Which one of the following compounds contains β-C₁-C₄ glycosidic linkage?

Solution

Chemistry ma’am · 2 steps

Check two things for each sugar: which carbons are joined, and whether the C1 end is α or β.

1. Read the label. In β-C1–C4, C1 of the first sugar bonds through an oxygen to C4 of the next. The β means the C1 –OH that formed the bond pointed up, on the same side as the CH₂OH group.

   Compound	Units joined	Glycosidic linkage
   Lactose	β-D-galactose + β-D-glucose	β-C1 of galactose to C4 of glucose
   Sucrose	α-D-glucose + β-D-fructose	C1 (α) of glucose to C2 (β) of fructose
   Maltose	α-D-glucose + α-D-glucose	α-C1 to C4
   Amylose	long chain of α-D-glucose	α-C1 to C4, unbranched

2. So. Only lactose, the milk sugar, joins its units by a β-C1–C4 glycosidic linkage. The answer is (a).

The idea to keep. Maltose and amylose (starch) use α-1,4 links. Lactose and cellulose use β-1,4 links. Sucrose is the odd one out, with a C1–C2 link that ties up both reducing ends.

Answer from the NTA final answer key. Solution by PrepWiser.