Chapter 22

Carbohydrates

Chapter 22 suggested problems: none

Class Notes

  1. An introduction to carbohydrates

    1. The single most abundant class of organic compounds

      1. Metabolism and energy sources (glucose, glycogen)

      2. Structure and coverings (cellulose, chitin)

      3. Precursors in the formation of other essential substances

      4. Various other biological functions: cellular recognition, etc.

      5. That there is more carbohydrate matter in the world (on a mass basis) than all the other organic matter combined is attributable to the ubiquity of glucose in starch and in cellulose (biomass, 1015 kg/yr)

    2. Polyfunctional molecules

      1. Either polyhydroxyaldehydes or polyhydroxyketones

      2. Most naturally occurring monosaccharides do not have free carbonyl groups but exist instead as polyhydroxy acetals or ketals

    3. General categorizations of carbohydrates

      1. Monosaccharides

      2. Disaccharides

      3. Oligosaccharides: as many as 10-20 monosaccharides, but often fewer

      4. Polysaccharides: generally consist of a single or two alternating monosaccharides

    4. Nomenclature: most carbohydrates end in "ose"

  2. Monosaccharides

    1. General

      1. Empirical formula (CH2O)n

      2. 3 - 7 carbon atoms in backbone

      3. The carbon backbone is unbranched

      4. All of the carbons but the carbonyl carbon are bonded to a hydroxyl group

      5. White crystalline solids at room temperature, (relatively) high M.P., highly soluble in water, insoluble in nonpolar solvents, sweet to taste

    2. Categorization based on on the nature of the carbonyl group and on numbers of carbons in backbone

      1. Aldoses and ketoses

      2. Trioses, tetroses, pentoses, hexoses, heptoses

      3. Group names combine carbonyl name then backbone number: ketopentoses, aldohexoses, ketohexoses, etc.

    3. Stereoisomerism

      1. There are 2n possible stereoisomers in a compound with n tetrahedral stereocenters

      2. All of the monosaccharides except the smallest ketotriose (dihydroxyacetone) have at least one chiral carbon

      3. D- and L-glyceraldehyde are the reference compounds for assigning the absolute configuration of all optically active compounds

      4. For monosaccharides having two or more chiral carbons, D- and L- assignments are based on the chiral carbon located furthest away from the carbonyl carbon

      5. Nearly (but not all, e.g. L-fucose, L-rhamnose, L-sorbose) all biologically important monosaccharides are of D- configuration; L- forms are possible but generally not synthesized

      6. Epimers: stereoisomers with multiple chiral carbons that vary only in the configuration around one of the chiral carbons

    4. Aldoses
      D-aldoses

    5. Ketoses

      1. D-ketoses

      2. The common names of some ketoses were derived from adding an "ul" to the name of the corresponding aldose e.g., ribose and ribulose

    6. Deoxy sugars - replace one or more hydroxyl groups with a hydrogen atom, e.g., ribose and 2-deoxyribose

  3. Cyclic hemiacetal structures

    1. Furan and pyran
      furan and pyran

    2. Only 0.2% of the monosaccharides in aqueous solution are of the open chain form, i.e., most naturally occurring monosaccharides do not have free carbonyl groups

    3. Aldohexoses (actually aldoses with five or more carbons) tend to form pyran-like structures - pyranoses - by forming cyclic hemiacetals through the reaction of the carbonyl group and the hydroxyl group on one of the backbone carbons (usually C-5)

      1. Draw the acyclic structure and then rotate it 90° clockwise

      2. Rotate around the C-4 - C-5 bond to orient the hydroxyl group near the carbonyl oxygen

      3. Form the cyclic hemiacetal

      4. Systematic names reflect conversion from acyclic to cyclic structures: glucose to glucopyranose

      5. This process results in formation of an additional chiral carbon, C-1 and the potential for two ring structures - alpha (a-, below the ring plane) and beta (b-, above the ring plane)

      6. Anomers: isomeric forms of monosaccharides that differ only in their configuration around the carbonyl carbon

      7. The carbonyl carbon is referred to as the anomeric carbon

      8. All aldoses (and ketoses) with five or more carbons form stable pyranose rings and can exist as various anomers

    4. Ketoses with five or more carbons tend to form furan-like structures - furanoses - by forming cyclic hemiacetals (hemiketals) through the reaction of the carbonyl group and the hydroxyl group on one of the backbone carbons (usually C-5)

      1. Draw the acyclic structure and then rotate it 90° clockwise

      2. Rotate around the C-4 - C-5 bond to orient the hydroxyl group near the carbonyl oxygen

      3. Form the cyclic hemiacetal

      4. Systematic names reflect conversion from acyclic to cyclic structures: fructose to fructofuranose
        conversion of fructose to fructofuranose

