Antifreezes in fish work quite similarly.

Antifreezes in fish work quite similarly

Though any ordinary cold-blooded creature would freeze solid in the 28.5[deg.]F to 32[deg.]F polar oceans, some fish species manage to stay fluid and flexible in the supercooled waters by carrying antifreeze in their blood (SN: 11/22/86, p. 330). These fish come equipped with a variety of proteins or protein-sugar compounds that stick to forming ice crystals and stunt their growth. Scientists are still puzzling over the mechanism, but a report in the February PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES (Vol. 86, No. 3) provides new clues.

Arthur L. DeVries of the University of Illinois in Urbana and his colleagues compared the effects of antifreezes from six different fish. By observing the way the stunted ice crystals grew, the scientists could see which crystal faces the antifreezes attacked. Though they differed in structure, the six compounds worked in strikingly similar ways.

The antifreezes don't lower the temperature at which ice melts -- only the temperature at which ice rapidly crystallizes -- so there are a few degrees between the melting point and freezing point of such an antifreeze solution. The scientists set the temperature somewhere in this margin and exposed a single ice crystal to each antifreeze.

The exposed ice crystals changed shape as they grew. Normally, the crystals grow in hexagonal prisms -- like hexagonal hat boxes. More water molecules tend to stick to the sides of the box than to the top or bottom, making it wider. Antifreeze molecules mainly attacked the sides, allowing only the top and bottom (basal planes) to add layers, so the boxes got tall and skinny.

The antifreezes also changed the crystals in more complicated ways. In all of the six cases, sloping, pyramidal faces emerged and in several the original prism-shaped crystal became a pyramid. In five of the six cases, the antifreeze also slowed growth on the basal planes by causing hexagonal pits to develop. The pits got deeper and wider as new layers built up around them.

The researchers propose that these six quite different molecules could act so similarly if they all have partially charged, or polar, areas the same distances apart. If that were the case, each antifreeze would have the same preferred sites where the polar areas would fit into the crystal lattice. However, another experiment looking at where the individual molecules attached showed that the six types attach to ice differently on a molecular level. Any proposed answer to the antifreeze puzzle will have to take into account both the observed similarities and the differences.