TY  - THES
AB  - Heterogeneous ice nucleation plays a very important role in the atmosphere by forming clouds influencing both the albedo and the hydrological cycle of the earth. Here, the significance of biological ice nucleators for atmospheric processes was confirmed on the examples of birch pollen and the fungus M. alpina that trigger the nucleation of ice at -18 °C or -6 °C, respectively. Due to the high concentration of the ice nucleating molecules that are easily released from its host by contact with water their impact on atmospheric ice nucleation is not negligible. The pollen release about 104 ice nucleating molecules per grain and accordingly 1012 per gram, the fungal spores up to 1011 per gram mycelium.
The ice affinity purification (IAP) method was established in this workgroup to specifically extract ice binding molecules. By means of the IAP the presence of ice structuring molecules in the birch pollen washing water as well as in the mycelium of M. alpina was shown. The ice crystals grew into a star-like shape induced by the molecules adsorbing at the secondary prism plane of hexagonal ice. The birch pollen macromolecules also inhibit the recrystallization of ice crystals (IRI) at a minimal effective concentration ci of 7 nmol l-1 that is comparable to the activity of the very efficient IRI agents. An IRI activity of the M. alpina extract can merely be assumed.<br />
Experiments using centrifugal filters with a molecular cutoff size of 100 kDa suggest that the ice nucleators and ice structuring molecules are not the same. The ice nucleating molecules are larger than 100 kDa while the antifreeze molecules pass the filter and must be smaller than 100 kDa. These sizes agree well with that of the known antifreeze proteins with sizes of maximum 30 kDa and that of the known biological ice nucleators with much larger sizes. To trigger ice nucleation at moderate supercoolings (Tnuk > -20 °C) an ice nucleus needs a minimal size of about 50 kDa. In contrast, several antifreeze molecules cooperate to inhibit the growth of an ice crystal so that their small size is sufficient for the antifreeze activity.<br />
In order to characterize the chemical nature of the ice binding macromolecules infrared spectroscopy was applied. The ice nucleating and ice structuring macromolecules in birch pollen both seem to be carboxylated polysaccharides with very similar structures. It is possible that the larger ice nucleators consist of clusters of the smaller antifreezes. The structural identification of M. alpina macromolecules could not be entirely resolved. However, the spectra indicate a proteinaceous identity of the ice nucleators and polysaccharides as ice structuring molecules.
In previous studies the high thermal hysteresis (TH) activity of the hyperactive antifreeze protein from the freeze avoiding organism of T. molitor was shown. Here this high TH activity was confirmed in experiments with a cryomicroscope, which was newly set up for TH measurements. The T. molitor antifreeze proteins showed freezing depressions of about 1 °C in a 100 μM solution. Further, an IRI activity with an
effective concentration ci of 1.6 μmol l-1 was obtained. For the first time ice nucleation measurements show a dual function of this 8 kDa protein. The measured heterogeneous ice nucleation temperature of about -31 °C is actually expected from Classical Nucleation Theory for a critical ice embryo of this size.<br />
In summary, these results show that the predominant function of an ice binding molecule in an organism as an ice nucleating or antifreeze molecule is determined by its size.
DA  - 2019
DO  - 10.4119/unibi/2934018
LA  - ger
PY  - 2019
TI  - Heterogene Eisnukleations- und Antigefriereigenschaften von Biomolekülen
UR  - https://nbn-resolving.org/urn:nbn:de:0070-pub-29340188
Y2  - 2024-11-24T13:09:22
ER  -