Chapter 20 Carbohydrates

Carbohydrates Carbohydrate: Carbohydrate: A polyhydroxyaldehyde or polyhydroxyketone, or a substance that gives these compounds on hydrolysis. Monosaccharide: Monosaccharide: A carbohydrate that cannot be hydrolyzed to a simpler carbohydrate. C n H 2n O n n Monosaccharides have the general formula C n H 2n O n, where n varies from 3 to 8. Aldose: Aldose: A monosaccharide containing an aldehyde group. Ketose: Ketose: A monosaccharide containing a ketone group.

Monosaccharides -ose The suffix -ose indicates that a molecule is a carbohydrate. tri-tetrapenta The prefixes tri-, tetra, penta, and so forth indicate the number of carbon atoms in the chain. aldoses. Those containing an aldehyde group are classified as aldoses. ketoses. Those containing a ketone group are classified as ketoses. There are only two trioses:

Monosaccharides ◦ Often aldo- and keto- are omitted and these compounds are referred to simply as trioses. ◦ Although “triose” does not tell the nature of the carbonyl group, it at least tells the number of carbons.

Monosaccharide Monosaccharides with ◦ three carbons: trioses ◦ Five carbons: pentose ◦ Six carbons: hexose ◦ And so on …

Monosacharides Figure 12.1 Glyceraldehyde, the simplest aldose, contains one stereocenter and exists as a pair of enantiomers.

Enantiomers Enantiomers: a molecule has a nonsuperimposable mirror image ◦ Chiral molecule – has four different groups

Monosaccharides Fischer projection: Fischer projection: A two-dimensional representation for showing the configuration of tetrahedral stereocenters. Horizontal lines represent bonds projecting forward from the stereocenter. Vertical lines represent bonds projecting to the rear. Only the stereocenter is in the plane.

Monosacharides In 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde. D-monosaccharide: D-monosaccharide: the –OH is attached to the bottom-most assymetric center (the carbon that is second from the bottom) is on the right in a Fischer projection.

Monosacharides L-monosaccharide: L-monosaccharide: the -OH is on the left in a Fischer projection.

D,L-Monosaccharides The most common D-tetroses and D-pentoses are:

D,L-Monosaccharides The three most common D-hexoses are:

Amino Sugars Amino sugars contain an -NH 2 group in place of an -OH group. Only three amino sugars are common in nature: D-glucosamine, D-mannosamine, and D-galactosamine. N-acetyl-D-glucosamine is an acetylated derivative of D-glucosamine.

Cyclic Structure hemiacetals Aldehydes and ketones react with alcohols to form hemiacetals Cyclic hemiacetals form readily when the hydroxyl and carbonyl groups are part of the same molecule and their interaction can form a five- or six-membered ring.

Epimers Diastereomers that differ in configuration at only on asymmetric center

Table 20-1 p532

Table 20-2 p532

Examples Draw Fisher projections for all 2-ketopentoses. Which are D- 2-ketopentoses, which are L-2-ketopentoses? Prefer to table 12.2 (your textbook) to write their names

Haworth Projections Figure 12.2 D-Glucose forms these two cyclic hemiacetals.

Haworth Projections A five- or six-membered cyclic hemiacetal is represented as a planar ring, lying roughly perpendicular to the plane of the paper. Groups bonded to the carbons of the ring then lie either above or below the plane of the ring. anomeric carbon The new carbon stereocenter created in forming the cyclic structure is called the anomeric carbon. anomers Stereoisomers that differ in configuration only at the anomeric carbon are called anomers. The anomeric carbon of an aldose is C-1; that of the most common ketose is C-2.

Haworth Projections In the terminology of carbohydrate chemistry, ◦ ◦ means that the -OH on the anomeric carbon is on the same side of the ring as the terminal -CH 2 OH. ◦ ◦ means that the -OH on the anomeric carbon is on the side of the ring opposite from the terminal -CH 2 OH. pyranose furanose ◦ A six-membered hemiacetal ring is called a pyranose, and a five- membered hemiacetal ring is called a furanose because these ring sizes correspond to the heterocyclic compounds furan and pyran.

Haworth Projections ◦ Aldopentoses also form cyclic hemiacetals. ◦ The most prevalent forms of D-ribose and other pentoses in the biological world are furanoses. deoxy ◦ The prefix “deoxy” means “without oxygen.” at C2

Haworth Projections D-Fructose (a 2-ketohexose) also forms a five-membered cyclic hemiacetal.

Examples Give structure of the cyclic hemiacetal formed by ◦ 4-hydroxybutanal ◦ 5-hydroxypentanal

Chair Conformations strain-free chair conformation For pyranoses, the six-membered ring is more accurately represented as a strain-free chair conformation.

Chair Conformations In both Haworth projections and chair conformations, the orientations of groups on carbons 1- 5 of  -D-glucopyranose are up, down, up, down, and up.

Chair Conformations

Examples Which OH groups are in the axial position in β-D-mannopyranose β-D-idopyranose

Mutarotation Mutarotation: Mutarotation: The change in specific rotation that accompanies the equilibration of  - and  -anomers in aqueous solution. ◦ Example: When either  -D-glucose or  -D-glucose is dissolved in water, the specific rotation of the solution gradually changes to an equilibrium value of +52.7°, which corresponds to 64% beta and 36% alpha forms.

Formation of Glycosides Treatment of a monosaccharide, all of which exist almost exclusively in cyclic hemiacetal forms, with an alcohol gives an acetal.

Formation of Glycosides glycoside A cyclic acetal derived from a monosaccharide is called a glycoside. glycosidic bond The bond from the anomeric carbon to the -OR group is called a glycosidic bond. Mutarotation is not possible for a glycoside because an acetal, unlike a hemiacetal, is not in equilibrium with the open-chain carbonyl-containing compound.

Formation of Glycosides Glycosides are stable in water and aqueous base, but like other acetals, are hydrolyzed in aqueous acid to an alcohol and a monosaccharide. e- ide Glycosides are named by listing the alkyl or aryl group bonded to oxygen followed by the name of the carbohydrate in which the ending -e is replaced by - ide.

Examples Draw a Haworth projection and a chair conformation for methyl -D-mannopyranoside. Label the anomeric carbon and glycosidic bond