Transcript Carbohydrates

Chapter 7: Outline
Monosaccharides
Monosaccharide stereoisomers
Cyclic structures
Reactions
Examples and derivatives
Di and oligosaccharides
Polysaccharides
Homo and heteropolysaccharides
Glycoconjugates
7P1-1
Originally, carbohydrates were
those compounds having the
formula Cn(H2O)n. Only
monosaccharides or simple
sugars fit the formula. As
more complex carbohydrates
were discovered, the term
came to mean compounds
associated with polyhydroxy
aldehydes and ketones.
7P1-2
7.1 Monosaccharides
polyhydroxy
Aldehydes
are
aldoses
Number of carbons
Ketones
are
ketoses
3=triose
4=tetrose
5=pentose
6=hexose
7P1-3
Monosaccharides: generic names
The generic name for a simple sugar
begins with the carbonyl prefix aldo or
keto and ends with the term for the
number of carbons.
An aldose with three carbons is called
an aldotriose. A ketose with three
carbons is a ketotriose.
What is the name for a six carbon
aldehyde sugar? aldohexose
What is the name for a five carbon
ketone sugar? ketopentose
7P1-4
Dihydroxyacetone
Dihydroxyacetone
CH2OH
C O
CH2OH
Is a ketotriose
7P1-5
Glyceraldehyde
Glyceraldehyde
Is an aldotriose
O
C
H
H C OH
CH2OH
Glyceraldehyde exists in two
stereoisomeric forms because the
starred carbon is a stereocenter: it has
four different groups attached.
7P1-6
The stereoisomers of glyceraldehyde are designated D or L.
The D isomer has the OH on the
stereocenter to the right. The L
isomer has the OH on the left .
O
C
H
H C OH
stereocenter
CH2OH
the D isomer
O
C
H
HO C H
CH2OH
the L isomer
7P1-7
Glyceraldehyde: 3
The stereoisomeric forms of glyceraldehyde are enantiomers: nonsuperimposable mirror image molecules.
Perspective drawings of the two
enantiomers of glyceraldehyde are on
the next slide. A stereo view is on slide
11.
Remember, barred bonds ( ) recede
behind the plane of the screen and
wedge ( ) bonds project in front of
the plane.
7P1-8
Perspective View
7P1-9
View with blue lens on the left eye.
Carbonyl group
H atom
OH group
CH2OH group
Stereoscopic view of glyceraldehyde
7P1-10
Fischer Projections
In a Fischer projection, the
sugar molecule is
oriented so that the most O
oxidized carbon is to the
top. The stereocenter
carbons are arranged so H
that the groups not part of
H
the main chain project
horizontally toward the
viewer.
The molecule is in the all
eclipsed form.
H
C
C OH
C OH
CH2OH
7P1-11
Monosaccharides are drawn in Fischer
projections with the most oxidized
carbon closest to the top. The carbons
are numbered from the top. If the the
stereocenter with the highest number
has the OH to the right, the sugar is D.
If the OH is to the left, the sugar is L.
Most common sugars are in the D form.
Note: Fisher projections represent an all
eclipsed conformation.
7P1-12
CH
OH
2
H
2
C
C O
1
H 2 C OH
HO 3 C H
H 3 C OH
H 4 C OH
H 4 C OH
H 5 C OH
5 CH OH
6 CH OH
2
2
D-ribose
D-fructose
an aldopentose a ketohexose
O
1
7P1-13
CHO
CHO
H
C
OH
HO C H
H C OH
H
C
OH
HO C H
HO C H
H C OH
H C OH
CH2OH
CH2OH
D-glucose
D-galactose
an aldohexose an aldohexose
These diastereomers are also epimers, they
differ in configuration at only one stereo7P1-14
center (colored dot).
Cyclic forms for sugars
Most simple sugars of four or more
carbons exist in the cyclic (hemiacetal
or hemiketal) form.
A hydroxy group in the sugar reacts with
the carbonyl group.
The new OH bearing carbon is now a
stereo center and is called an anomeric
carbon.
If the OH on the ring is “up” the carbon
is b, if the OH is “down” it is a.
7P1-15
Cyclic forms for sugars-2
Fischer projections for a D glucose
H C OH
HO C H
CO
H C OH
H C OH
C OH
HO C H O HO C H O
CH
H C OH
H C OH
C OH
HC
HC
C OH
CH2OH
CH2OH
CH2OH
D-glucose a D-glucose b D-glucose
H
H
HO
H
H
cyclic form
cyclic form
7P1-16
Cyclic forms for sugars-3 Haworth
1. Draw a five- or six-membered ring
O
O
pyranose form furanose form
2. Anomeric C to right of O. Place OH
up or down. Left on Fischer, up on
ring.
