The temperature below 0 °C occurs commonly on earth . How can organisms survive in conditions where — Analytical Chemistry Chemistry Question
Antifreeze proteins
The temperature below 0 °C occurs commonly on earth . How can organisms survive in conditions where water freezes? For many insects exposed to prolonged subfreezing conditions, cryoprotectant synthesis begins in their bodies during early autumn, and these substances are cleared in early spring. Figure 1 shows the seasonal content of glycerol in a freeze–avoiding insect.
[VISUAL]
Figure 23.1. Daily maximum and minimum temperatures and glycerol content in the freeze–avoiding larvae of the goldenrod gall moth. The structure of glycerol is given on the right. (Reference: K.B. Storey and J.M. Story 1988 Physiol. Rev. 68:27)
What is the mass percent of glycerol in the insect if it could avoid freezing at –20.0 °C due to glycerol concentration alone? Assume the insect behaves as an ideal solution. What would be the osmotic pressure at this glycerol concentration? Comment on the resulting values. The freezing point depression constant for water is Kf = 1.86 °C (mol/kg)–1.
Model Answer
For freezing point depression, ∆T = −Kf m
The molal concentration (molality) m is obtained by
– 20.0 = – 1.86 m,
m = 10.75 mol kg–1
The mass of the glycerol in 1 kg of water is
m(glycerol) = molality × M(glycerol) = 10.75 mol kg–1 × 92.10 g mol–1 = 990 g.
Therefore, glycerol would be about 50 % by mass.
This is a ridiculously large number indicating that something else is also required to avoid freezing.
Osmotic pressure is obtained from the van’t Hoff equation, π = c R T . Assuming that the molar concentration c is approximately the same as the molality obtained above,
osmotic pressure π = 10.75 mol dm–3 × 8.314 J mol–1K–1 × 253 K = 22 612 kPa
The osmotic pressure is extremely high, and the organism may not be stable.
What is the actual fraction of glycerol in the insect’s wet mass in January? What would be the freezing point of water in the insect in January considering the amount of glycerol alone?
Model Answer
From Figure 1, the glycerol content in January is
2500 µmol g–1 = 2.5 mol kg–1 = 230 g of glycerol / 1 kg water.
Therefore, the glycerol mass fraction is 0.187 (18.7 mass %)
Freezing point depression at this concentration is – 2.5 × 1.86 = – 4.7 °C,
i. e. significantly higher than the temperature in January.
In addition to freezing point depressants, “antifreeze proteins” are known to act to avoid freezing in animals including cold–water fish and many insects. The colligative freezing point depression due to antifreeze proteins is quite small. Experiments suggest that antifreeze proteins inhibit the growth of small ice particles. If some threonine or aspartate side chains composing the protein are chemically modified, the antifreeze activity disappears. What kind of interactions between antifreeze proteins and ice particles are probably responsible for the antifreeze activity?
Model Answer
[VISUAL]
Threonine and aspartate side chains can contact each other through their oxygen and hydrogen groups to form stable hydrogen bonds. The hydrogen bonds of these side chains might preferably interact with water molecules of the ice particle surfaces, thus inhibiting ice crystal growth.