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Chemistry 263
Carbohydrates
Chapter 22 in Solomons and Fryhlie, 11th edition
Carbohydrates
In the beginning…
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2 H2O + 2 NADP+ + 3 ADP + 3 Pi + light → 2 NADPH + 2 H+ + 3 ATP + O2
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Electron transport in plants bears a remarkable similarity to oxidative phosphorylation
in mitochondria
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So what happens to this energy trapped as ATP and NADPH? Enter the dark side
of the Calvin Cycle1
One of the molecules of 3-Phosphoglycerate is then used for synthesis of needed
molecules such as glucose and on to cellulose (look at glycolysis in reverse below)
1 Actually, light independent is better since the enzymes involved are activated in the light. The
Calvin cycle is more fully known as the Calvin-Benson-Bassham (CBB) after its discoverors Melvin
Calvin, Andrew Benson, and James Bassham of UC Berkeley
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Below is shown the Fischer projection for Ribulose-1,5-bisphosphate. How does the
formation of an enolate allow CO2 to bond and ultimately give rise to 2 molecules of 3-
phosphoglycerate on hydrolysis? Hint: If you count the oxygens in ribulose-1,5-
bisphosphate + CO2 and compare to the number of oxygens in 2 3-phosphoglycerate,
you will see the need for 1 more oxygen. Now what do you suppose could provide one
oxygen?
3-PhosphoglycerateRibulose-1,5-bisphosphate
Interestingly, 3-phosphoglycerate is a central intermediate in glycolysis as well
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So why do we say energy is trapped in ATP? For the same reason we consider acid
anhydride to be a reactive species – the presence of an acid anhydride linkage
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22.1, 22.2: Structural Formulas for Monosaccharides & Mutarotation
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22.4: Glycoside Formation
Mechanistically
Glycoside Formation
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Glycoside Hydrolysis
Mechanistically
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22.5: Further Reactions of Carbohydrates
Enolization, Tautomerization, and Isomerization
Carbohydrate Protecting Groups
Benzyl Ether Formation
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Benzyl Ether Cleavage
Exhaustive Methylation with Dimethyl Sulfate
Regioselective Silyl Ether Formation
Regioselective Silyl Ether Cleavage
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[Acetate] Ester Formation
Formation of Cyclic Acetals
Question: How would you avoid forming the usual acetal between monosaccharides?
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22.6: Oxidation Reactions of Monosaccharides
Reducing Sugars Revisited
Benedict’s Reagent
Question: What’s wrong with this picture where ketoses are concerned?
Question: Tollen’s test uses [Ag(NH3)2]+ and generates a silver mirror as a positive
test for aldehydes and is used to distinguish between aldehydes and ketones.
However, Tollen’s test gives the same result as Benedict’s reagent where reducing
sugars are concerned. What’s up with that?
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Aldonic Acids From Monosaccharides & Bromine Water
The mildly acidic conditions of the bromine water avoid complicating reactions.
However, things are a tad more complicated in that oxidation is selective for the
anomer, which can subsequently hydrolyse and reform a 5-membered lactone
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Aldaric Acids From Monosaccharides & HNO3
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22.7: Reduction Reactions of Monosaccharides – Alditols
22.8: Reactions of Monosaccharides with Phenylhydrazine - Osazones
Mechanistically
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22.8: Synthesis and Degradation of Monosaccharides
Kiliani-Fischer Synthesis
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Ruff Degradation
22.10: The “D” Family of Aldoses
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22.11: Fischer’s Proof of the Structure of D-(+)-Glucose
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22.12: Disaccharides
Sucrose
Maltose
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So how do we know how monosaccharides are linked? Well, we do have to rely on
and glycosidic enzymes to clue us in to the nature of the glycosidic linkage, but
we can also employ a little organic chemistry fancy footwork to make our
determination. Using maltose as an example
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Lactose
Cellobiose
Interestingly, lactose can be broken down by the -glycosidic enzyme lactase; an enzyme
that is feedback regulated (almost a pun there) and diminishes as a child matures.
Cultures that have a tradition of using milk as a major source of calories have evolved the
capacity to maintain lactase through adulthood. Europeans, West Asians, India, and
parts of East Africa are among those who tolerate lactose in the adult population > 70 %.
Those who’s genetic roots trace to other parts of Africa, Eastern and Southeast Asia,
the native peoples of Australia, and New Zealand, and Pacific Islanders have tolerance
rates < 30 %. Further, cellobiose is not digestible but is being mass produced
enzymatically by Nippon Paper Chemicals, Ltd. in part to help livestock gain weight.
Apparently, as a non-digestible disaccharide cellobiose helps promote increased
populations of beneficial hydrolytic gut flora consequently inproving digestion of foods in
the lower intestine. These bacteria also compete against bacteria that are potentially
harmful (such clostridium difficile which may take over during clindamycin treatment). As
such these bacteria are known as probiotics, actually being beneficial to the health of the
organism. Is it possible lactose intolerant individuals are improving the composition of
their gut flora by drinking milk products? I think most would just rather opt for Lactaid®
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22.13: Polysaccharides
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Cotton vs. Rayon