3. In D- sugars, the last C is always up.
7P1-17
Cyclic forms for sugars-4 Haworth
CH2 OH
Anomeric C
b-OH
C O
HO C H
OH
O
HO CH2
H C OH
HO
CH2OH
H C OH
OH
CH2OH b-D-fructofuranose
D-fructose + a isomer
7P1-18
Cyclic forms for sugars-5 Glucose
CH2OH
arrows show
O H a form
H
electron movement
H
(alpha)
OH H
CH2OH
HO
O
H
OH
H
H
OH
Pyranose
H
H
OH H O
ring
form
CH
OH
2
HO
O
H
OH
H OH
H
b
form
H
OH
HO
H (beta)
H OH
7P1-19
Cyclic forms for sugars-6
The alpha and beta forms of cyclic
sugars are said to be anomers. They
differ in configuration about the
hemiacetal or hemiketal carbon.
7P1-20
Cyclic forms for sugars-7
Ribose also exists mainly in the cyclic
form.
arrows show
electron movement
CH2OH
O H
H
H
H
H
OH
OH
b D-ribose
b D-ribofuranose
(furanose ring form)
CH2OH
O H
O
O
H
H
OH
OH
H
H
7P1-21
a D-glucose: the chair conformer
H
HO
4
H
CH2OH
O
6
5
HO
H
2
3
H
H
1
OH
OH
Four of the five bulky groups (OH and CH2OH on C 2,3,4,5)
on the ring are in the more stable equitorial positions!
7P1-22
Oxidation of Monosaccharides
Aldoses react with Tollen’s reagent (Ag(NH3)2+) to give a
lactone (cyclic ester). The silver ion plates out as a mirror.
+ Ag(mirror)
CH2OH
O H
H
H
+
Ag(NH3)2
H
OH H
2+
Cu
HO
HO
OH
H
OH
CH2OH
O
H
OH H
H
O
OH
+ Cu2O (red-orange)
Benedict’s reagent (a blue copper ion solution) also gives a
lactone. The blue color fades as reaction occurs.
7P1-23
Oxidation of Monosaccharides-2
Aldehyde oxid’n
aldonic acid
H
HO
H
H
CHO
C OH
CH
C OH
C OH
COOH
D-glucuronic acid
H
HO
H
H
COOH
C OH
CH
C OH
C OH
CH2OH
D-gluconic acid
Term CH2OH oxid’n
uronic acid
7P1-24
Oxidation of Monosaccharides-3
Aldehyde + term CH2OH oxid’n 
aldaric acid
COOH
H C OH
HO C H
H C OH
H C OH
COOH
D-glucaric acid
7P1-25
Reduction of Monosaccharides
The most important reduced sugar is
deoxyribose. (In DNA)
CH2OH
CH2OH
O
H
C
OH
H
C
H C OH H C OH
H C H
HO C H HO C H
H C OH
H C OH
H
C
OH
H C OH
CH2OH
CH
OH
CH2OH
2
D-xylitol
D-sorbitol
D-deoxyribose
When the carbonyl of a sugar is reduced to
an alcohol, alditols are produced. The two
shown above are used to sweeten nonsugar
gum.
7P1-26
Isomerization
H
H
HO
H
H
CO
C OH
CH
C OH
C OH
CH2OH
H C OH
C OH
HO C H
H C OH
H C OH
CH2OH
Isomerization of monosaccharides occurs
through an enediol.
CH2 OH
C O
HO C H
H C OH
H C OH
CH2OH
H
HO
HO
H
H
CO
CH
CH
C OH
C OH
CH2OH
7P1-27
Esters of Monosaccharides
The OH groups of sugars can react with
phosphoric acid to give phosphate
2esters.
CH2OPO3
O OH
H
H
OH H
HO
H
H
OH
b D-glucose-6-phosphate
7P1-28
Glycosides
The anomeric OH can react with another
OH on an alcohol or sugar. Water is
lost to form an acetal/ketal
CH2OH
H
O OH
H
OH H
HO
H
H
OH
+ CH3 O H
Acetal link: R-O-C-O-R
CH2OH
H
O O CH3
H
OH H
HO
H
H
OH Acetal
+ H2O
carbon
7P1-29
Important Monosaccharides
CH2 OH
O
H
H
H
OH H
HO
OH
H
CH2 OH
O
HO
H
H
OH H
H
OH
OH
H
OH
HOCH2 O
OH
H HO
CH2OH
H
OH
H
7P1-30
Amino Sugars
CH2 OH
CH2 OH
O
O
H
H
H
H
H
H
OH H
OH H
HO
OH HO
OH
H
+
NH3
a-D-glucosamine
H
NH
CH3C O
N-acetyl-a-D-glucosamine
7P1-31
Amino Sugars-2
O
CH3C NH
HO
O
R
-
COO
H C OH
OH R= H C OH
CH2OH
OH
N-acetylnuraminic acid
sialic acid
7P1-32