Figure 1: A computer-generated image of the beetle antifreeze xylomannan reveals that one face bristles with oxygen atoms (red), forming a polar surface that helps it to cling to ice crystals. Image: Yukishige Ito |
Animals
and plants have evolved all sorts of chemical tricks that allow them to
colonize extreme environments. For species that call Antarctica or the Arctic home, surviving sub-zero temperatures is an
essential ability, and chemists have isolated many natural antifreeze compounds
from these organisms. The antifreeze called xylomannan, which is produced by
the freeze-tolerant Alaskan beetle Upis
ceramboides, is being studied by Akihiro Ishiwata and Yukishige Ito
at the RIKEN Advanced Science Institute at Wako and their colleagues. Their
findings to date show that xylomannan is a particularly unusual antifreeze.
Xylomannan
was first reported in 2009, and has been shown to be amongst the most active
insect antifreezes found to date. Antifreeze compounds, which are also known as
thermal hysteresis factors (THFs), protect the insects’ cells from damage as
temperatures fall and ice crystals begin to form. THFs seem to work by sticking
to the surface of nascent ice crystals and somehow stopping them from growing,
protecting nearby cell membranes from being punctured by needles of ice.
The
unusual thing about xylomannan is its constituents. Every natural THF isolated
to date is protein based, but xylomannan is a glycan, a long-chain sugar-based
compound. “Xylomannan is the first example of a THF biomolecule with little or
no protein component,” says Ishiwata. “Its mode of action is not entirely
clear, but it should be different to those of common THFs such as antifreeze
proteins and glycoproteins.”
To
confirm the proposed structure of xylomannan, so that they can begin to study
how it interacts with ice crystals, Ishiwata, Ito and their colleagues
synthesized what they thought to be a key component of the compound’s
sugar-based backbone. Their structural analysis, using nuclear magnetic
resonance techniques and molecular modeling, confirmed that the structure
matches that of the natural compound. It also hints at the way that xylomannan
might stick to ice crystals: one face of xylomannan is much more polar than the
other face, making one face hydrophilic and the other hydrophobic (Fig. 1).
“We propose that the hydrophilic phase of xylomannan might
bind to the ice crystal, exposing the hydrophobic phase on the ice crystal’s
surface,” says Ishiwata. This hydrophobic surface should repel water molecules
away from the ice crystal, stopping it from growing any further. “However, the
binding mode is still not clear from our structural analysis,” he adds. To test
the theory further, the team now plans to synthesize longer fragments of
xylomannan to examine their ice-binding ability.
