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Carbohydrates

Building Bridges to Knowledge

Photo of chocolate on bananas Michaelle Cadet Carbohydrates constitute a class of compounds containing carbon, hydrogen, and oxygen. Carbohydrates are classified as monosaccharides, disaccharides, and polysaccharides. Polysaccharides are naturally occurring carbohydrates that hydrolyze to produce three or more monosaccharides. Monosaccharides have an empirical formula equal to CH2O. Disaccharides have an empirical formula equal to C1.09H2O. The term saccharide originates from the Latin word for sugar, saccharon. Some examples of polysaccharide carbohydrates are starches, amylose, amylopectin, and cellulose. Cellulose is formed in structural materials of organic systems. The term carbohydrate means hydrated carbon, i.e., carbon that is hydrated,

For example, glucose, a monosaccharide, has the molecular formula C6H12O6, and as hydrated carbon, it could be represented as

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The term hydrated carbon atoms is misleading and is not an accurate description of carbohydrates. A more scientific description of carbohydrates is that they are polyhydroxy aldehydes or ketones. Therefore, carbohydrates can be more accurately described as organic molecules with several OH functional groups (alcohols), and latent (potentially free) carbonyl groups. In addition, carbohydrates contain chiral centers where each chiral center may be designated as R or S. The rules of stereochemistry described in the paper titled Stereochemistry, Building Bridges to Knowledge apply to carbohydrates and their chiral centers. Some carbohydrates exhibit reactions of aldehydes or ketones, because a small amount of the latent (or hidden) carbonyl group exists in equilibrium with the potentially free aldehyde or ketone group. The driving force to the potentially free aldehydic group is based on Le Chateliers Principle. As the small quantity of free aldehyde (about 0.02%) is consumed, the equilibrium continues to favor the direction of the free aldehyde. This shift in equilibrium toward the free aldehyde produces more aldehyde that can undergo further reaction. Complex carbohydrates containing hemiacetal or hemiketal function can be hydrolyzed to free aldehydes or ketones. Carbohydrates are chemically classified into two groups, reducing sugars and nonreducing sugars. Reducing sugars give positive results with Tollens reagent, Benedicts reagent, and Fehlings reagent. Nonreducing sugars give negative results with Tollens reagent, Benedicts reagent, and Fehlings reagent. The Tollens test is a two-step laboratory test that uses freshly prepared reagents. The first step involves the preparation of silver oxide from aqueous silver nitrate and aqueous NaOH. 2 AgNO3 (aq) + 2 NaOH (aq) Ag2O (s) + 2 NaNO3 (aq) + H2O (l) The second step is the preparation of the Tollens reagent and involves the addition of ammonia to the freshly prepared silver oxide to produce diaminesilver nitrate.

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Ag2O (s) + 4 NH3 (aq) + 2 NaNO3 (aq) + H2O (l) 2 Ag(NH3)2NO3 (aq) + 2NaOH (aq) Diaminesilver nitrate (the Tollens reagent) reacts with the potentially free aldehyde group or the alpha-hydroxy ketone of the carbohydrate to form elemental silver. Since silver metal coats the inside of the reaction vessel, the positive results have been referred to as the silver mirror test.

For alpha-hydroxy ketones such as fructose, tautomerism occurs to produce an -hydroxy aldehyde. Then the -hydroxy aldehyde reacts with the diaminesilver complex to form elemental silver producing a silver mirror within the reaction vessel.

Monosaacharides are classified on the basis of the number of carbon atoms in the sugar as listed in table 22.1.

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Number of Carbon Atoms Formula Classification 3 C3H6O3 triose 4 C4H8O4 tetrose 5 C5H10O5 pentose 6 C6H12O6 hexose 7 C7H14O7 heptose

Table 22.1 The suffix ose indicates that the system is a carbohydrate. Polyhydroxyaldehydes are referred to as aldoses, and polyhydroxyketones are referred to as ketoses. Consequently, an aldopentose is a polyhydroxyaldehyde containing five carbon atoms (including an aldehyde group), and a ketopentose is a polyhydroxyketone containing five carbon atoms (including a ketone group). As indicated in Table 22.1, the simplest carbohydrates (sugars) are the trioses. Dihydroxyacetone and glyceradehyde, two trioses, can be derives from the oxidation of glycerol.

dihydroxyacetone Glycerol glyceraldehyde (a ketotriose) (an aldotriose) Glycerol is optically inactive, and dihydroxyacetone is optically inactive; however, glyceraldehyde is a chiral molecule, and is, therefore, optically active. Two stereoisomers of glyceraldehyde exist. One isomer rotates plane polarized light to the right, and the other isomer rotates plane polarized light to the left. Dihydroxyacetone and glyceraldehyde are important intermediates in muscle metabolism. As previously mentioned, gyceraldehyde, a chiral molecule, has a chrial center. The structure of glyceraldehyde with an R designation at its chiral center, (2R)-2,3-dihydroxypropanal, can be represented by the following structure.

