Re: why use alcohol to precipitate polysaccharides (levan)

Hello Feng,

Thanks for your question. Yes, water is denser than alcohol (ethanol), but that is not the mechanism behind the purification.

Levan^1,2 is a homopolysaccharide (the monomeric unit is fructose) consisting of branching chains. The chains vary in length, but can be very long indeed and can interact with one another. Despite the large size of the molecules, they are somewhat water-soluble, or at least form suspensions³ that do not precipitate readily. This is due to the large number of hydroxyl groups in levan, which interact with the water solvent.

Ethanol is quite miscible with water and so adding it to a solution of levan disrupts the interactions between levan molecules, water molecules and any charged ions in the solution. Basically, ethanol disrupts the hydration of the polysaccharide and the charged ions by water. This allows the polysaccharide molecules to come closer together, interact and form "clumps" that are too large to stay in solution. They can then be removed by centrifugation. Ethanol precipitation of large, negatively-charged molecules from aqueous solution is a common biochemical procedure for other molecules besides polysaccharides.

I hope that answers your question,
Neil

[1] Levans at Wikipedia
[2] Levan PubChem record
[3] Levan properties