Chapter 7

Sugar Alcohols In the monosaccharide derivatives known as sugar alcohols, the carbonyl oxygen is reduced to a hydroxyl group. For example, \(\mathrm{D}\)-glyceraldehyde can be reduced to glycerol. Why can the sugar alcohol glycerol no longer be designated D or L?

Sugar Structures Compare and contrast the structural features of each pair: (a) Cellulose and glycogen (b) DGlucose and D-fructose (c) Maltose and sucrose

Hemiacetal and Glycosidic Linkages Explain the difference between a hemiacetal and a glycoside.

A Taste of Honey The sweetness of honey gradually decreases at a high temperature. Also, high-fructose corn syrup (a commercial product in which much of the glucose in corn syrup is converted to fructose) is used for sweetening cold drinks but not hot drinks. What chemical property of fructose could account for both of these observations?ns?

Gluconolactone and Glucose Oxidation States The cyclic glucose derivative 6-phosphogluconolactone is an intermediate in the pentose phosphate pathway (discussed in Chapter 14). Compare the oxidation state of \(C-1\) for the cyclic form of both gluconolactone and \(\beta\)-D-glucose.

Invertase "Inverts" Sucrose As sweet as sucrose is, an equimolar mixture of its constituent monosaccharides, \(\mathrm{D}-\) glucose and \(\mathrm{D}\)-fructose, is sweeter. Besides enhancing sweetness, fructose has hygroscopic properties that improve the texture of foods, reducing crystallization and increasing moisture. In the food industry, hydrolyzed sucrose is called invert sugar, and the yeast enzyme that hydrolyzes it is called invertase. The hydrolysis reaction is generally monitored by measuring the specific rotation of the solution, which is positive \(\left(+66.4^{\circ}\right)\) for sucrose but becomes negative (inverts) as more \(\mathrm{D}-\mathrm{glucose}\left(\right.\) specific rotation \(\left.=+52.7^{\circ}\right)\) and \(\mathrm{D}\)-fructose (specific rotation \(=-92^{\circ}\) ) form. From what you know about the chemistry of the glycosidic bond, how would you hydrolyze sucrose to invert sugar nonenzymatically in a home kitchen?

Manufacture of Liquid-Filled Chocolates The manufacture of chocolates containing a liquid center is an interesting application of enzyme engineering. The flavored liquid center consists largely of an aqueous solution of sugars rich in fructose to provide sweetness. The technical dilemma is the following: the chocolate coating must be prepared by pouring hot, melted chocolate over a solid (or almost solid) core, yet the final product must have a liquid, fructose-rich center. Suggest a way to solve this problem. (Hint: Sucrose is much less soluble than a mixture of glucose and fructose.)

Anomers of Sucrose? Lactose exists in two anomeric forms, but no anomeric forms of sucrose have been reported. Why?

Physical Properties of Cellulose and Glycogen The almost pure cellulose obtained from the seed threads of Gossypium (cotton) is tough, fibrous, and completely insoluble in water. In contrast, glycogen obtained from muscle or liver disperses readily in hot water to make a turbid solution. Despite their markedly different physical properties, both substances are \((1 \rightarrow 4)\)-linked D-glucose polymers of comparable molecular weight. What structural features of these two polysaccharides underlie their different physical properties? Suggest possible biological advantages of their respective properties.

Glycoproteins versus Proteoglycans Which characteristics describe glycoproteins and which describe proteoglycans? a. Exclusively located at the cell surface and in the extracellular matrix b. May contain \(N\)-linked glycosidic bonds c. Found in Golgi complexes, secretory granules, and lysosomes d. Include the heparan sulfate family e. Sulfated glycosaminoglycan chains can only be covalently linked to a Ser residue f. Form highly specific sites for recognition and highaffinity binding by lectins