Dear Robin,
There are two kinds of Cryoprotenctants one is permeating (generally smaller) and another is non permeating (generally larger). Permeating cryoprotectants get distributed on both side of membrane and bound water molecules to reduce large ice crystal formation and protect the membrane from physical damage. on the other hand the non permeating cryoprotectants pulls the water out from the cells and makes it slightly dehydrated and protects the cell from outside by bounding water molecules.
this is regarding the size of cryoprotectant and its permeability across the cell. now the action of cryoprotectant depend on its water bounding capacity so the molecule that can bound more water will protect cell more efficiently irrespective of the molecular weight. But smaller molecule rapidly changes the tonicity of the solution where as the larger molecule do not change the tonicity of the solution if equal amount of both the substance is taken. as the tonicity depends on the number of molecules.
so cell viability depends on so many factors and it would be not appropriate just to correlate the molecular size of cryoprotectant and cell viability.
hope your query is answered.
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