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(2R)-2,3-dihydroxypropanal (2R)-2,3-Dihydroxypropanal is dextrorotatory; therefore, its complete IUPAC name would be (+)-(2R)-2,3-dihhydroxypropanal. (+)-(2R)-2,3-Dihhydroxypropanal is also called (+)-D-glyceraldehyde, and it has a specific rotation of +13.5o g-1mL dm-1. The structure of glyceraldehyde that exhibits an S designation at its chiral center, (2S)-2,3-dihydroxypropanal, can be represented by the following structure.

(2S)-2,3-dihydroxypropanal (2S)-2,3-Dihydroxypropanal is levorotatory; therefore, the complete IUPAC name would be (-)-(2S)-2,3-dihydroxypropanal. This molecule is also called (-)-L-glyceraldehyde, and it has a specific rotation of -13.5o g-1mL dm-1. Traditionally, the enantiomers R-glyceraldehyde and S-glyceraldehyde have been referred to respectively as D-glyceraldehyde and L-glyceraldehyde. The D and L notations have led to the conventional representation of the biologically active carbohydrates with a D configuration on the carbon atom adjacent to the CH2OH group. D monosaccarides can be synthesized from D-glyceraldehyde, and L monsaccharides can be synthesized from L-glyceraldehyde. The D and L designations in the nomenclature go back to the late nineteen and early twentieth century with the work of Emil Fischer, who received the 1902 Nobel Prize for determining the structure of glucose.

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The D-sugars and L-sugars can be structured by following the series of reactions for building the carbon skeleton of monosaccharides. The carbon skeleton of carbohydrates can be built one carbon atom at a time using the Kiliani-Fischer synthesis. The Kiliani-Fischer synthesis is a three-step sequence of reactions that lead to an increase in the carbon-chain of the carbohydrate by an additional carbon atom and the generation of a diastereomeric pair. (1)

(2)

(3)

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The elementary steps that rationalize these three reactions follow similar unimolecular and/or bimolecular steps found in the paper titled Stereochemistry, Building Bridges to Knowledge. The Kiliani-Fischer synthesis would lead to the following conclusions for extending the carbon chains of carbohydrates by one more carbon atom.

The result, D-erythrose and D-threose, are diastereomers. Both structures rotate plane polarized light. The observed direction of rotation can only be obtained experimentally using a polarimeter. Using the Fischer-Projection Model, D-aldotetroses resulting from the D-aldotriose, (2R)-2,3-dihydroxypropanal, would possess two chiral centers. The D-aldotetrose, (2R,3R)-2,3,4-trihydroxybutanal or D-erythrose, would exhibit R,R designations at the two chiral centers, and another D-isomer, (2S,3R)-2,3,4-trihydroxybutanal or D-threose would exhibit S,R designations at its two chiral centers.

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The D-aldotetrose, (2R,3R)-2,3,4-trihdroxybutanal or D-erythrose, would produce two aldopentoses, (2R,3R,4R)-2,3,4,5-tetrahydroxypentanal or D-ribose and (2S,3R,4R)-2,3,4,5-tetrahydroxypentanal or D-arabinose. The L-aldotetrose would produce two aldopentoses, (2R,3S,4R)-2,3,4,5-tetrahydroxypentanal or D-xylose and (2R,3S,4S)-2,3,4,5-tetrahydroxypentanal or L-arabinose. These aldopentoses can be viewed and understood by studying the Killani-Fischer synthesis. The D-aldotetrose would produce two D designated aldopentoses, and the L designated aldopentose would produce two L designated aldopebtoses. For example, the Killani-Fischer synthesis using D-erythrose as a precursor produces D-ribose, (2R, 3R, 4R)-2,3,4,5-tetrahydroxybutanol and D-arabinose, (2S, 3R, 4R)-2,3,4,5-tetrahydroxybutanol as products. The Killani-Fischer synthesis can be used to prepare the eight aldopentose isomers from the aldotetrose isomers. The biologically active sugar, D-ribose, (2R,3R.4R)-2,3,4,5-tetrahydroxypentanal, has a D designation.

