A 0.4 g sample of amylopectin was exhaustively methylated and then hydrolyzed to

Question

A 0.4 g sample of amylopectin was exhaustively methylated and then hydrolyzed to component methylated monosaccharides. The yields of some of the methylated monosaccharides were as follows: 0.4 micromol 1,2,3,6-tetramethylglucose, 45 micromol 2,3-dimethylglucose, and 62 micromol 2,3,4,6-tetramethylglucose. What percent (to the nearest tenth) of the amylopectin was non-reducing ends? Remember that glycogen is composed entirely of glucose residues, which have a molecular mass of 162 g/mol. ***The answer is NOT 3%***

Explanation / Answer

Reducing ends form tetrametylglucose.

dimethylglucose is the branch points (non - reducing ends) = 45 micromoles = 7.3 *10-3 gm

percentage of amylopectin having non-reducing ends = (7.3 *10-3 / 0.4) *100 = around 1.8%

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