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In the disaccharide lactose (milk sugar), what type of glycosidic linkage connects the two monosaccharide units?

1. C1 - C4 alpha-glycosidic linkage
2. C1 - C2 alpha-glycosidic linkage
3. C1 - C4 beta-glycosidic linkage
4. C1 - C2 beta-glycosidic linkage

Correct answer: C1 - C4 beta-glycosidic linkage

Solution

Lactose (milk sugar) is a disaccharide made of beta-D-galactose linked to D-glucose. The bond is between C1 of galactose (in the beta configuration) and C4 of glucose, forming a C1-C4 beta-glycosidic (also written beta-1,4-glycosidic) linkage. This is distinct from sucrose (alpha-1,2) and maltose (alpha-1,4).

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