chapter 8 (part 1) carbohydrates. most abundant class of biological molecules on earth originally...
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Chapter 8 (part 1)
Carbohydrates
Carbohydrates
• Most abundant class of biological molecules on Earth
• Originally produced through CO2 fixation during photosynthesis
Roles of Carbohydrates
• Energy storage (glycogen,starch)
• Structural components (cellulose,chitin)
• Cellular recognition • Carbohydrate derivatives
include DNA, RNA, co-factors, glycoproteins, glycolipids
Carbohydrates
• Monosaccharides (simple sugars) cannot be broken down into simpler sugars under mild conditions
• Oligosaccharides = "a few" - usually 2 to 10
• Polysaccharides are polymers of the simple sugars
Monosaccharides• Polyhydroxy ketones
(ketoses) and aldehydes (aldoses)
• Aldoses and ketoses contain aldehyde and ketone functions, respectively
• Ketose named for “equivalent aldose” + “ul” inserted
• Triose, tetrose, etc. denotes number of carbons
• Empirical formula = (CH2O)n
C
C*
O
C*
C*
CH2OH
H OH
HO H
H OH
H
CH2OH
C
C*
C*
CH2OH
O
HO H
H OH
D-ribose D-ribulose
Monosaccharides are chiral
• Aldoses with 3C or more and ketoses with 4C or more are chiral
• The number of chiral carbons present in a ketose is always one less than the number found in the same length aldose
• Number of possible steroisomers = 2n (n = the number of chiral carbons)
C
C*
O
C*
C*
C*
CH2OH
H OH
HO H
H OH
H OH
H
CH2OH
C
C*
C*
C*
CH2OH
O
HO H
H OH
H OH
D-glucose D-fructose
Stereochemistry
•Enantiomers = mirror images
•Pairs of isomers that have opposite configurations at one or more chiral centers but are NOT mirror images are diastereomers
•Epimers = Two sugars that differ in configuration at only one chiral center
C
C*
O
C*
C*
C*
CH2OH
H OH
HO H
H OH
H OH
H
C
C*
O
C*
C*
C*
CH2OH
HO H
H OH
HO H
HO H
H
C
C*
O
C*
C*
C*
CH2OH
H OH
HO H
H OH
H OH
H
C
C*
O
C*
C*
C*
CH2OH
HO H
HO H
H OH
H OH
H
C
C*
O
C*
C*
C*
CH2OH
HO H
HO H
H OH
H OH
H
C
C*
O
C*
C*
C*
CH2OH
H OH
HO H
HO H
H OH
H
D-glucoseL-glucose
Enantiomers
D-glucose D-mannose
Epimers
D-mannose D-galactose
Diastereomers
Cyclization of aldose and ketoses introduces additional
chiral center• Aldose sugars (glucose) can cyclize to
form a cyclic hemiacetal
• Ketose sugars (fructose) can cyclize to form a cyclic hemiketal
HC
R1
O
OH R2
HO
C*
OR2
R1H
H
ALCOHOL
ALDEHYDE
HEMIACETAL
NEW CHIRAL CARBON
RC
R1
O
OH R2
HO
C*
OR2
R1R
H
ALCOHOL
KETONE
HEMIKETAL
NEW CHIRAL CARBON
Haworth Projections
Anomeric carbon(most oxidized)
-OH up = beta-OH down = alpha
1
23
4
5
6
For all non-anomeric carbons, -OH groups point down in Haworth projections if pointing right in Fischer projections
C1
C2 OHH
HO
C3
C4
C5
CH2OH
HHO
OHH
OHH
Monosaccharides can cyclize to form Pyranose / Furanose
forms = 64% = 36%
= 21.5% = 58.5%
= 13.5% = 6.5%
Conformation of Monosaccharides
Pyranose sugars not planar molecules, prefer to be in either of the two chair conformations.
Reducing Sugars
• When in the uncyclized form, monosaccharides act as reducing agents.
• Free carbonyl group from aldoses or ketoses can reduce Cu2+ and Ag+ ions to insoluble products
Derivatives of Monosaccharides
Sugar Phosphates
Deoxy Acids
Amino Sugars
Sugar alcohols
Monosaccharide structures you need to
know• Glucose• Fructose• Ribulose• Glyceraldehyde• Dihydroxyacetone
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