chapter 20: carbohydrates
DESCRIPTION
Chapter 20: Carbohydrates. Carbohydrate: A polyhydroxyaldehyde or polyhydroxyketone , or a substance that gives these compounds on hydrolysis. Chapter Overview: Monosaccharides Fischer projections Haworth projections Hemiacetals and Acetals Oxidation and reduction Disaccharides - PowerPoint PPT PresentationTRANSCRIPT
Chapter 20: Carbohydrates
Carbohydrate:Carbohydrate: A polyhydroxyaldehyde or polyhydroxyketone, or a substance that gives these compounds on hydrolysis.
Chapter Overview:Chapter Overview:•Monosaccharides
• Fischer projections• Haworth projections• Hemiacetals and Acetals• Oxidation and reduction
•Disaccharides•Polysaccharides
Monosaccharide:Monosaccharide: A carbohydrate that cannot be hydrolyzed to a simpler carbohydrate.• Monosaccharides have the general formula CCnnHH2n2nOOnn, where nn varies from 3 to 8.• Aldose:Aldose: A monosaccharide containing an aldehyde group.• Ketose:Ketose: A monosaccharide containing a ketone group.• The prefixes tri-tri-, tetratetra, pentapenta, and so forth indicate the number of carbon atoms in the chain.
Dihydroxyacetone (a ketotriose)
Glyceraldehyde (an aldotriose)
CHO
CHOH
CH2OH
CH2OH
C=O
CH2OH
Chapter 20: Monosaccharides
Fructose( ____________ )
Chapter 20: Fischer projections
Fischer projection:Fischer projection: A two-dimensional representation for showing the configuration of tetrahedral stereocenters.• Horizontal lines represent bonds projecting forward from the
stereocenter. • Vertical lines represent bonds projecting to the rear.
CHO
CH OH
CH2OH
H OHCHO
CH2OH
convert to a Fischerprojection
Chapter 20: Monosaccharides
In 1891, Emil Fischer made the arbitrary assignments of D- and L- to the enantiomers of glyceraldehyde.
• D-monosaccharide:D-monosaccharide: the -OH on its penultimate carbon is on the right in a Fischer projection.
• L-monosaccharide:L-monosaccharide: the -OH on its penultimate carbon is on the left in a Fischer projection.
L-GlyceraldehydeD-Glyceraldehyde
CHOCHO
H OH
CH2OH CH2OH
HHO
[]25 = +13.5°D
[]25 = -13.5°D
Chapter 20: Monosaccharides
The most common monosaccharides:The most common monosaccharides:
D-Ribose D- Glucose D- Galactose D- Fructose
Chapter 20: Cyclic Structure
• Aldehydes and ketones react with alcohols to form hemiacetalshemiacetals (Chapter 17).• Cyclic hemiacetals form readily when the hydroxyl and carbonyl
groups are part of the same molecule and their interaction can form a five- or six-membered ring.
O-HH
O
CO O
H
H
O O-H
H4-Hydroxypentanal
A cyclic hemiacetal
14
14
redraw to show -OH and -CHO
close to each other
Chapter 20: Haworth Projections
• D-Glucose forms these two cyclic hemiacetals.
CHO
OH
H
OH
H
HO
H
H OH
CH2OH
HH OH
HHO
HOH
OH
H
CH2OHO
C
H OH
HHO
HOH
H
CH2OHOH
O
H
OHH OH
HHO
HH
OH
H
CH2OHO
D-Glucose
-D-Glucopyranose (-D-Glucose)
()
()
-D-Glucopyranose (-D-Glucose)
+
anomericcarbon
5
5
1
1
redraw to show the -OH on carbon-5 close to thealdehyde on carbon-1
anomericcarbon
Chapter 20: Anomers
• The anomeric carbon of an aldose is C-1; • The anomeric carbon of most ketoses is C-2. • ββ means that the -OH on the anomeric carbon is on the same
side of the ring as the terminal -CH2OH.
• αα means that the -OH on the anomeric carbon is on the side of the ring opposite from the terminal -CH2OH.
• A six-membered hemiacetal ring is called a pyranosepyranose, and a five-membered hemiacetal ring is called a furanosefuranose because these ring sizes correspond to the heterocyclic compounds furan and pyran.
PyranFuranOO
Chapter 20: Monosaccharides
OH ()
H
HOH OH
H HOHOCH2
H
OH ()
HOH H
H HOHOCH2
-D-Ribofuranose(-D-Ribose)
-2-Deoxy-D-ribofuranose(-2-Deoxy-D-ribose)
The prefix “deoxydeoxy” means “without oxygen.”
Chapter 20: Monosaccharides
HO
HOCH2 OH
HHO
CH2OH
OHH
H
C=O
CH2OH
HOH
CH2OH
OHH
HO HOH
HOHOCH2
HO HCH2OH
OH
D-Fructose
1
2
5
5
5
1
2
2
()
-D-Fructofuranose(-D-Fructose)
-D-Fructofuranose(-D-Fructose)
()
1
Fructose Fructose is a ketose and it also forms cyclic hemiacetals
Chapter 20: Monosaccharides
• Mutarotation: Mutarotation: The change in specific rotation that accompanies the equilibration of a- and b-anomers in aqueous solution.• Example: When either a-D-glucose or b-D-glucose is dissolved in
water, the specific rotation of the solution gradually changes to an equilibrium value of +52.7°, which corresponds to 64% beta and 36% alpha forms.
