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Chapter 8 Opener

Carbohydrates. General formula: ~(CH 2 O)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

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Chapter 8 Opener

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  1. 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

  2. Hierarchy of aldose (aldehyde-based) sugars Chapter 8 Opener

  3. Text, Figure 8-1

  4. Each new carbon adds a new stereocenter 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 D vs L indicates stereochemistry at the penultimate (i.e. next-to-last) carbon Figure 8-1 part 1

  5. L-Glucose Page 221

  6. 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 Figure 8-2

  7. Hemiacetals and hemiketals: hydroxyl attack at the carbonyl Text, page 221 Page 221

  8. hemiacetal/hemiketal formation leads to circular form of monosaccharides by internal attack (often by penultimate hydroxyl) Text, Figure 8-3 Figure 8-3

  9. Basis for nomencalture of 5 and 6-membered sugar rings (furanoses and pyranoses) Text, page 222 Page 222

  10. Ring closure introduces a new stereocenter at what was the carbonyl carbon • The a and b 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 b form has the hydroxyl on the anomeric carbon pointing up, and pointing down for a. [N.B. The linear and cyclic forms can equilibrate, but the cyclic forms typically dominate] Figure 8-4

  11. The cyclic forms of pyranoses typically have two major alternative conformational (chair) forms Text, Figure 8-5 In alternative chair forms, axial groups become equatorial, and vice-versa. The dominant form is the one with the bulkiest groups in equatorial positions. Figure 8-5

  12. Examples of oxidized sugars Text, page 224 Page 224

  13. Examples of reduced sugars Text, page 224 Page 224

  14. Important example of a modified (deoxy) sugar Text, page 224 Page 224

  15. The glycosidic bond: Text, Figure 8-7 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) Figure 8-7

  16. Example of a glycosidic bond between the sugar ribose and a nucleotide base Text, page 221 Page 226

  17. Oligosaccharides are monosaccharides connected by glycosidic bonds Text, page 227 Example of a common disaccharide. Note the drawing style and notation: b(14) Page 227

  18. Oligosaccharides are monosaccharides connected by glycosidic bonds Text, page 227 Example of a common disaccharide. Note the drawing style and notation: a1b2 (note that in a reasonable configuration one of the rings would be flipped over) Page 227

  19. Cellulose: the major plant structural polysaccharide Text, Figures (various) • A globally abundant carbon compound • Current 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 Page 229

  20. Amylose and amylopectin (plant starch) Text, page 230 Page 230

  21. 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. Page 231

  22. Various highly charged, biologically relevant polysaccharides Text, Figure 8-12 Figure 8-12

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