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The number of stereoisomers theoretically possible for compounds containing chiral centers can be obtained from the formula 2n where n equals the number of chiral centers in the molecule. Since aldopentoses contain three chiral centers, there should be 23 or eight (8) isomers, including the D and L designations for the five-carbon polyhydroxyaldehyde compounds. Eight isomers of the aldopentose have been identified, and they are D-ribose, (2R, 3R, 4R)-2,3,4,5-tetrahydroxypentanal; D-arabinose, (2S, 3R, 4R)-2,3,4,5-tetrahydroxypentanal; D-xylose, (2R, 3S, 4R)-2,3,4,5-tetrahydroxypentanal; D-lyxose, (2S, 3R, 4R)-2,3,4,5-tetrahydroxypentanal; L-arabinose, (2R, 3S, 4S)-2,3,4,5-tetrahydroxypentanal; L-ribose, (2S, 3S, 4S)-2,3,4,5-tetrahydroxypentanal; L-lyxose, (2R, 3R, 4S)-2,3,4,5-tetrahydroxypentanal; and L-xylose, (2S, 3R, 4S)-2,3,4,5-tetrahydroxypentanal. All eight isomers would rotate plane polarized

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light; however, the observed rotation would be obtained using a polarimeter. Only the D-isomers, (2R,3R,4R)-aldopentose; (2R,3S,4R)-aldopentose; (2S,3R,4R)-aldopentose; and (2S,3S,4R)-aldopentose are biologically active. However, as previously mentioned, the most important biologically active polyhydroxyaldopentose is D-ribose, (2R, 3R, 4R)-pentanal. D-ribose is an important chemical component of ribonucleic acid, RNA. The specific rotation of D-ribose is -18o mL g-1 dm-1. A naturally modified molecule of D-ribose, D-2-deoxyribose (compound I in which the OH on carbon atom number 2 has been replaced with a hydrogen atom, and has the IUPAC name (3R,4R)-3,4,5-trihydroxypentanal), is an important component of deoxyribonucleic acid, DNA.

Compound I Analogous arguments can be used to build the carbon structures for aldohexoses. There are four chiral centers for polyhydroxyaldehydes; therefore, sixteen (16) aldohexoses are possible. Eight (8) of these isomers are biologically active, and the eight structures are D-polyhydroxyaldehydes. These D-aldohexoses are D-allose, (2R,3R,4R,5R)-2,3,4,5,6-tetrahydroxyhexanal; D-altrose, (2S,3R,4R,5R)-2,3,4,5,6-tetrahydroxyhexanal; D-glucose, (2R,3S,4R,5R)-2,3,4,5,6-tetrahydroxyhexanal; D-mannose, (2S,3S,4R,5R)-2,3,4,5,6-tetrahydroxyhexanal; D-gulose, (2R,3R,4S,5R)-2,3,4,5,6-tetrahydroxyhexanal; D-idose, (2S,3R,4S,5R)-2,3,4,5,6-tetrahydroxyhexanal; D-galactose,

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(2R,3S,4S,5R)-2,3,4,5,6-tetrahydroxyhexanal; and D-talose, (2S,3S,4S,5R)-2,3,4,5,6-tetrahydroxyhexanal. The Kiliani-Fischer synthesis can be used to add an additional chiral carbon to aldopentoses to produce aldohexoses. The following aldohexoses can be synthesized from the corresponding aldopentoses.

The free aldehyde formula for D-lyxose, D-galactose, and D-talose showing the stereochemistry at the chiral centers are

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D-lyxose D-galactose D-talose In the late nineteenth century, Emil Fischer used what has become known as the Fischer Projection formula to represented D-glyceraldehyde and L-glyceraldehyde

D-glyceraldehyde L-glyceraldehyde as

D-glyceraldehyde L-glyceraldehyde Since the monosaacharides can be constructed from D-glyceraldehyde and L-glyceraldehyde, the Fischer Projection formulas for the free aldehyde structures like D-lyxose, D-galactose, and D-talose can be represented as

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This paper uses a slightly different perspective of the Fischer Projection formula. The Fischer Projection formula for D-arabinose is

and the notation used in this paper is

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Three of the eight biologically active aldohexoses are frequently encountered. Those monosaccharides are glucose, mannose, and galactose. Of the eight stereoisomeric ketohexoses possible, fructose (compound II) is the most familiar.

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Compound II Glucose Glucose, a crystalline solid, is considered to be the most important aldohexose. It is sometimes referred to as dextrose. Glucose is blood sugar that constitutes approximately 6.5 x10-2 % to 1.10 x 10-1 % of blood. It is essential to life since it is the only sugar the cells can use directly for the production of energy. Other sugars must convert to glucose before they are useful to the body. Glucose can be obtained from several sources. It can be obtained from the hydrolysis of starches, and it can be obtained from the hydrolysis of cellulose. Glucose and fructose are components of sucrose. Sucrose and fructose are found in plant sap. Honey, invert sugar, is comprised of a mixture of glucose and fructose. Approximately half of the carbon atoms in the biosphere constitutes some form of glucose of which most is in the form of cellulose, and cellulose cannot be digested by human beings. Mannose and Galactose Mannose and galactose, crystalline solids, are natural sugars that are widely distributed in nature. Mannose is a component of mannon, a polysaccharide found in certain berries. Mannon is also found in the endosperm of palm nuts. Galactose can be obtained from the hydrolysis of lactose, a disaccharide called milk sugar. Galactose and glucose are the two monosaccharide-constituents of lactose. Also, galactose is a

