Chapter 20

Work out a mechanism for the acid-induced hydrolysis of N-glycosides. Pay special attention as to where a proton can be added to be most effective in assisting the reaction. Would you expect that adenosine would hydrolyze more, or less, readily than \(\mathrm{N}\) -methyl- \(\alpha\) -ribosylamine? Give your reasoning.

Draw Haworth and conformational structures for each of the following disaccharides: a. \(6-\mathrm{O}-\beta-D\) -glucopyranosyl- \(\beta-D\) -glucopyranose b. \(4-\mathrm{O}-\beta-D\) -galactopyranosyl- \(\alpha-D\) -glucopyranose c. \(4-\mathrm{O}-\beta-D\) -xylopyranosyl- \(\beta-L\) -arabinopyranose d. \(6-\mathrm{O}-\alpha-D\) -galactopyranosyl \(-\beta-D\) -fructofuranose

Show how the structure of maltose can be deduced from the following: (1) The sugar is hydrolyzed by yeast \(\alpha-D\) -glucosidase to \(D\) -glucose. (2) Maltose mutarotates and forms a phenylosazone. (3) Methylation with dimethyl sulfate in basic solution followed by acid hydrolysis gives \(2,3,4,6\) -tetra- \(\mathrm{O}\) -methyl- \(D\) glucopyranose and \(2,3,6\) -tri-O-methyl- \(D\) -glucose. (4) Bromine oxidation of maltose followed by methylation and hydrolysis gives \(2,3,4,6\) -tetra- \(\mathrm{O}\) -methyl- \(D\) -glucopyranose and a tetramethyl- \(D\) -gluconic acid, which readily forms a \(\gamma\) -lactone.

Show how the structure of lactose may be deduced from the following: (1) The sugar is hydrolyzed by \(\beta-D\) -galactosidase to a mixture of equal parts of \(D\) -glucose and \(D\) -galactose. (2) Lactose mutarotates and forms a phenylosazone. (3) Bromine oxidation of lactose followed by hydrolysis gives \(D\) -gluconic acid and \(D\) -galactose. (4) Methylation and hydrolysis of lactose gives a tetra-O-methyl- \(D\) -galactose and \(2,3,6\) -tri-O-methyl- \(D\) -glucose. The same galactose derivative can be obtained from the methylation and hydrolysis of \(D\) -galactopyranose. (5) Bromine oxidation of lactose followed by methylation and hydrolysis yields tetra-O-methyl-1,4-gluconolactone and the same galactose derivative as in (4).

Explain how the \(\beta-D\) -glucoside units of cellulose produce a polymer with a stronger, more compact physical structure than the \(\alpha-D\) -glucose units of starch. Models will be helpful.

Explain how you could account for the fact that ascorbic acid is most stable in the enediol form rather than having its \(\mathrm{C}_{3}\) and \(\mathrm{C}_{2}\) carbons arranged either as \(-\mathrm{C}(=\mathrm{O})-\mathrm{CH}(\mathrm{OH})-\) or as \(-\mathrm{CH}(\mathrm{OH})-\mathrm{C}(=\mathrm{O})-\).

The heat of combustion of glucose(s) to \(\mathrm{CO}_{2}(g)\) and \(\mathrm{H}_{2} \mathrm{O}(l)\) is \(670 \mathrm{kcal} \mathrm{mol}^{-1}\), whereas that of 2 oxopropanoic acid \((l)\) is \(280 \mathrm{kcal} \mathrm{mol}^{-1}\). Neglecting the heats of solution of the compounds in water, estimate the energy of glucose \((a q)+\mathrm{O}_{2} \rightarrow 2 \mathrm{CH}_{3} \mathrm{COCO}_{2} \mathrm{H}(a q)+2 \mathrm{H}_{2} \mathrm{O}(l) .\)

The following interconversion is catalyzed by the enzyme triose phosphate isomerase: Explain how you might use bond energies to estimate whether the equilibrium constant, \(K\), for this reaction would be greater, or less, than unity.

The reaction \(\mathrm{ADP}+\mathrm{RCO}-\mathrm{SR}^{\prime}+\mathrm{PO}_{4}^{3-} \rightarrow \mathrm{ATP}+\mathrm{RCO}_{2} \mathrm{H}+\mathrm{HSR}^{\prime} \quad\) is substantially more favorable than the corresponding reaction with \(\mathrm{RCO}_{2} \mathrm{R}\). On the basis of the valence-bond treatment, explain why this should be so.

A first step in unraveling the mechanism of the metabolism of fatty acids was made in 1904 by F. Knoop, who found that dogs metabolized 4-phenylbutanoic acid to phenylethanoic acid and 3 -phenylpropionic acid to benzoic acid. What does this pattern of results indicate about the mechanism of degradation of fatty acids? Give your reasoning.

A very strong man can lift \(225 \mathrm{~kg}(500 \mathrm{lb}) 2\) meters \((6.5 \mathrm{ft})\). Muscle action gets its energy from the reaction \(\mathrm{ATP}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{ADP}+\mathrm{H}_{2} \mathrm{PO}_{4}^{-}\), a process with a \(\Delta G^{0}\) of \(-7 \mathrm{kcal}\) a. Assuming \(50 \%\) efficiency in the use of the hydrolysis free energy, how many grams of ATP (MW 507 ) would have to be hydrolyzed to achieve this lifting of the weight? (One \(\mathrm{kg}\) raised one meter requires \(2.3\) calof energy.) b. How many grams of glucose would have to be oxidized to \(\mathrm{CO}_{2}\) and water to replenish the ATP used in Part a on the basis of a \(40 \%\) conversion of the energy of combustion to ATP? ( \(\Delta G^{0}\) for combustion of glucose is \(-686 \mathrm{kcal}\) )

Draw Haworth- and conformation-type formulas for each of the following: a. methyl \(2,3,4,6-\mathrm{O}\) -tetramethyl- \(\alpha-D\) -glucopyranoside b. \(\beta-D\) -arabinofuranosyl- \(\alpha-L\) -arabinofuranoside c. \(L\) -sucrose

Write a mechanism for the interconversion of an aldohexose and a ketohexose that is catalyzed by hydroxide ion. What products would you expect starting with \(D\) -glucose?