4.carbohydrate
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Transcript of 4.carbohydrate
Carbohydrates
CARBOHYDRATES
polyhydroxy aldehydes and ketones or substances that hydrolyze to yield polyhydroxy aldehydes and ketones.
Most abundant class of biological molecules on Earth
Originally produced through CO2 fixation during photosynthesis
Empirical formula = (CH2O)n
ROLES OF CARBOHYDRATES Energy storage (glycogen,starch) Structural components (cellulose,chitin) Cellular recognition Carbohydrate derivatives include DNA, RNA,
co-factors, glycoproteins, glycolipids
CARBOHYDRATES CLASSIFICATION
Monosaccharides (simple sugars) cannot be broken down into simpler sugars under mild conditions
Oligosaccharides = "a few" - usually 2 to 10 monosaccharides. It is further divided into disaccharides or trisaccharides.
Polysaccharides are polymers of the simple sugars with high molecular weight. It is of two types, Homopolysaccharides and heteropolysaccharides
CLASSIFICATION OF MONOSACCHARIDES
Monosaccharides are classified according to:(1) The number of carbon atoms present in the
molecules.(2) whether they contain an aldehyde or keto group.
Monosaccharides
Aldoses (-HC=O)
Ketoses(-C=O)
Trioses (C3H6O3) Glyceraldehyde Dihydroxyacetone
Tetroses (C4H8O4)
Erythrose Erythrulose
Pentoses (C5H10O5)
Ribose Ribulose
Hexoses (C6H12O6)
Glucose Fructose
MONOSACCHARIDES
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
D VS L DESIGNATION
D & L designations are based on the configuration about the single asymmetric C in glyceraldehyde.
The lower representations are Fischer Projections.
CHO
C
CH2OH
HO H
CHO
C
CH2OH
H OH
CHO
C
CH2OH
HO H
CHO
C
CH2OH
H OH
L-glyceraldehydeD-glyceraldehyde
L-glyceraldehydeD-glyceraldehyde
SUGAR NOMENCLATURE
O H O H
C C
H – C – OH HO – C – H
HO – C – H H – C – OH
H – C – OH HO – C – H
H – C – OH HO – C – H
CH2OH CH2OH
D-glucose L-glucose
For sugars with more than one chiral center, D or L refers to the asymmetric C farthest from the aldehyde or keto group.
Most naturally occurring sugars are D isomers.
CYCLIZATION OF ALDOSE AND KETOSES INTRODUCES ADDITIONAL CHIRAL CENTER Aldose sugars (glucose) can cyclize to form a
cyclic hemiacetal An aldehyde can react with an alcohol to form a hemiacetal.
Ketose sugars (fructose) can cyclize to form a cyclic hemiketal
A ketone can react with an alcohol to form a 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
Pentoses and hexoses can cyclize as the ketone or aldehyde reacts with a distal OH.Glucose forms an intra-molecular hemiacetal, as the C1 aldehyde & C5 OH react, to form a 6-member pyranose ring, named after pyran.
These representations of the cyclic sugars are called Haworth projections.
H O
OH
H
OHH
OH
CH2OH
H
OH
H H O
OH
H
OHH
OH
CH2OH
H
H
OH
-D-glucose -D-glucose
23
4
5
6
1 1
6
5
4
3 2
H
CHO
C OH
C HHO
C OHH
C OHH
CH2OH
1
5
2
3
4
6
D-glucose (linear form)
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
Fructose forms either a 6-member pyranose ring, by reaction of
the C2 keto group with the OH on C6, or a 5-member furanose ring, by reaction of
the C2 keto group with the OH on C5.
CH2OH
C O
C HHO
C OHH
C OHH
CH2OH
HOH2C
OH
CH2OH
HOH H
H HO
O
1
6
5
4
3
2
6
5
4 3
2
1
D-fructose (linear) -D-fructofuranose
MONOSACCHARIDES CAN CYCLIZE TO FORM PYRANOSE / FURANOSE FORMS
= 64% = 36%
= 21.5% = 58.5%
= 13.5% = 6.5%
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 ALCOHOLS - LACKS AN ALDEHYDE OR KETONE; E.G…
COOH
C
C
C
C
H OH
HO H
H OH
D-gluconic acid D-glucuronic acid
CH2OH
OHH
CHO
C
C
C
C
H OH
HO H
H OH
COOH
OHH
sugar acid - the aldehyde at C1, or OH at C6, is oxidized to a carboxylic acid; e.g., gluconic acid, glucuronic acid.
SUGAR DERIVATIVES
amino sugar - an amino group substitutes for a hydroxyl. An example is glucosamine.
The amino group may be acetylated, as in N-acetylglucosamine.
H O
OH
H
OH
H
NH2H
OH
CH2OH
H
-D-glucosamine
H O
OH
H
OH
H
NH
OH
CH2OH
H
-D-N-acetylglucosamine
C CH3
O
H
AMINO SUGARS
MONOSACCHARIDE STRUCTURES YOU NEED TO KNOW
Glucose Fructose Ribulose Glyceraldehyde Dihydroxyacetone
GLYCOSIDIC BONDS
The anomeric hydroxyl and a hydroxyl of another sugar or some other compound can join together, splitting out water to form a glycosidic bond:
R-OH + HO-R' R-O-R' + H2O
GLYCOSIDIC LINKAGE
O
CH2OH
OH
OH
OHOH
O
CH2OH
OH
OH
OH
OH
H2OH2O
O
CH2OH
OH
OH
OH
O
O
CH2OH
OH
OH
OH
alcohol
hemiacetal
glycosidic linkage
Hydrolysis
Condensation
acetal
DIASACCHARIDES
H O
O H
H
O HH
O H
CH 2O H
H
O H
O H
H
O HH
O H
CH 2O H
H
O
HH
1
23
5
4
6
1
23
4
5
6
m altose
-D-glucosyl-(1->4)--D-glucopyranose)
DIASACCHARIDES
-D-glucosyl-(1->4)--D-glucopyranose)
H O
O H
H
O HH
O H
CH 2O H
H
O O H
H
H
O HH
O H
CH 2O H
H
H
H
O1
23
4
5
6
1
23
4
5
6
cellobiose
DISACCHARIDES
O
CH2OH
OH
OH
H
O
O
CH2OH
OH
OH
OH
H
OH
H
H
H
H
H
H
lactose
sucrose
-D-galactosyl-(1->4)--D-glucopyranose)
-D-glucosyl-(1->2)--D-fructofuranose)
O
CH2OH
OH
OH
OH
H
H
H
H
OCH2OH
H
H
OH
OH
H
O
CH2OH