Bettelheim, Brown, Campbell and Farrell Chapter 20 Carbohydrates Bettelheim, Brown, Campbell and Farrell Chapter 20

Carbohydrates Carbohydrate: polyhydroxyaldehyde or polyhydroxyketone, or a substance that can be hydrolyzed to form these compounds Monosaccharide: a carbohydrate that cannot be hydrolyzed to a simpler carbohydrate (simple sugar)

Monosaccharide: Monosaccharides have the general formula CnH2nOn, where n varies from 3 to 8 aldose: a monosaccharide containing an aldehyde group ketose: a monosaccharide containing a ketone group

Monosaccharides Monosaccharides are classified by their number of carbon atoms “ose” ending for sugars

Monosaccharides There are only two trioses Often simply call these trioses Tells the number of carbons

Monosaccharides Glyceraldehyde, the simplest aldose, contains a stereocenter and exists as a pair of enantiomers

Monosaccharides Fischer projection: a two dimensional representation for showing the configuration of tetrahedral stereocenters Horizontal lines represent bonds projecting forward Vertical lines represent bonds projecting to the rear

D,L Monosaccharides In 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde D-monosaccharide: the -OH on its penultimate (next to last) carbon is on the right L-monosaccharide: the -OH on its penultimate (next to last) carbon is on the left

D,L Monosaccharides the most common D-tetroses and D-pentoses the three common D-hexoses

Review: Addition of Alcohols to Carbonyls Addition of an alcohol to the carbonyl group of an aldehyde or ketone forms a hemiacetal (a half-acetal) Functional group of a hemiacetal is a carbon bonded to one -OH group and one -OR group H of the alcohol adds to the carbonyl oxygen and -OR adds to the carbonyl carbon

Addition of Alcohols to Carbonyl Hemiacetals are generally unstable and are only minor components of most equilibrium mixtures Major exception: When both the carbonyl group and the hydroxyl group are in the same molecule and can form a cyclic hemiacetal with a 5- or 6-member ring Cyclic hemiacetals predominate

Cyclic Structure 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

Haworth Projections D-Glucose forms these cyclic hemiacetals

Carbohydrate Rings -OH group can react with C=O group to give hemiacetal Fischer Fischer ring Haworth

Haworth Projections 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 New carbon stereocenter created in forming the cyclic structure is called an anomeric carbon Stereoisomers that differ in configuration only at the anomeric carbon are called anomers Anomeric carbon of an aldose is C-1; that of the most common ketoses is C-2

Haworth Projections In the terminology of carbohydrate chemistry, b form: -OH on the anomeric carbon is on the same side of the ring as the terminal -CH2OH (up) a form: -OH on the anomeric carbon is on the side of the ring opposite from the terminal -CH2OH (down) Pyranose: 6-member ring sugar containing O Furanose: 5-member ring sugar containing O

Haworth Projections aldopentoses also form cyclic hemiacetals the most prevalent forms of D-ribose and other pentoses in the biological world are furanoses

Haworth Projections D-fructose also forms a five-membered cyclic hemiacetal

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

Chair Conformations in both a Haworth projection and a chair conformation, the orientations of groups on carbons 1- 5 of b-D-glucopyranose are up, down, up, down, and up

Mutarotation Mutarotation: a- and b-anomers can have the ring open and then form the other anomer. A change in specific optical rotation accompanies the equilibration of in aqueous solution example: when either a-D-glucose or b-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

Fig 19.2, p.472

Mutarotation

Mutarotation Rings can open to form open chain compound. New ring can be either - or b-anomer. Equilibrium among - and b- forms (and open chain) will be established. Change in specific optical rotation accompanies this equilibration If either a-D-glucose or b-D-glucose is dissolved in water, specific rotation changes to an equilibrium value of +52.7° 64% beta and 36% alpha forms at equilibrium Very little will exist in open chain form

Another representation of ring opening and closure Fig. 17.6 Another representation of ring opening and closure

Fig 19.2, p.472

Mutarotation Shown in chair conformation

Physical Properties Monosaccharides Colorless crystalline solids Very soluble in water Only slightly soluble in ethanol Sweet taste