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component of the galactan polysaccharides found in the cell walls of plants. Galactose is found in the brain and nerve tissue where it is a component of cerebrosides and gangliosides. Fructose Fructose, a crystalline solid, is a component of sucrose and honey. It is also a component of fruit juices, and it is found in animal and human semen. Glucose, galactose, and mannose are monosaccharides that exist in nature as hemiacetals (potential free aldehydes) rather than the free aldehyde. Fructose exists in nature as a hemiketal. Galactose, glucose, and mannose hemiacetals and fructose hemiketal exist as cyclic molecules that exhibit anomerization. A clearer understanding of hemiacetal formation may be obtained if you imagine a bridge forming between the aldehydic carbon atom and the oxygen atom on the fifth carbon atom in glucose, mannose, and galactose. Analogously, a clearer understanding of fructose hemiketals formation can be visualized if you imagine a bridge forming between the carbonyl group on the second carbon atom of fructose and the oxygen atom on the fifth carbon atom of fructose. For example, the electron in a nonbonding molecular orbital of the oxygen atom on the fifth carbon atom can intramolecularly undergo a nucleophilic attack with the aldehydic carbonyl group as discussed in the paper titled Aldehydes and Ketone in the section concerning acetal formation. Upon intramolecular nucleophilic attack, the carbonyl carbon can form two compounds, -D-glucose and -D-glucose.

D-glucose -D-glucose -D-glucose

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The difference between these two compounds is the spatial arrangements of the OH group formed after nucleophilic attack of the oxygen attached to the fifth carbon atom on the carbonyl group of the aldehyde. This carbon atom is referred to as the anomeric carbon atom. The anomeric carbon atom is accompanied by a new chiral center in the molecule. The resulting hemiacetal forms an oxygen bridge that is represented in the Fischer Projection formula that appears as if it has two additional carbon atoms that somehow magically appear, but that is not the case. The Fischer Projection formula is a two-dimensional effort to demonstrate that intramolecular oxygen bridges exist in the molecule. A better representation of aldohexoses is the Haworth Model. Even though the Haworth Model is a two dimensional model that represents the cyclic structure of the aldohexose, it doesnt give the impression that extra carbon atoms have been added to the system The Haworth Model show that the hemiacetal model for the Haworth structure can lead to two different structures, -D-glucopyranose and -D-glucopyranose.

D-glucose -D-glucopyranose -D-glucopyranose -D-glucopyranose has a specific rotation of + 112o g-1 mL dm-1, and -D-glucopyranose has a specific rotation of +18.7 g-1 mL dm-1. Note that in the Haworth Model, the OH in the down position represents the alpha anomer and the OH in the up position represents the beta anomer. -D-glucopyranose melts at 146oC and -D-glucopyranose melts at 150oC. A solution of -D-glucopyranose and -D-glucopyranose will undergo mutarotation by conversion to the free aldehyde. In solution, approximately 36% of the glucose exists as -D-glucopyranose and approximately 64% exist as -D-glucopyranose.

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Mutarotation (the equilibrium between the -D-glucopyranose, the free aldehyde, and the -D-glucopyranose) results in a specific rotation of approximately +52.3o g-1 mL dm-1 [ ]D

20o C = 0.36 x 112o mLg dm + 0.64 x 18.7o mL

g dm [ ]D

20o C = 52.3o mLg dm About 0.02% of D-glucose exists as the free aldehyde. A chemical structure with a six-membered ring exists in the chair conformation to alleviate any potential angle strain in the molecule (see paper on Cyclic Aliphatic Hydrocarbons, Building Bridges to Knowledge). The structures of -D-glucopyranose and -D-glucopyranose have chair conformations. Therefore, the best structural formula for the aldohexose -D-glucopyranose can be represented by

The linkage around the anomeric carbon atom has the OH in the axial position. The best structural formula for the aldohexose -D-glucopyranose can be represented by

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The linkage around the anomeric carbon atom has the OH in the equatorial position. The anomer is more stable, because the OH group on the anomeric carbon atom is in the equatorial position; whereas, the anomer has the -OH group on the anomeric carbon atom in the axial position. If -D-glucopyranose is placed in solution, mutarotation occurs, i.e., the alpha compound would equilibrate via the free aldehyde to form an equilibrium mixture of the -D-glucopyranose and the -D-glucopyranose where the percent of the alpha anomer is approximately 36% and the percent of the beta anomer is 64%.

