For question #113 (Biological Sciences) of TBR #1, it discusses a hypothetical carbohydrate (Compound X) that is made of 2 molecules (mannose + galactose connected by an alpha 1,4 linkage).
Question: Treatment of Compound X in Figure 2 with excess H3COSO3C6H5 results in the complete methylation of its hydroxyl groups. What is the final product after complete hydrolysis of the methylated disaccharide?
A. 2,3,4,6-tetra-O-methyl-D-mannose and 2,3,4,6-tetra-O-methyl-D-galactose
B. 2,3,6-tri-O-methyl-D-mannose and 2,3,4,6-tetra-O-methyl-D-galactose
C. 2,3,4,6-tetra-O-methyl-D-mannose and 2,3,6-tri-O-methyl-D-galactose
D. 2,3,6-tri-O-methyl-D-mannose and 2,3,6-tri-O-methyl-D-galactose
Correct Answer is C.
Explanation: C is the best answer. You are told in the passage that the aldohexapyranose in sucrose is glucose, and that lactose is composed of glucose and galactose. This means that the structure for galactose can be deduced. But is the left sugar mannose and the right sugar galactose, or is the right sugar mannose and the left sugar galactose? Before hydrolysis, the right sugar is methylated at sites 1, 2, 3, and 6, while the left sugar is methylated at sites 2, 3, 4, and 6. The final products after hydrolysis are 2,3,4,6-tetra-O-methyl-D-mannose and 2,3,6-tri-O-methyl-D-galactose.
Although I don't disagree with the answer, I am a bit confused as to why the methyl from carbon 1 of the galactose would be replaced by a hydroxyl group...Wouldn't the correct answer be 2,3,4,6-tetra-O-methyl-D-mannose and 1,2,3,6-tetra-O-methyl-D-galactose?
In short, why does hydrolysis only occur on carbon 1 of the galactose and the alpha 1,4? Why doesn't it occur anywhere else?
Question: Treatment of Compound X in Figure 2 with excess H3COSO3C6H5 results in the complete methylation of its hydroxyl groups. What is the final product after complete hydrolysis of the methylated disaccharide?
A. 2,3,4,6-tetra-O-methyl-D-mannose and 2,3,4,6-tetra-O-methyl-D-galactose
B. 2,3,6-tri-O-methyl-D-mannose and 2,3,4,6-tetra-O-methyl-D-galactose
C. 2,3,4,6-tetra-O-methyl-D-mannose and 2,3,6-tri-O-methyl-D-galactose
D. 2,3,6-tri-O-methyl-D-mannose and 2,3,6-tri-O-methyl-D-galactose
Correct Answer is C.
Explanation: C is the best answer. You are told in the passage that the aldohexapyranose in sucrose is glucose, and that lactose is composed of glucose and galactose. This means that the structure for galactose can be deduced. But is the left sugar mannose and the right sugar galactose, or is the right sugar mannose and the left sugar galactose? Before hydrolysis, the right sugar is methylated at sites 1, 2, 3, and 6, while the left sugar is methylated at sites 2, 3, 4, and 6. The final products after hydrolysis are 2,3,4,6-tetra-O-methyl-D-mannose and 2,3,6-tri-O-methyl-D-galactose.
Although I don't disagree with the answer, I am a bit confused as to why the methyl from carbon 1 of the galactose would be replaced by a hydroxyl group...Wouldn't the correct answer be 2,3,4,6-tetra-O-methyl-D-mannose and 1,2,3,6-tetra-O-methyl-D-galactose?
In short, why does hydrolysis only occur on carbon 1 of the galactose and the alpha 1,4? Why doesn't it occur anywhere else?