      5. This process results in formation of an additional chiral carbon, C-1 and the potential for two ring structures - alpha (a-, below the ring plane) and beta (b-, above the ring plane)

      6. Aldohexoses may also exist as furanoses but since the pyranose form is more stable it is the predominant form in aqueous aldohexose solutions

    5. Haworth projections (Haworth structures) - used to convey the 3-dimensionality of cyclic hemiacetals

  4. Chemical and physical properties of monosaccharides

    1. Mutarotation

      1. Pure a-D-glucose and b-D-glucose have different physical and chemical properties
        Property
        a-D-glucose
        b-D-glucose
        specific rotation
        +112.2°
        +18.7°
        melting point (°C)
        146
        150
        solubility in water, g per 100 mL
        82.3
        178
        relative rate of oxidation by glucose oxidase
        100
         <1

      2. Pure a-D-glucose and b-D-glucose can be isolated, although they exist in a 36/64 ratio (a/b) in nature, which results in the solution having an optical rotation of +52.7°

      3. If one of the pure anomers is dissolved in water, the optical rotation of the solution changes until the value of +52.7° is reached

      4. The change is called mutarotation as is the result of an equilibrium between the a- and b- anomers

      5. Interconversion between anomers takes place through the straight-chain forms

      6. This is relevant because it contributed to the understanding of monosaccharides existing as cyclic rather than straight-chain compounds

    2. Acetal formation and the production of glycosides

      1. The acid-catalyzed conversion of hemiacetals (hemiketals) to acetals (ketals) occurs in the presence of any alcohol

        1. This means that monsaccharide hemiacetals can be methylated, ethylated, etc.

      2. The anomeric carbon loses its hydroxyl group, the alcohol loses its hydroxyl proton

      3. The ether linkage between between the anomeric carbon and the alkoxy group is called an O-glycosidic bond (as compared to a N-glycosidic bond)

      4. The bond will be an a- or b-glycosidic linkage depending on the configurations of the anomeric carbon involved in the bond

        1. These bonds can be referred to briefly as (e.g.) b(1->4) glycosidic linkages, which means that a b-anomer shares a glycosidic linkage between its C-1 and the C-4 of another anomer which may be either a- or b-

        2. a- linked polysaccharides are digestible to humans while b- linked are not

        3. Example

          1. maltose

      5. The bonds between the monosaccharides in disaccharides and polysaccharides are glycosidic bonds

      6. Can also form N-glycosidic bonds between anomeric carbons and the nitrogen atoms of amines

  5. Some important monosaccharides

    1. Glucose: "the most important simple carbohydrate in human metabolism" (M&C: 642); a component of the disaccharide sucrose

    2. Galactose: commonly found in plant gums and resins, a component of the disaccharide lactose (milk sugar)

    3. Fructose: found in honey and many different fruits; also known as fruit sugar

    4. Ribose and 2-deoxyribose: aldopentoses found in nucleic acids, etc.

  6. Disaccharides

    1. Maltose: two glucose molecules, a(1->4) glycosidic linkage: (glucose-a(1->4)-glucose)

    2. Cellobiose: two glucose molecules, b(1->4) glycosidic linkage: (glucose-b(1->4)-glucose)

    3. Lactose: galactose and glucose, b(1->4) glycosidic linkage: (galactose-b(1->4)-glucose)

    4. Sucrose: between the hemiacetal groups of a-D-glucose and a-D-fructose: (glucose-a(1->2)-fructose)

      1. This is an a,b(1->2) linkage

      2. It is a-glycosidic w.r.t. glucose

      3. It is b-glycosidic w.r.t. fructose

  7. Polysaccharides

    1. Starch and glycogen: based on glucose

      1. Most starches are 10-30% amylose and 70-90% amylopectin

      2. Amylose is linear and unbranched with a backbone of a(1->4) glycosidic linkages

        1. Chains may range in mass from a few thousand to 500,000 amu

        2. Not truly soluble in water, forms hydrated micelles in which the polysaccharide chain is twisted into a helical coil of maltose units

      3. Amylopectin is linear with a backbone of a(1->4) glycosidic linkages and highly branched with a(1->6) glycosidic linkages

        1. Average branch length is 24-36 glucose residues (12-18 maltose residues)

        2. Not very soluble in water

        3. Molecular weights as high as 100 million

        4. Forms either micelles or colloids

      4. Glycogen is similar to amylopectin but more extensively branched

        1. Branches occur about every 8-12 glucose residues

        2. This results in a more highly branched, compact molecule than amylopectin

        3. Molecular weights up to several million

        4. Up to 10% wet weight of the liver is glycogen; up to 1-2% wet weight of muscle cells

    2. Cellulose is based on glucose and is linear and unbranched with b(1->4) glycosidic linkages; a single molecule may contain from 300 to 15,000 glucose residues (molecular weight of 50,000 to 2.5 million)
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