36% 0.02% 64% The difference between the alpha anomer and the beta anomer is relatively small, but extremely important in biological systems. Reactions of monosaccharides We have already been introduced to the Tollens test or silver mirror test for aldehydes and alpha-hydroxy ketones. There are two other popular tests for aldehydes, the Benedicts test and the Fehlings test.

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The Benedicts Test for Potentially Free Aldehydes and Alpha-hydroxyl Ketones The Benedict reagent is a blue solution consisting of sodium carbonate, copper(II) sulfate, and sodium citrate. A positive Benedicts test results if the carbohydrate has a latent, potentially free, aldehyde or alpha-hydroxy keto group that can be oxidized to a carboxylate anion. In the REDOX reaction, the copper is reduced to copper(I) oxide, a bright red precipitate. The carbohydrate, sugar, undergoing the Benedicts test must have a potentially free aldehyde group to act as the reducing agent. Sugars possessing latent aldehydes or alpha-hydroxy ketones are referred to as reducing sugars. For example, glucose is a reducing sugar, because 0.02% of the hemiacetal is in the free aldehydic state and 99.08% is in the alpha and beta hemiacetal forms. In aqueous solution, Le Chataliers Principle drives the equilibrium toward the free aldehyde as more of the hemiacetal is converted into the free aldehyde by oxidation to the carboxylate anion. Sucrose, a disaccharide that consists of fructose and glucose, doesnt have potentially free aldehydes; therefore, it cannot form a free aldehyde. Since sucrose cannot form a free aldehyde, it gives a negative Benedict test. A positive Benedicts test leads to the formation of copper(I) oxide, a bright red precipitate, by the following reaction.

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The Benedicts solution (blue color) Copper in the benedicts solution will be reduced to copper(I) oxide, a red precipitate. Carbonate is an anionic base that produces hydroxide ions in aqueous solution.

The base produced by the carbonate anion, the anionic base, will provide the basic medium to facilitate the formation of the red precipitate, copper(I) oxide and the carboxylate anion.

bright red precipitate The positive test for a latent alpha-hydroxy ketone, e.g, fructose, may be represented by the following equation.

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The Fehlings Test for Potentially Free Aldehydes and Ketones The Fehlings Test works in an analogous manner as the Benedicts solution; however, the Fehlings test consists of two solutions, Fehlings A and Fehlings B solutions. Fehlings A is a blue aqueous solution of copper(II) sulfate and Fehlings B is a clear aqueous solution of potassium sodium tartrate and sodium hydroxide. When equal volumes of these two solutions are mixed, a fresh Fehlings solution is produced that has a deep blue color. Following is a chemical equation representing the Fehlings reagent.

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deep blue solution The copper in the Fehlings solution will be reduced to copper(I) oxide, a red precipitate as described in the following reaction.

bright red precipitate The positive test for a latent alpha-hydroxy ketone, e.g, fructose, can be represented by the following equation.

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The Silver Mirror Test (the Tollens Test), the Benedicts Test, and the Fehlings Test are important for detecting reducing sugars. For example, for years these tests were used to detect sugar in the urine (an indication of diabetes). Fructose, that has a potentially reactive -hydroxy ketone, can give positive results for the Tollens reagent, the Benedicts reagent, and the Fehlings reagent, because fructose will undergo tautomerism in base to form an -hydroxy aldehyde which can be oxidized to an -hydroxy carboxylate. - D-Fructofuranose and -D-fructofuranose can be represented by the following Haworth Models. The Haworth model shows that fructose exhibits a hemiacetal structure.

- D-fructofuranose

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-D-fructofuranose The cyclic hemiketal structure of fructose can form when the oxygen atom attached to carbon atom 5 undergoes an intramolecular nucleophilic attack on the carbon atom that contains the carbonyl group of fructose.

- D-fructofuranose -D-fructofuranose The hemiketal form of fructose, -D-fructofuranose or -D-fructofuranose, can exhibit tautomerism that will allow oxidation to occur to give a positive test for a reducing sugar, the bright red Cu2O precipitate and the silver mirror test. All monosaacharides act as reducing sugars; however this is not the case for all disaacharides. Some dissacharides are reducing sugars and others are not. Dissacharides that are actals or ketals are non-reducing sugars. Dissacharides that are hemiacetal or hemiketals are reducing sugars and will give positive results with Tollens reagents, Benedicts reagents, and Fehlings reagents. Hemiacetals and hemiketals react with alcohols to form acetals and ketals. Carbohydrate acetals and ketals are called glycosides. Glycosides are stable whereas hemiacetals and hemiketals are unstable. Glycosides do not undergo mutarotation and they dont give positive results with Tollens reagent, Benedicts reagent, or Fehlings reagent.

