found that one of the strongest such proteins, from an insect called the spruce budworm, sticks to a developing ice crystal on its broadest face, where it restricts the crystal's growth like a vice at cold temperatures. The results may aid in designing molecules that keep organs alive longer and prevent freezer burn.

Antifreeze proteins, also called ice-structuring proteins (ISPs), help keep animals alive at temperatures at which their tissues would normally freeze full of jagged ice crystals. Ice cream manufacturers already add ISPs to some of their products to improve the texture of low-fat ice cream. To confirm where such molecules stick to ice crystals, biophysicist Ido Braslavsky of Ohio University and his co-workers linked different ISPs to a protein that fluoresces green under a microscope. "If you want somehow to improve antifreeze proteins, you need to know better how they act," Braslavsky says.

In their latest work, reported here at the annual meeting of the American Physical Society, the researchers soaked small ice crystals in a solution of ISP from the spruce budworm. As they lowered the temperature, the ISP-covered particles remained the same size until rapidly expanding into larger crystals when the temperature became too cold for the protein to work anymore [click here for video].

Ice particles stack together in sheets that resemble honeycombs. The group found that weaker ISPs from fish prevent these particles from expanding sideways, apparently by attaching to the edges of the crystal sheets. Because the ice can only attach to the tops or bottoms of these sheets, the disk- or blob-shaped particles grow into six-sided diamonds [see photo above]. The group observed that the budworm protein collects on both the corners and the flat surfaces of the ice crystals, likely cutting off both directions in which ice normally grows, Braslavsky reports.

The result reinforces the proposed explanation of strong ISPs, says Charles Knight, an ice crystal expert at the National Center for Atmospheric Research in Boulder, Colo. "It places the knowledge on a solider ground," he says. Braslavsky says that researchers still want to understand how individual ISP molecules stick to ice. "It's still not clear," he says, "how they work exactly."

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