Biomolecules Survey Part 1: Carbohydrates Lecture Supplement page 76

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Biomolecules Survey Part 1CarbohydratesLecture
Supplement page 76
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Why Should I Study This?

• Why is this topic important?
• All organisms utilize carbohydrates ? important
  biomolecules
• Nutrition carbos are more than just starch and
  sugar
• Application of previous concepts

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Origin of Carbohydrate
Before 1900
Glucose C6H12O6
no change
Sucrose C12H22O11
glucose fructose
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Origin of Carbohydrate
Sugar general formula CnH2nOn
Cn(H2O)n
carbon hydrate
carbohydrate
Confirmation
sucrose H2SO4
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Monosaccharide Molecular Structure
All common/ important monosaccharides have...

• Chain of three to six carbons

C3 triose C4 tetrose C5 pentose C6 hexose
an aldohexose

• One aldehyde or ketone

aldehyde aldose ketone ketose Aldoses more
common than ketoses

• All other carbons are alcohols
• H-C-OH or CH2OH

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The (D)-Aldose FamilyThe (D)-Aldotrioses
One stereocenter ? two enantiomers
(L)-(-)-glyceraldehyde
(D)-()-glyceraldehyde

• Stereochemical configuration
• D OH above CH2OH on the right
• Configuration of natural aldoses
• L-aldoses generally unimportant
• No correlation of /- and D/L

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The (D)-Aldose FamilyFischer Projections

• Determined relative structure of (D)-aldoses
• Nobel Prize in Chemistry 1902
• (D)-glyceraldehyde R configuration (x-ray
  crystallography 1950)

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The (D)-Aldose FamilyThe (D)-Aldotetroses
Two stereocenters ? four stereoisomers
Two (D) and two (L)
(D)-(-)-erythrose
(D)-(-)-threose Not found in nature
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The (D)-Aldose FamilyThe (D)-Aldopentoses
Three stereocenters ? eight stereoisomers
Four (D) and four (L)
(D)-(-)-ribose RNA (ribonucleic acid) DNA
(deoxyribonucleic acid)
(D)-()-xylose
(D)-(-)arabinose
(D)-(-)-lyxose Not found in nature
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The (D)-Aldose FamilyThe (D)-Aldohexoses
Four stereocenters ? 16 stereoisomers ? eight
(D) and eight (L)
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The (D)-Aldose FamilyThe (D)-Aldohexoses
Four stereocenters ? 16 stereoisomers ? eight (D)
and eight (L)

• Must I memorize all of these structures?
• Most important aldoses glucose, ribose,
  galactose
• Learn by doing problems

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Cyclic Monosaccharides

• Many acyclic monosaccharides in equilibrium with
  more stable cyclic isomers
• Example glucose

a-D-glucopyranose
OH axial less stable
36 at equilibrium in H2O
b-D-glucopyranose
OH equatorial more stable
64 at equilibrium in H2O
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Cyclic MonosaccharidesAnomeric Carbon
Stereochemistry
a trans CH2OH, anomeric OH
b cis CH2OH, anomeric OH
Anomeric carbon

• Was carbon of carbonyl in acyclic form
• Anomeric OH variable stereochemistry
• Anomeric ether has fixed stereochemistry
  (glycosides)

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Cyclic MonosaccharidesHaworth Projections
a-D-glucopyranose
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Cyclic MonosaccharidesFuranoses
X OH D-ribose X H D-2-deoxyribose
X OH b-D-ribofuranose (RNA) X H
b-D-2-deoxyribofuranose (DNA)
Which ribose OH becomes ribofuranose ether?
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Cyclic MonosaccharidesFuranoses in DNA
A short segment of the DNA double helix
a or b?
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Disaccharides

• Composed of two monosaccharide molecules
• Useful vocabulary
• Linked by glycoside (an ether), part of acetal
  functional group
• Other anomeric carbon hemiacetal
  functional group

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DisaccharidesCarbohydrate Ring Numbering

• Anomeric carbon receives lowest number
• Carbon 1 in aldoses
• Carbon 2 (rarely 3) in ketoses
• All other carbons numbered in order

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DisaccharidesMaltose
1,4-a-D-glucopyranosyl-D-glucopyranose

• Product of partial hydrolysis of starch
• Present in juice of cereal grains (from which
  beer is brewed)
• Compare with cellobiose (from partial hydrolysis
  of cellulose)
• An a-glycoside easily digested by mammals

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DisaccharidesLactose
1,4-b-D-galactopyranosyl-D-glucopyranose

• Present in mammalian milk (up to 8 by weight
  varies with species)
• Readily digested by infant mammals requires
  enzyme lactase
• Adults often less tolerant due to low levels of
  lactase

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DisaccharidesSucrose
1,2-b-D-fructofuranosyl-a-D-glucopyranose

• Unusual structure 1,2-a-glycoside
• Most common disaccharide in nature
• Produced only by plants such as sugar cane, sugar
  beats
• Annual commercial world production 9 x 109 kg
  yr-1 (4 x 1013 sugar packets)
• An a-glycoside readily digested by mammals

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Polysaccharides

• Polysaccharide hydrolysis yields many
  monosaccharide molecules

• Most important are glucose polymers cellulose,
  starch (high molecular weight)

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PolysaccharidesCellulose

• Linear 1,4-b-D-glucopyranose polymer

• Most abundant organic substance in nature
• Function support structure in plants
• Wood is 50 cellulose by weight
• Strength due to intermolecular hydrogen bonding

• Not easily digested by mammals

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Polysaccharides

• Starch
• Two forms amylose, amylopectin

• 1,4-a-D-glucopyranose polymer
• Function plant glucose/energy storage
• Hydrolysis ? glucopyranose
• Easily digested by mammals

• Amylose
• Linear polymer of 1K-6K glucopyranose
• 20-25 of starch

• Amylopectin
• Branched polymer containing 106 glucopyranose
• 75-80 of starch

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End Exam 1