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Following is a reaction between -D-glucopyranose and methyl alcohol to form the acetal methyl -D-glucopyranoside. The aldose hemiacetals are called pyranoses and the acetals are called pyranosides.

-D-glucopyranoside In reality, the acetal formation leads to both the -D-glucopyranoside and -D-glucopyranoside, because the mechanism of the reaction proceeds through the free aldehyde.

-D-glucopyranoside -D-glucopyranoside Disaccharides Maltose, lactose, and sucrose are familiar examples of disaccharides.

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Maltose Maltose is found in sprouting grain and is obtained from the partial hydrolysis of starch. The disaccharide, maltose, consists of two glucose molecules linked together by an -1,4 oxygen bridge. The Haworth structure for maltose is

The numbering system shows that an oxygen atom on carbon atom 1 of one glucose molecule undergoes a nucleophilc reaction with carbon atom 4 of a second glucose molecule. However, carbon number 1 of the second glucose molecule has a hemiacetal formation; therefore, maltose is a reducing sugar and will give positive results with Tollens reagent, Benedicts reagent, and Fehlings reagent. The following equation represents the formation of the 1,4 oxygen bridge in maltose.

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The chair conformation of this system would provide a more accurate visualization of the alpha linkage of the oxygen bridge. Notice that maltose has an alpha 1,4 glycoside link not a beta 1,4 linkage.

The wavy line on the hemiacetal end of the molecule means that the OH on the anomeric carbon atom can be or . The IUPAC name for maltose is -D-glucopyranosyl-(14)-D-glucose. The chair conformation of maltose clearly shows that the 1,4 oxygen bridge results in the formation of a glycosidic linkage at carbon atom 1. The OH group on the fourth carbon atom of another glucose molecule leaves the anomeric carbon atom on the second molecule of glucose in a hemiacetal form. This gives the disaccharide its reducing capabilities. Maltose is formed from starch by an enzymatic reaction. The enzyme, a biological catalyst, used to produce maltose is amylase. Amylase is found in malt, yeast, and saliva. Maltose can be hydrolyzed into two glucose molecules by the enzyme maltase. A similar reaction can be accomplished invitro with dilute mineral acids such as HCl. This reaction is similar to the hydrolysis of an acetal or a himiacetal with dilute hydrochloric acid.

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Lactose Lactose, milk sugar, constitutes 5% - 7% of human milk and 4% -6% of cows milk. Lactose is a disaccharides composed of glucose and galactose linked together by a -1,4 oxygen bridge. Since there is a hemiacetal linkage at the anomeric carbon atom on glucose, then lactose is a reducing sugar.

The chair conformation of glucose and galactose show the -linkage more clearly than the Haworth model of lactose.

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galactose glucose and

The IUPAC name for lactose is -D-galactopyranosyl-(14)-D-glucose. Sucrose Sucrose, cane sugar or beet sugar, is the most important disaccharide. Sucrose is composed of glucose and fructose linked together by an -1,2 oxygen bridge or glycosidic linkage. Sucrose is an acetal with no potentially free aldehydic group or -hydroxy acetal available for oxidation; therefore, sucrose is a nonreducing sugar and will not give positive results with Tollens reagent, Benedicts reagent, or Fehlings reagent. Following is the Haworth structure for sucrose.

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The following chair conformation of glucose illustrates the -linkage clearer than the Haworth model for sucrose. In this model, fructose has a glycosidic linkage with glucose, and glucose has an glycosidic linkage with fructose.

Sucrose can be hydrolyzed in a dilute acidic medium to produce glucose and fructose.

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A mixture of glucose and fructose is called invert sugar. Invert sugar is sweeter than sucrose. Honey is primarily comprised of invert sugar. The yearly per capita consumption of sucrose (in the form of drinks, sweets, processed foods, etc.) is about one hundred pounds. Table 22.2 lists the sweetness of substances relative to the sweetness of sucrose where the sweetness of sucrose is designated as 1.00.

Sweetener Relative Sweetness saccharin 450 aspartame 160 fructose 1.74

Invert sugar 1.25 honey 0.97

glucose 0.74 maltose 0.33

galactose 0.32 lactose 0.16

Starch Starch is a polysaccharide of the glucose monomer. Starches are found in plants that produce glucose by photosynthesis. Starches can be separated into two factions - amylose and amylopectin.

