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©1999 Thomas Poon
Carbohydrates
Learning Objectives
As we study this chapter, you should...
1)
Know how to draw and interpret Fisher projections. Straight-chain (acyclic) carbohydrates are
usually drawn using Fisher Projections. In a Fisher projection, the horizontal lines come out of the page
and the vertical lines go behind the page.
the most oxidized carbon is
drawn at the top
* each intersection is a
stereocenter
the lowest stereocenter
is called the penultimate
carbon
2)
1 CHO
The numbering system
starts at the topmost
OH
*
carbon
H
*
=
* OH
* OH
CH
2OH
6
H2
HO 3
H4
H5
Fisher projection
(with key parts labeled)
CHO
OH
H
rotate
OH
180°
about
the
OH
z-axis
CH2OH
HO
HO
H
HO
CH2OH
H
H
OH
H
CHO
These two molecules are the same
H
HO
HO
H
HO
=
perspective view
(viewed from your perspective)
H OH
CHO
H
CH2OH
perspective view
(side view)
CHO
OH
H
OH
OH
CH2OH
H
HO
H
H
rotate
180°
about the
y-axis
HO
H
HO
HO
CHO
H
OH
H
H
CH2OH
These two molecules are different
H
HO
H
H
CHO
OH
H
OH
OH
CH2OH
rotate
180°
about the
x-axis
H
H
HO
H
CH2OH
OH
OH
H
OH
CHO
y
x
z
These two molecules are different
Know the “Nomenclature” and classifications of acyclic carbohydrates. Nearly all carbohydrates
have common names. For example, the carbohydrate used in the item #1 (above) is known as "glucose"
and is rarely called “(2R,3S,4R,5R)-2,3,4,5,6-pentahydroxyhexanal.” Straight chain carbohydrates are
given the prefix “aldo” (for aldehydes) or “keto” (for ketones). They are then classified according to their
number of carbon atoms (see table). So glucose (shown above in #1) is an aldohexose. Other examples
are shown below:
CH2OH
No. of carbons
3
4
5
6
4)
CHO
OH
H
OH
OH
CH2OH
Know the rules for manipulating Fisher projections. The fact that horizontal lines in a Fisher
projection come out of the page and vertical lines go behind the page limits what you can do with the
drawing. Essentially, Fisher projections may only be rotated 180° on the plane of the paper. Any other
rotation that lifts the molecule off the paper yields a different molecule.
H
HO
H
H
3)
H
HO
H
H
Classification
triose
tetrose
pentose
hexose
H
H
H
CHO
OH
OH
OH
CH2OH
Ribose
(an aldopentose)
O
H
HO
H
OH
H
OH
CH2OH
Fructose
(a 2-ketohexose)
H
CHO
OH
CH2OH
Glyceraldehyde
(an aldotriose)
Understand the definition of D and L carbohydrates. Capital “D” and “L” are another classification of
carbohydrates. In straight chain carbohydrates, a compound is classified as a D-sugar if the OH at the
penultimate carbon is on the right side, and a compound
CHO
CHO
CHO
is classified as an L-sugar if the OH at the penultimate
H
H
H
OH
HO
OH
carbon is on the left side. These are not to be confused HO
H
H
H
OH
HO
with lower case “d” and “l,” which are used to indicate
H
H
H
OH
HO
HO
the rotation of plane polarized light. One thing the D/L
H
H
H
OH
HO
OH
classification has in common with the d/l classification
CH2OH
CH2OH
CH2OH
is that they both can be used to describe pairs of D-Glucose
L-Glucose
D-Galactose
enantiomers.
enantiomers
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©1999 Thomas Poon
5) Understand how carbohydrates come to be cyclic carbohydrates. Although we initially study the
straight-chain form of carbohydrates, most carbohydrates are in equilibrium with two possible cyclic
forms which arise from an intramolecular hemiacetal-forming reaction between one of the alcohols and
the carbonyl carbon. The two possible forms are called “alpha” and “beta” depending on the relative
relationship (anti and syn, respectively) between the former carbonyl oxygen and the CH2OH group. The
former carbonyl carbon (C-1) is called the anomeric carbon. The α and β isomeric forms of the cyclic
carbohydrates are called anomers. Because hemiacetal formation is reversible, all three forms of glucose
are in equilibrium with each other.
