chapter 8 opener

Chapter 8 Opener

Carbohydrates

• General formula: ~(CH2O)n• Biological Roles

• Structural (e.g. cellulose in plants)• Molecular recognition (modification of cell

surface proteins)• Energy storage – reduced carbon (e.g. starch

in plants, glycogen in animals)• Key intermediates in central metabolism

• Facile chemistry• compared to hydrocarbons, for example• formation and cleavage of C-C bonds in

carbohydrates promoted by hydroxyl (and carbonyl) substituents

Chapter 8 Opener

Hierarchy of aldose (aldehyde-based) sugars

Text, Figure 8-1

Figure 8-1 part 1

Each new carbon adds a new stereocenter

D vs L indicates stereochemistry at the penultimate (i.e. next-to-last) carbon

For drawings of sugars, stereochemistry at each carbon is based on a Fisher projection

The red carbon is the new one added in going from the triose to the tetroses

L-Glucose

Figure 8-2

Hierarchy of ketose

(ketone-based) sugars

Note that all the carbons are chiral except those at the end, and the one attached to the carbonyl.For aldoses, that is a total of n-2, for ketoses that is n-3 (since the carbonyl is not one of the terminal carbons)

Text, Figure 8-2

Hemiacetals and hemiketals: hydroxyl attack at the carbonyl

Text, page 221

Figure 8-3

hemiacetal/hemiketal formation leads to circular form of monosaccharides by internal attack (often by

penultimate hydroxyl)

Text, Figure 8-3

Basis for nomencalture of 5 and 6-membered sugar rings (furanoses and pyranoses)

Text, page 222

Figure 8-4

Ring closure introduces a new stereocenter at what was the carbonyl carbon

The and are called anomeric forms. The new stereocenter is called the anomeric carbon.•draw the ring with the anomeric carbon at the rightmost point and the lone ring oxygen in the backward position. Then, the form has the hydroxyl on the anomeric carbon pointing up, and pointing down for . [N.B. The linear and cyclic forms can equilibrate, but the cyclic forms typically dominate]

Figure 8-5

The cyclic forms of pyranoses typically have two major alternative conformational (chair) forms

In alternative chair forms, axial groups become equatorial, and vice-versa.The dominant form is the one with the bulkiest groups in equatorial positions.

Text, Figure 8-5

Examples of oxidized sugars

Text, page 224

Examples of reduced sugars

Text, page 224

Important example of a modified (deoxy) sugar

Text, page 224

Figure 8-7

The glycosidic bond:

Transformation (forwards or backwards) generally depends on acidic conditions.Stable at neutral pH (so the monosaccharide whose anomeric carbon is involved gets trapped in the cyclic form)

Text, Figure 8-7

Example of a glycosidic bond between the sugar ribose and a nucleotide base

Text, page 221

Oligosaccharides are monosaccharides connected by glycosidic bonds

Example of a common disaccharide.Note the drawing style and notation: (14)

Text, page 227

Oligosaccharides are monosaccharides connected by glycosidic bonds

Example of a common disaccharide.Note the drawing style and notation: 12(note that in a reasonable configuration one of the rings would be flipped over)

Text, page 227

Cellulose: the major plant structural polysaccharide

A globally abundant carbon compoundCurrent efforts to degrade it efficiently (so it can be converted to various biofuels)• Numerous bacteria and termites (actually microbes in the gut) have evolved to do this

Text, Figures (various)

Amylose and amylopectin (plant starch)

Text, page 230

Amylopectin: an example of polysaccharide branching

Note that the degree or frequency of branching in glycogen, the primary form of carbohydrate storage in animals) is very high. This gives the polymeric molecule a much larger number of ‘free’ ends. This allows for more rapid degradation when monosaccharide units are required for energy production.

The structure and linkage is otherwise similar.

Figure 8-12

Various highly charged, biologically relevant polysaccharides

Text, Figure 8-12

chapter 8 opener

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