Chapter 24: Carbohydrates
Photosynthesis: : Energy storage Sun, chlorophyll 6 CO 2 + 6 2 O C 6 ( 2 O) 6 + 6 O 2 Metabolism Glucose
Land plants Universal Chlorophyll 3% to 6% of the total incident solar radiation is used for chemical conversion of 2 O (oxidation to O 2 ) and CO 2 (reduction to glucose).
Carbohydrate = C n ( 2 O) n ydrated carbon Carbohydrates occur in nature in nucleic acids, fats, cellulose, fibers, starch, table sugar, antibiotics, and other biological molecules. A pentahydroxyaldehyde
Naming The simplest carbohydrates are the sugars or saccharides.. They constitute polyhydroxy- aldehydes (aldoses)) or -ketones (ketoses); they form oligomers by ether bridges (hence di-, tri-, tetrasaccharide,, etc.). The simplest sugars, both C 3 ( 2 O) 3 : Chiral 2,3-Dihydroxypropanal (Glyceraldehyde) An aldotriose 1,3-Dihydroxyacetone A ketotriose Chain length: Triose ose, tetrose ose, pentose, etc.
Some important monosaccharides saccharides: * * * 4 Stereo- centers * * 3 Stereo- centers * * * * * Dextrose, blood sugar, grape sugar Sweetest natural sugar; fruits Ribonucleic acids Fischer projections: Review chapter 5
Fischer Projection: A flat stencil Br Br C 3 C C 2 C 3 Br Eyes in the plane of the board Rules reminder: 180 º turn or double exchange leaves stereochemistry intact C 3 C 2 C 3 Depending on your starting dashed-wedged line structure, several Fischer projections are possible for the same molecule.
Most sugars are chiral and occur enantiomerically pure. Simplest case, one stereocenter: D and L is an older nomenclature (predates the knowledge of the absolute configuration of glyceraldehyde). The dextrorotatory enantiomer was called D, the other L. Later, D was found to be R,, L therefore S. Almost all natural sugars have stereocenter furthest away from carbonyl (drawn at the bottom) with the same absolute configuration as D-glyceraldehyde: D-sugars
Rules for arranging the Fischer stencil: Carbonyl on top, places bottom C*O on the right in the D sugars. Natural Unnatural D-Erythrose L-Erythrose D-Threose L-Threose
The Family of Natural Aldoses
The Family of Natural Ketoses
Fischer Projections and Dashed-Wedged Line Structures Recall: Fischer projections represent an unrealistic model of the molecule: All eclipsed; ; carbon chain curves curves. It s usefulness is in stereochemical bookkeeping. D-Glucose
Five- membered ring Six- membered ring Two diastereomers: Anomers Two diastereomers: Anomers
aworth Structures Groups that lie on the right in the Fischer projection point downward in the aworth projection. At the anomeric carbon: O down is called the α-anomer,, O up is called the β-anomer.
Other ways of drawing cyclic structures: Not a carbon atom
Best are conformational pictures: Anomeric carbon Anomeric carbon O down: α-anomer; O up: β-anomer; crystallizes more stable because all-equatorial Mutarotation: : Change in observed optical rotation when a sugar molecule equilibrates with its anomer.
Reactions of Sugars 1. Oxidation a. CO COO (aldose( aldonic acid) b. α-ydroxy group of ketoses α-dicarbonyl Tests for reducing sugars; all ordinary saccharides are reducing.
Large scale preparation of aldonic acids: Br 2, 2 O Dehydration gives lactones, typically five-membered (γ-( lactones). Mechanism of bromine oxidation: O RC 2 O, + R C O Br 2 Br Br R C O : : O O RCO O O
c. Oxidation of both ends of aldoses aldaric acid Note selectivity of nitric acid: Picks on primary O function (after oxidizing the formyl group): Less hindered. Mechanism: O RC 2 O, O-NO 2 O C R O N O O: :: O RCO + O-NO Nitrous acid
For some sugars, this oxidation may give meso (achiral) aldaric acid. Can be used for proof of stereochemistry. CO COO O O O O NO 3 O O Mirror plane O O C 2 O COO D-(+) (+)-Allose Allaric acid (meso( meso) Symmetry becomes obvious also in NMR, e.g. 13 C NMR: 6 peaks 3 peaks
d. Oxidative cleavage: IO 4 This reagent causes the rupture of vicinal diols to dialdehydes. ow?
