For a class project, I would like to demonstrate the presence of mannose in A. vera leaf gel. To do this, I would like to perform Benedict's reagent testing (no quantitative testing will be required). However, I have worries about the lipids in the gel interfering with the test, as the presence of lipids may prevent the sugars from dissolving in water.
To get rid of this possibility, I have decided to submerge the gel in some sort of nonpolar medium, then decant, leaving behind a water-soluble fraction that can be used in testing. I have two questions about this method:
1.) Would this method be worth performing at all? In other words, will the lipids really interfere with Benedict's testing?
2.) What nonpolar medium should I use to extract the lipids?
To the best of my knowledge, most of the mannose in Aloe exists in the form of mannans (i.e., mannose-containing polysaccharides). These would not react with reducing-sugar reagents like Benedict's solution unless the mannan was first hydrolyzed with an enzyme or strong acid. Furthermore, the mannans are soluble in water but precipitated by organic solvents. If you are really interested in demonstrating mannose in aloe, I would isolate the polysaccharide and then hydrolyze it. One approach is to macerate the plant tissue or somehow dissolve the gel in water. The mannan can then be precipitated by adding ethanol until you get a sticky, gummy precipitate. Isolate the precipitate by centrifugation or decantation, wash it by redissolving and reprecipitating, then dry it. To hydrolyze it will require something like 1M-2M strong acid, (e.g., sulfuric or trifluoroacetic), with heating for 1-2 hours. Neutralize this, and you will have a solution containing mannose, along with any other sugars that may have been present, and salts of the acid. Note that Benedict's solution is not specific for mannose, so you would get similar results from any plant polysaccharide.
Dear Dr. Cote,
I am very grateful for your advice. I will try to add some gel to water and then add ethanol to precipitate the polysaccharides as you have suggested. Are there any specific ways of demonstrating the presence of mannose exclusively?
If not, I will attempt to do the Benedict's test and relay the results to you. Demonstrating the presence of reducing sugars in general should be sufficient for my class.
Dear Dr. Cote,
Thank you for your correspondence. I will take your advice into account.
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