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Amylose consists of 60 to 300 units of glucose, and amylopectin consist of 300 to 6,000 glucose units. The structure of amylose may be represented by the following structure:

amylose where n= 60 to 300 glucose units The structure of amylopectin can be represented by the following structure:

amylose

where n = 300 to 6,000 glucose units

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Cellulose Cellulose is a polymer of glucose with -1,4 glycoside linkages. Since the glycoside is around the anomeric carbon atom, then cellulose is not digestible in humans. Cellulose is the glucose polysaccharide found in grass; consequently, grazing animals and termites can digest this natural polymer of glucose since they have enzymes that can hydrolyzed -1,4 glycoside linkages. Cellulose in plant walls is arranged as fibrils, i.e., bundles of parallel chains. These chains run in opposite directions. Cotton is an example of this type of cellulose structure. Following is an illustration of cellulose showing its -1,4 glycoside linkages.

Reactions of Carbohydrates Ruffs Degradation Otto Ruff, in 1898, published his synthesis of D-arabinose from D-glucose. His synthesis was later referred to as the Ruff Degradation. The synthesis involves the conversion of glucose to aldonic acid followed by decarboxylation to D-arabinose. Following are three reactions that demonstrate this process. Reaction 1 would be the conversion of D-glucose to D-gluconic acid, an aldonic acid.

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The mechanism for this reaction probably follows a free radical process that could involve the following possible steps.

(1) Br2 + light 2 Br

(2) Br2 + H2O HOBr + HBr

(3)

(4) H + HOBr HO + HBr

(5)

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The second reaction is the conversion of the product, D-gluconic acid, formed in the first reaction to calcium D-gluconate.

The third, and final reaction is the decarboxylation of calcium D-gluconate to D-Arabinose.

The decarboxylaion of D-gluconate to D-arabinose probably follows a free-radical process. Following is a proposed partial mechanism for the free radical decarboxylation of D-gluconate to D-arabinose.

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The Ruff Degradation could be applied to any of the aldotrioses, aldotetroses, aldopentoses, and aldohexoses. Monosaccharides can be reduced to alditols by the reduction of the latent aldehyde using sodium borohydride.

free aldehyde D-sorbitol D-Sorbitol is also called D-glucitol. Monosaccharides can be oxidized to dicarboxylic acids, aldaric acids, by nitric acid.

free aldehyde D-glucaric acid

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D-glucaric acid is an aldaric acid. Periodic acid, HIO4, (discussed in the paper titled Alcohols, Building Bridges to Knowledge) has the ability to cleave vicinal (vic) glycols to produce carbonyl compounds. In addition, periodic acid can cleave -hydroxyketones as well. Since monosaccharides such as glucose contain many OH groups on adjacent carbon atoms as well as the aldehyde group, the cleavage reaction is rather extensive. For example, one mole of D-glucose would react with five moles of HIO4 to yield one mole of formaldehyde and five moles of formic acid.

Periodic acid reacts with monosaccharides through the following steps. Step 1

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Step 2

Step 3

Step 4

Step 5

The sum of steps1-5 gives

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The following mechanism rationalizes the carbon-carbon cleavage of polydroxyaldehydes with periodic acid, HIO4. (1)

(2)

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(3)

This occurs four more times to give a total of 5 moles of HIO3 producing a total of 5 moles of formic acid and one mole of formaldehyde. Osazones The potentially free aldehydic form of monosaccharides reacts with phenylalanine to form osazone.

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Emil Fischer used osazones to identify aldose sugars that are epimers. Epimers are diastereoisomeric carbohydrates that have only one chiral center that is different and the remaining chiral centers are identical. For example D-glucose and D-mannose are epimers, because D-glucose and D-mannose have two chiral centers that have the same R/S designation and one chiral center with a different R/S designation. Since D-glucose and D-mannose are epimers, they form the same osazone. Carbon atom 2 is the epimer carbon for D-glucose and D-manose.

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The formation of osazones occurs in an acidic medium. Following is a proposed series of elementary steps (mechanism) that rationale the formation of the osazone from D-altrose.

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(1)

(2)

(3)

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(4)

(5)

(6)

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(7)

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(8)

(9)

Adding steps (1)-(9) would give the following equation:

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The anomeric carbon atom of -D-ribofuranose is susceptible to nucleophilic attack by the ring nitrogen atom of pyrimidine and purine bases to form N-glycosides. The ring nitrogen atom must contain a hydrogen atom. For example, the biochemical reaction of cytosine, a pyrimidine base, with -D-ribose produces cytidine.