Sweetness relative to Sucrose

Formation of Glycosides Reaction of a hemiacetal with alcohol gives an acetal Acetals Hemiacetal

Formation of Glycosides Cyclic acetal derived from a monosaccharide is called a glycoside Bond from the anomeric carbon to the -OR group is called a glycosidic bond Mutarotation is not possible in a glycoside Ring can’t open up No equilibrium with the open-chain form Can be hydrolyzed in acidic solution to form alcohol and a monosaccharide

Oxidation and Reduction of simple sugars Aldehyde and ketone groups can be reduced to form alcohols Requires reducing agents such as NaBH4 or H2 with a transition metal catalyst Aldehyde group can be oxidized to produce a carboxylic acid

Reduction to Alditols The carbonyl group of a monosaccharide can be reduced to an hydroxyl group by a variety of reducing agents, including NaBH4 and H2 in the presence of a transition metal catalyst the reduction product is called an alditol

Reduction to Alditols sorbitol is found in the plant world in many berries and in cherries, plums, pears, apples, seaweed, and algae it is about 60 percent as sweet as sucrose (table sugar) and is used in the manufacture of candies and as a sugar substitute for diabetics Other common alditols

Oxidation to Aldonic Acids the aldehyde group of an aldose is oxidized under basic conditions to a carboxylate anion the oxidation product is called an aldonic acid any carbohydrate that reacts with an oxidizing agent to form an aldonic acid is classified as a reducing sugar (it reduces the oxidizing agent)

Common Disaccharides Sucrose (table sugar) sucrose is the most abundant disaccharide in the biological world; it is obtained principally from the juice of sugar cane and sugar beets sucrose is a nonreducing sugar (ring can’t open)

Fig. 17.10

Common Disaccharides Lactose Principal sugar present in milk; it makes up about 5 to 8 percent of human milk and 4 to 6 percent of cow's milk it consists of D-galactose bonded by a b-1,4-glycosidic bond to carbon 4 of D-glucose lactose is a reducing sugar

Common Disaccharides Maltose present in malt, the juice from sprouted barley and other cereal grains maltose consists of two glucose units joined by an a-1,4-glycosidic bond maltose is a reducing sugar

Polysaccharides Polysaccharide: Polymer containing many monosaccharide units Starch: a polymer of D-glucose Two forms: amylose and amylopectin Amylose is composed of unbranched chains of up to 4000 D-glucose units joined by a-1,4 bonds Amylopectin contains chains up to 10,000 D-glucose units also joined by a-1,4-glycosidic bonds Branched compound New chains of 24 to 30 units attached by a-1,6 linkages

Polysaccharides Glycogen is the energy-reserve carbohydrate for animals Glycogen is a branched polysaccharide of approximately 106 glucose units joined by a-1,4- and a-1,6-glycosidic bonds Total amount of glycogen in the body of a well-nourished adult human is about 350 g, divided almost equally between liver and muscle

Amylose

Branched

Polysaccharides Cellulose is a chain of D-glucose units joined by b-1,4-glycosidic bonds Average chain has approximately 2200 glucose units per molecule cellulose chain is much stiffer than starch Chains line up side by side into well-organized water-insoluble fibers in which the OH groups form numerous intermolecular hydrogen bonds Arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength Cellulose is insoluble in water

Polysaccharides Cellulose (cont’d) humans and other animals cannot digest cellulose Can only digest starch and glycogen to get glucose Many bacteria and microorganisms can digest cellulose Termites have such bacteria in their intestines and can use wood as their principal food Ruminants (cud-chewing animals) and horses can also digest grasses and hay

Cellulose

Blood Groups Several sugar groups attached to red blood cell (RBC) surface Difference in one sugar accounts for Types A, B, AB and O blood.

Sugars attached to RBC (α 1,4) β (1,3) X –-------- D-Galactose-----------N-Acetyl-D- RBC | glucosamine | (α 1,2) | L-fucose

Blood Groups -- Type A N-Acetyl-D-Galactosamine on RBC surface

Blood Groups -- Type B galactose on RBC surface

Blood Groups -- Type O Neither N-Acetyl-D-galactosamine nor Galactose attached to rest of sugar groups on RBC surface