O
H
HO
H
H
C
1
H
these grps are
anti to each
other
CH2OH grp
OH
H
3
OH
4
OH
5
6 CH2OH
2
H OH
H2O
4
HO
HO
6
5
3
H
H
CH2OH grp
H OH
6
4
O
1
2
H HO
HO
HO
H
3
H
OH
α-hemiacetal
H
5
H
2
these grps are
syn to each
other
O
1 OH
OH
H
β-hemiacetal
To convert an acyclic D-carbohydrate into a cyclic D-carbohydrate:
a) decide which hydroxyl group will be attacking the carbonyl
b) rotate the carbohydrate 90° clockwise
c) transpose the hydroxyl groups to the cyclohexyl ring as shown below (Any OH group that is “up” in
the rotated Fisher projection, will be “up” in the ring. Any OH group that is “down” in the rotated
Fisher projection, will be “down” in the ring).
eventually becomes
the anomeric OH
O
group
CHO
H 2 OH
H
HO
3
make the
HO 4 H
hemiacetal
bond
H 5 OH
6 CH2OH
1
1
H
HO
HO
H
D-galactose
6)
C
HO
OH
OH
H OH OH H 1 OH
4 6 5
6
O
5 4 3 2
H
HOCH
C
=
2
3
1
rotate 90°
HO 3 2
clockwise
H
H
H
O
OH
4
OH
HO
O
5
OH at C-1 can be
H eventually
becomes
up (β) or down (α)
CH
OH
deprotonated
2
6
2
OH at C-2 is down
OH at C-3 is up
OH at C-4 is up
for all D-sugars, the
CH2OH group is up
Know the classifications of cyclic carbohydrates and be able to draw and interpret Haworth
projections. Aside from the α and β designations, a cyclic carbohydrate is also named according to ring
size. The two most common ring sizes are 5 and 6, furanoses and pyranoses, respectively. The drawings
shown below are called Haworth projections. They are used to give a 3-D perspective of the
carbohydrate, while also being quick to draw. Note that the pyranose rings don’t provide information
regarding the axial or equatorial orientation of the bonds.
HOCH2
O
H
H
H
H
OH
OH OH
α-ribose
(an α-furanose)
6
HOCH2
H
4
HO
5
O
H
OH
3
H
H OH
OH
1
H
=
4
HO
HO
2
H
OH
β-glucose
(a β-pyranose)
6
5
3
H
H
H
2
O
1
OH
H
OH
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©1999 Thomas Poon
7) Understand the formation of glycosides. Carbohydrates can also cyclize in the presence of an alcohol
or amine to form acetals known as glycosides (glycoside: any O- or N-acetal of a carbohydrate). As we
know from our studies on ketones & aldehydes, acetals are more stable than hemiacetals, so glycosides
are not generally in equilibrium with their open chain forms (unless under acidic conditions).
Examples of glycosides
H OH
HO
HO
H OH
HO
H
glycosidic
H HO
H
N(CH3)2 bond
HO
HO
H
glycosidic bond
H
HO
HO
H HO
HO OH
OCH3
H
NH2
glycosidic bond
H OH
HO
H HO
O
HO
H
H
N
HO
H
H HO
Lactose
8)
N
HO
H
O
HO H
OH
H
glycosidic
bond
H
OH OH
Cytosine (from RNA)
Be familiar with the concept of complex carbohydrates. Sugars can exist as dimers (a molecule made
up of 2 sugars covalently bonded), oligomers (a molecule made up of 3-10 sugars covalently bonded), or
polymers (a molecule made up of more than 10 sugars covalently bonded). These are known as
disaccharides, oligosaccharides, and polysaccharides, respectively. The individual sugar units can be
connected at various locations, but are usually connected via a glycosidic bond.
Examples of complex sugars
H OH
HO
HO
H OH
HO
H
H HO
H
O
HOCH2 H O HO
H
OH
H
Sucrose
9)
HO
HO
H
H OH
HO
H HO
H
H OH
O
HO
CH2OH
OH
H
Maltose
HO
H HO
HO
O
H
HO
H HO
OH
H
H
HO
HO
HO
O
O
HO
HO
H HO
H HO
H
H
Cellulose
H OH
O
HO
HO
O
H
H HO
H
Be familiar with these definitions.
Epimers: Sugars that differ only in the configuration of one stereocenter). Epimers are also
diastereomers.
Anomeric Carbon: In cyclic sugars, this is the carbon that used to be the ketone or aldehyde.
Anomers: Cyclic sugars that differ only in the configuration at the anomeric carbon. Each anomer is
referred to as either α or β. α and β are relative designations and refer to the relationship
between the anomeric OH and the stereochemistry at the penultimate carbon.
Mutarotation: Conversion of one anomer to another and establishment of an equilibrium between
the two such that a constant optical rotation is obtained.
Pyranoside: A 6-membered ring glycoside.
Furanoside: A 5-membered ring glycoside.
N-Glycoside: a glycoside in which the anomeric OR group has been replaced by a bond to an amine.