-D-ribose cytosine cytidine

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The IUPC name for cytidine is 4-amino-1-[3,4-dihydroxy-5-(hydoxymethyl)tetrahydrofuran-2-yl]-2-one. The pyrimidine bases are thymine, cytosine, and uracil.

thymine cytosine uracil The group designated as N-H in the pyrimidine bases attaches to the anomeric carbon of - D-ribose to form the pyrimidine glycosides. The purine bases are adenine and guanine.

adenine guanine The group designated as N-H in the purine bases attaches to the anomeric carbon of -D-ribose to form the purine glycosides. The five pyrimidine and purine bases are the components of DNA and RNA.

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Problems Carbohydrates 1. A non-reducing sugar with the molecular formula C18H22O16 reacts with aqueous mineral acids to produce three reducing sugars.

The non-reducing sugar, C18H22O16 ,produces galactose and sucrose when treated with an enzyme that cleaves galactose from a polycarbohydrate.

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The non-reducing sugar, C18H22O16, produces produces fructose and melibiose when treated with an enzyme that cleaves sucrose.

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Suggest a structure for C18H22O16. (2) Hydrolysis of one mole of a biochemical polymer produced nine

moles of compound A, 1 mole of compound B, and one mole of sulfuric acid. Methylation of one mole of the polymer followed by hydrolysis produced nine moles of compound C, one mole of compound D, and 1 mole of sulfuric acid.

Suggest a structure for the biochemical polymer.

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and forms of Compound A

and forms of compound B

and forms of Compound C

and forms of Compound D (3) Acid hydrolysis of a carbohydrate with the molecular formula

C18H22O16 gives compound A as the only product. The carbohydrate is a non-reducing sugar. Methylation of the carbohydrate followed by hydrolysis produces compound B. Suggest a structure for C18H22O16.

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and forms of Compound A

and forms of Compound A (4) Suggest syntheses for the following from D-(+)-glucose.

(a)

(b)

(5) Suggest structures for A through G

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Following is 1HNMR and 13CNMR spectra for Compound E

1HNMR

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13CNMR (6) An optically inactive carbohydrate with the molecular formula

C6H10O6 reduces benedicts solution; however, C6H10O6 does not react with Br2 and H2O. Reaction of C6H10O6 with sodium borohydride and water produces two compounds (compounds A and B) with the same molecular formula, C6H12O6 . Compounds A and B are oxidized by HIO4 to six moles of formic acid, HCOOH. In addition, compounds A and B react with acetic anhydride to produce C18H24O12.

Compound A Compound B Suggest a structure for the optically inactive carbohydrate C6H10O6, and determine the R/S notation at each chiral center. Also, determine the R/S notation at the chiral centers in compounds A and B. Suggest a structure for C18H24O12.

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Solutions to Carbohydrate Problems 1. A non-reducing sugar with the molecular formula C18H22O16 reacts with aqueous mineral acids to produce three reducing sugars.

The non-reducing sugar, C18H22O16 ,produces galactose and sucrose when treated with an enzyme that cleaves galactose from a polycarbohydrate.

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The non-reducing sugar, C18H22O16, produces produces fructose and melibiose when treated with an enzyme that cleaves sucrose.

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Suggest a structure for C18H22O16.

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2. Hydrolysis of one mole of a biochemical polymer produced nine moles of compound A, 1 mole of compound B, and one mole of sulfuric acid. Methylation of one mole of the polymer followed by hydrolysis produced nine moles of compound C, one mole of compound D, and 1 mole of sulfuric acid.

Suggest a structure for the biochemical polymer.

and forms of Compound A

and forms of compound B

and forms of Compound C

and forms of Compound D

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3. Acid hydrolysis of a carbohydrate with the molecular formula C18H22O16 gives compound A as the only product. The carbohydrate is a non-reducing sugar. Methylation of the carbohydrate followed by hydrolysis produces compound B. Suggest a structure for C18H22O16.

and forms of Compound A

and forms of Compound A

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4. Suggest syntheses for the following from D-(+)-glucose.

(a)

(b)

5. Suggest structures for A through G

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72

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Following is 1HNMR and 13CNMR spectra for Compound E

1HNMR

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13CNMR

6. An optically inactive carbohydrate with the molecular formula C6H10O6 reduces benedicts solution; however, C6H10O6 does not react with Br2 and H2O. Reaction of C6H10O6 with sodium borohydride and water produces two compounds (compounds A and B) with the same molecular formula, C6H14O6 . Compounds A and B are oxidized by HIO4 to six moles of formic acid, HCOOH. In addition, compounds A and B react with acetic anhydride to produce C18H24O12.

Compound A Compound B Suggest a structure for the optically inactive carbohydrate C6H10O6, and determine the R/S notation at each chiral center. Also, determine the R/S notation at the chiral centers in compounds A and B. Suggest a structure for C18H24O12.

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C6H10O5

The molecule has a point of symmetry; therefore, it is optically inactive. Reduction of the molecule produces compound A and B

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C18H24O12