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Ch. 23 - Carbohydrates and Nucleic Acids
Wade - Organic Chemistry 9th Edition
Wade9th EditionOrganic ChemistryISBN: 9780135213728Not the one you use?Change textbook
All textbooksWade 9th EditionCh. 23 - Carbohydrates and Nucleic AcidsProblem 41
Chapter 23, Problem 41

Raffinose is a trisaccharide (C18H32O16) isolated from cottonseed meal. Raffinose does not reduce Tollens reagent, and it does not mutarotate. Complete hydrolysis of raffinose gives D-glucose, D-fructose, and D-galactose. When raffinose is treated with invertase, the products are D-fructose and a reducing disaccharide called melibiose. Raffinose is unaffected by treatment with a β-galactosidase, but an α-galactosidase hydrolyzes it to D-galactose and sucrose. When raffinose is treated with dimethyl sulfate and base followed by hydrolysis, the products are 2,3,4-tri-O-methylglucose, 1,3,4,6-tetra-O-methylfructose, and 2,3,4,6-tetra-O-methylgalactose. Determine the complete structures of raffinose and melibiose, and give a systematic name for melibiose.

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Step 1: Analyze the given information about raffinose. Raffinose is a trisaccharide (C18H32O16) that does not reduce Tollens reagent, indicating it does not have a free aldehyde or ketone group. It also does not mutarotate, meaning it does not have a free anomeric carbon. This suggests that all the anomeric carbons in raffinose are involved in glycosidic bonds.
Step 2: Understand the hydrolysis products of raffinose. Complete hydrolysis of raffinose yields d-glucose, d-fructose, and d-galactose. This indicates that raffinose is composed of these three monosaccharides linked by glycosidic bonds. Treatment with invertase produces d-fructose and melibiose, a reducing disaccharide. This suggests that the glycosidic bond between d-fructose and the other two monosaccharides is cleaved by invertase.
Step 3: Examine the action of specific enzymes. Raffinose is unaffected by β-galactosidase, but α-galactosidase hydrolyzes it to d-galactose and sucrose. This indicates that the d-galactose is linked to the rest of the molecule via an α-glycosidic bond. The sucrose portion (d-glucose and d-fructose) is linked by a β-glycosidic bond between the anomeric carbons of glucose and fructose.
Step 4: Analyze the methylation and hydrolysis data. Treatment of raffinose with dimethyl sulfate and base followed by hydrolysis yields 2,3,4-tri-O-methylglucose, 1,3,4,6-tetra-O-methylfructose, and 2,3,4,6-tetra-O-methylgalactose. This indicates that the hydroxyl groups at these positions are free and not involved in glycosidic bonds. The methylation pattern confirms the positions of the glycosidic linkages in raffinose.
Step 5: Determine the structure and systematic name of melibiose. Melibiose is a reducing disaccharide formed from d-galactose and d-glucose. Since it is reducing, the anomeric carbon of glucose is free, while the anomeric carbon of galactose is involved in an α(1→6) glycosidic bond with glucose. The systematic name for melibiose is α-d-galactopyranosyl-(1→6)-d-glucopyranose.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Structure of Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with a general formula of (CH2O)n. They can be classified into monosaccharides, disaccharides, oligosaccharides, and polysaccharides. Understanding the structure of carbohydrates, including the arrangement of their sugar units and glycosidic bonds, is essential for analyzing compounds like raffinose and melibiose.
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Enzymatic Hydrolysis

Enzymatic hydrolysis involves the breakdown of complex carbohydrates into simpler sugars through the action of specific enzymes. For instance, invertase catalyzes the hydrolysis of sucrose into glucose and fructose, while galactosidases target galactose-containing oligosaccharides. Recognizing the role of these enzymes is crucial for predicting the products of raffinose hydrolysis.
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Reducing and Non-reducing Sugars

Reducing sugars are carbohydrates that can donate electrons to reduce other molecules, typically characterized by the presence of a free aldehyde or ketone group. In contrast, non-reducing sugars lack this ability due to their glycosidic bonds. Understanding the distinction between these types of sugars is important for interpreting the behavior of raffinose with reagents like Tollens' reagent and for identifying the reducing nature of its hydrolysis products.
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