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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