Does this ring a bell? Remember OsO 4 oxidation of alkenes to vicinal diols: : Cleaves half a double bond. IO 4 does the same to a single bond (bearing Os). Similarly: Remember benzylic oxidation of alkylbenzenes to benzenecarboxylic acids by basic KMnO 4. O O Mn O :O: : K
C-C C bond broken via: O O O KMnO 4 does both the OsO 4 -type and IO 4 -type oxidations sequentially in the same reaction step.
ow does this work for sugars? Leads to complete degradation of the carbon chain. CO O O O C O O IO 4 O C O O I O O O O C O O O O O O Note: Each carbon fragment retains the same number of attached hydrogen atoms as were present in the original sugar.
Another way to think about this is as a dihydroxylative dihydroxylative cleavage of each chain C-C C C bond, e.g. fructose: O O O O O O O O O O Add Os to each side of the bond broken Note: Each carbon fragment retains the same number of attached hydrogen atoms as were present in the original sugar.
2. Reduction to alditols Note: Just as in the oxidation to aldaric acids, reduction may symmetrize the sugar. Sorbitol ( sugar alcohol ) is used as artificial sweetener in diet foods: 2.6 cal/g per versus 4 cal/g for normal sugar. Sorbitol also occurs naturally in many stone fruits.
3. Esters and Ethers: Protection emiacetal Acetal Acetal function can be deprotected selectively
Mild, does not touch normal ether Alternative exploitation of the special reactivity of the (hemi)acetal( function: Turn it into an acetal,, called glycoside for sugars. Protection of anomeric carbon no mutarotation,, no aldehyde oxidation (i.e. does not behave as reducing sugar), no reduction.
Protection as cyclic acetals Recall: -C 2 O often not engaged: Flexibility makes entropy of acetal formation worse O O 3 C C 3 O O O O O O, + 3 C O 3 C O O O O β-d-altrose β-d-altrose bisacetonide
4. Kiliani-Fischer Extension (Modified) einrich Kiliani 1855-1945 Lindlar type Emil Fischer 1852-1919
Example: O O CO O C 2 O 1. CN 2. 2, Pd-BaSO 4, 2 O, + O O O CO O C 2 O + O O CO O O C 2 O D-Lyxose D-Talose D-Galactose
5. Ruff Degradation Note: Both diastereomers (R or S at the top stereocenter) degrade to the same lower sugar. Fe 3+ Fe Fe 2+ Fe 3+ Fe 2+ 3+ Fe
Structure Determination of Sugars- The Fischer Proof Logical combination of 1. Sugar extension 2 diastereomers 2. Sugar degradation: 2 Diastereomers give same sugar) 3. Sugar symmetrization via aldaric acids or alditols 4. Recognition of intricate stereochemical relationships
A glimpse at the logic: Achiral Oxn.. or Redn. Ext. Ext. Oxn.. or Redn. Chiral Etc. Etc. Oxn. or Redn. Oxn. or Redn. Same compound
CO C 2 O C 2 O CO O O O O O O O O O O O O C 2 O C 2 O C 2 O C 2 O D-Arabinose Same! D-Lyxose CO COO COO CO O O O O O O O O O O O O O O O O C 2 O COO COO C 2 O D-Glucaric acid Enantiomers D-Gularic acid
D-Ketoses correlate with alditols by reduction E.g., Alditols of erythrose and threose Again, potential symmetrization helps in structural assignment Alditols of glucose and mannose
Di- and igher Saccharides Sucrose: Disaccharide derived from glucose and fructose Table sugar 150 lb/ person/ year Ether bridge between respective anomeric centers: Nonreducing
[α] D = +66.5 [α] D = -20
Lactose (milk sugar) Ether bridge between anomeric C and C 4 α β C 4 Reducing
Cellulose: Sugar Polymer Molecular weight 500,000 (~3000 units) 1 unit = 178 molecular weight. Used in cell wall material: Rigid structure due to multiple hydrogen bonds.
Nitrocellulose: Explosive Major component of smokeless gunpowder (guncotton); used in photographic film. Christian F. Schönbein 1799-1868 enri Braconnot: 1832: NO 3 + starch Théophile-Jules Pelouze: 1838: NO 3 + paper
Starch: Polyglucose with α acetal links Constitutes fuel reserve in plants: Corn, potatoes, wheat (bread), rice. ydrolyzes to glucose (sweat taste of bread in mouth). Two major components: Amylose
Amylopectin (has branches)
uman Fuel Tank: Glycogen Provides immediate glucose during strain MW = 100 million
Sugars as water solubilizing groups in nature: Anthracycline anticancer agents and antibiotics Anthracycline intercalates into DNA: Kills (tumor) cells Streptomycin binds to the ribosome, causes misreading of genetic information