A graduate student recently asked me this question and I stumbling to give a good answer. We freeze our purified proteins quickly (liquid nitrogen or dry ice/EtOH) to prevent ice crystals from forming and damaging the protein but we freeze our cells slowly (in DMSO in a container in -80C freezer to slow the freezing process). Why the difference or why don't ice crystals form and hurt the cells?
Snap freezing proteins prevents formation of water crystals when the water forms ice.
For cells it turns out to be a bit more complicated. In slow-freezing, cells in a medium are cooled to below freezing point. At some stage, ice masses containing pure crystalline water will form. What remains between the growing ice masses is the so-called unfrozen fraction, in which all cells and all solutes are confined. The concentrations of sugars, salts and cryoprotectant (e.g. DMSO) increase, while the volume of the unfrozen fraction decreases. The increase in osmotic strength causes an efflux of water from the cells. Slow cooling is needed in order to allow sufficient efflux of water to minimize the chance of intracellular ice formation. As cooling continues, the viscosity of the unfrozen fraction ultimately becomes too high for any further crystallization. The remaining unfrozen fraction turns into an amorphous solid that contains no ice crystals.
I plucked the latter from here: http://www.fao.org/docrep/016/i3017e/i3017e04.pdf
Snap freezing proteins prevents formation of water crystals when the water forms ice.
For cells it turns out to be a bit more complicated. In slow-freezing, cells in a medium are cooled to below freezing point. At some stage, ice masses containing pure crystalline water will form. What remains between the growing ice masses is the so-called unfrozen fraction, in which all cells and all solutes are confined. The concentrations of sugars, salts and cryoprotectant (e.g. DMSO) increase, while the volume of the unfrozen fraction decreases. The increase in osmotic strength causes an efflux of water from the cells. Slow cooling is needed in order to allow sufficient efflux of water to minimize the chance of intracellular ice formation. As cooling continues, the viscosity of the unfrozen fraction ultimately becomes too high for any further crystallization. The remaining unfrozen fraction turns into an amorphous solid that contains no ice crystals.
I plucked the latter from here: http://www.fao.org/docrep/016/i3017e/i3017e04.pdf
Thanks very much Michael. That pretty much makes sense and I would never have come up with such a detailed answer. I shared it with the lab.
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