Carbohydrates
Molecules with formula CnH2nOn.
Fisher projections:
≡
≡
D-glucose
(C6H12O6)
D-/L-
notation: After placing most-oxidized carbon on top,
sugars with last chiral –OH on right are D-sugars.
Monosaccharides: Aldoses and Ketoses
Aldoses are polyhydroxy-aldehydes.
D-glyceraldehyde
D-erythrose
D-arabinose
D-glucose
(C3H6O3)
(C4H8O4)
(C5H10O5)
(C6H12O6)
an aldotriose
an aldotetrose
an aldopentose
an aldohexose
Monosaccharides: Aldoses and Ketoses
Ketoses are polyhydroxy-ketones.
dihydroxyacetone
(C3H6O3)
D-erythrulose
D-ribulose
D-fructose
(C4H8O4)
(C5H10O5)
(C6H12O6)
a ketotriose
a ketotetrose
a ketopentose
a ketohexose
Epimers
Epimers differ in stereochemistry at one carbon.
1
6
D-mannose
the C2-epimer of
glucose
D-glucose
D-galactose
the C4-epimer of
glucose
Monosaccharides Equilibrate Between
Acyclic and Cyclic Forms
rotate
t t
C4-C5
bond
≡
D-glucose
cyclic
hemiacetals
stereoisomers,
“anomers”
Equilibrating Glucose Anomers
Haworth
projections
p
j
Timescale of reaction:
mins-hrs
< 1%
32%
-D-glucopyranose
-anomer
64%
-D-glucopyranose
-anomer
The Anomeric Effect
-D-glucopyranose
32%
64%
-D-glucopyranose
“mutarotation”
Advantage:
The anomeric effect.
Advantage:
Hydroxyl group is equatorial.
+
Interaction between Osp3
lone pair and C1-O *
antibonding orbital is
stabilizing,
t bili i
and
d orbitals
bit l
are oriented correctly.
Anomeric effect not possible
for -anomer; orbitals don’t
line up.
Ketoses Also Equilibrate Between
Acyclic and Cyclic Forms
≡
D-fructose


cyclic
h ik t l
hemiketals
Also forms pyranose
(6
(6-membered)
b d) rings.
i
-D-fructofuranose
70%
30%
-D-fructofuranose

, but found most often in complex sugars.
Glycosides
catalytic
H+
R –OH
-D-glucose
+
Timescale of reaction:
decades
R
R
-glycoside
-glycoside
((often
f
the major
product)
Glycosides do not
undergo mutarotation.
Disaccharides via Enzymatic Glycosidation
H
HO
 glycosidase
-glycosidase
D-glucose
CH2OH
(+ H2O)
O
H
H
HO H
OH
H
O
HO
HO CH H
2
HOH C
2
O
sucrose
+
D-fructose
H
OH
H
Enzyme catalyst increases reaction rate,
makes
k reaction
ti stereospecific.
t
ifi
In absence of catalyst, at pH 7, reaction
too slow to observe.