[]25 = + 18.7°-D-Glucopyranose-D-Glucopyranose
[]25 = +112°
OHOH
HOHO
CH2OHO HO OH
OC
CH2OH
HO
HOH
OCH2OH
HO
HOOH
HO
Open-chain form
D D
Chapter 20: Monosaccharides
Hemiacetal + Alcohol = Acetal
HH OH
HHO
HOH
OH
H
CH2OHO
CH3OHH+
-H2O
OCH2OH
H
OH
OCH3H
HOH
OHH
H
OCH2OH
H
OH
HH
HOH
OHH
OCH3
(-D-Glucose)-D-Glucopyranose
Methyl -D-glucopyranoside(Methyl -D-glucoside)
anomeric carbon
+
+
Methyl -D-glucopyranoside(Methyl -D-glucoside)
glycosidicbond
• The acetal obtained from a monosaccharide is called glycoside• Mutarotation is not possible in glycosides
Chapter 20: Disaccharides
Disaccharide:Disaccharide: a carbohydrate containing two monosaccharide units joined by a glycosidic bond.Sucrose (table sugar) = Glucose + Fructose• Sucrose is the most abundant disaccharide in the biological
world; it is obtained principally from the juice of sugar cane and sugar beets.
• Sucrose is a nonreducing sugar.
O
HOOH
OH
CH2OH
O
OH
HOO
CH2OH
HOCH2
OHO
HO
O
OH
CH2OH
OH
HOO
CH2OH
HOCH2
1
1
2
1
2
1
a unit of -D-glucopyranose
a unit of -D-fructofuranose
-1,2-glycosidic bond
Chapter 20: Monosaccharides
Lactose = Galactose + Glucose• Lactose is the principal sugar present in milk; it makes up about
5 to 8 percent of human milk and 4 to 6 percent of cow's milk.• It consists of D-galactopyranose bonded by a β-1,4-glycosidic
bond to carbon 4 of D-glucopyranose.• Lactose is a reducing sugar.
O
OH
HOOH
O
CH2OH
O
HOOH
OH
CH2OHOOH O
OH
OH
CH2OH
O OH
OH
OH
CH2OH
1
1
4
4
-1,4-glycosidic bond
Chapter 20: Monosaccharides
Maltose = Glucose + Glucose• Present in malt, the juice from sprouted barley and other cereal
grains. • Maltose consists of two units of D-glucopyranose joined by an a-
1,4-glycosidic bond.• Maltose is a reducing sugar.
OHO
HOOH
OOHO OH
OH
CH2OH
CH2OHO
OH
O
OHHO
O OH
HO
OH
CH2OH
HOCH2 1
4
-1,4-glycosidicbond1 4
Chapter 20: Physical Properties
Monosaccharides are colorless crystalline solids, very soluble in water, but only slightly soluble in ethanol.
Sweetness relative to sucrose:
Carbohydrate
fructose
glucose
galactose
sucrose (table sugar)
lactose (milk sugar)
honey
SweetnessRelative to Sucrose
1.741.000.970.74
0.320.16
Artificial Sweetener
SweetnessRelative to Sucrose
maltose 0.33
saccharin 450acesulfame-K 200aspartame 180sucralose 600
Chapter 20: Polysaccharides
Polysaccharide:Polysaccharide: A carbohydrate consisting of large numbers of monosaccharide units joined by glycosidic bonds.
Starch:Starch: A polymer of D-glucose.• Starch can be separated into amylose and amylopectin.• Amylose is composed of unbranched chains of up to 4000 D-
glucose units joined by α-1,4-glycosidic bonds.• Amylopectin contains chains up to 10,000 D-glucose units also
joined by α-1,4-glycosidic bonds; at branch points, new chains of 24 to 30 units are started by α-1,6-glycosidic bonds.
Chapter 20: Polysaccharides
Amylopectin, a branched polymer of approximately 10,000 units of D-glucose joined by -1,4-glycosidic bonds.
Chapter 20: Polysaccharides
GlycogenGlycogen is the energy-reserve carbohydrate for animals.• Glycogen is a branched polysaccharide of approximately 106
glucose units joined by α-1,4- and α-1,6-glycosidic bonds.• The total amount of glycogen in the body of a well-nourished
adult human is about 350 g, divided almost equally between liver and muscle.
Chapter 20: Polysaccharides
CelluloseCellulose is a linear polysaccharide of D-glucose units joined by β-1,4-glycosidic bonds.• It has an average molecular weight of 400,000
g/mol, corresponding to approximately 2200 glucose units per molecule.
• Cellulose molecules act like stiff rods and align themselves side by side into well-organized water-insoluble fibers in which the OH groups form numerous intermolecular hydrogen bonds.
• This arrangement of parallel chains in bundles gives cellulose fibers their high mechanical strength.
• It is also the reason why cellulose is insoluble in water.
Chapter 20: Polysaccharides
Cellulose is a linear polymer containing as many as 3000 units of D-glucose joined by β-1,4-glycosidic bonds.
• Humans and other animals can not digest cellulose because their digestive systems do not contain β-glycosidases, enzymes that catalyze the hydrolysis of β-glycosidic bonds.
• Termites have such bacteria in their intestines and can use wood as their principal food.
• Ruminants (cud-chewing animals) and horses can also digest grasses and hay. • Instead, we have only α-glucosidases; hence, the polysaccharides we use as
sources of glucose are starch and glycogen.• Many bacteria and microorganisms have β-glucosidases.