Carbohydrates III

Blood Group Type
http://anthro.palomar.edu/blood/Rh_system.htm
Involves a large number of different antigens on
the surface of red cells whch are genetically
controlled by 2 genes on chromosome #1.
Polysaccharides
Also, proteins on the cell surface: Rh factor, + or -
Cellulose: a glucose polymer
•Can animals in the isoptera family feed on
it?
•Can homo sapiens feed on it?
There are approximately 1,900 species of termites.
Adults briefly have two equal sized wings
(iso="equal", ptera="wings"). The adults retain their
wings for a brief mating -flight and then before
mating, the wings are detached and shed.
Termites not only feed, groom and protect each other
within colonies, but the offspring of one generation
assist the parents in raising the next generation.
Termites are social insects. Colonies are highly
organized societies of several hundred thousand to 1
million or more individuals within a loose collection
of underground tunnels and chambers.
The structures of cellulose (a) and chitin (b). In
both substances, all glycosidic linkages are of
the β-(1, 4) type.
Chemistry at a
Glance:
Biochemically
Important
Carbohydrates
Epimerization, Isomerization,
and Biological Aldol/RetroAldol Reactions of
Carbohydrates
Epimerization
Enol Forms of Carbohydrates
CH
• Enolization of an aldose scrambles the
stereochemistry at C-2.
• This process is called epimerization.
Diastereomers that differ in
stereochemistry at only one of their
stereogenic centers are called epimers.
• D-Glucose and D-mannose, for
example, are epimers.
CHOH
O
H
OH
HO
HO
H
H
OH
C
H
OH
O
HO
H
HO
H
H
OH
H
H
OH
H
OH
H
OH
CH2OH
OH
CH
CH2OH
D-Glucose
Enediol
CH2OH
D-Mannose
This equilibration can be catalyzed by hydroxide ion.
Isomerization
Enol Forms of Carbohydrates
CH
• The enediol intermediate on the preceding
slide can undergo a second reaction. It can
lead to the conversion of D -glucose or D mannose (aldoses) to D -fructose (ketose).
O
CHOH
HO
CHOH
CH2OH
C
C
OH
O
H
OH
HO
H
H
OH
HO
H
H
H
OH
H
OH
H
OH
H
OH
CH2OH
D-Glucose or
D-Mannose
CH2OH
Enediol
CH2OH
D-Fructose
Isomerization
Glycolysis / Retro-Aldol reactions
• D-fructose 6-phosphate undergoes
phosphorylation of its free CH2 OH group to give
D-fructose 1,6-diphosphate.
• D-Fructose 1,6-diphosphate is cleaved into two
3-carbon products by a reverse aldol reaction.
• This retro-aldol reaction is catalyzed by the
enzyme aldolase.
Glycolysis occurs metabolically in nearly
every organism, both aerobically and
anaerobically. The released energy is used
to form the high energy compounds, ATP
and NADH.
HO
CH2OP(O)(OH)2
CH2OP(O)(OH)2
C
C
O
H
H
OH
H
OH
aldolase
O
CH2OH
CH
CH2OP(O)(OH)2
D-Fructose
1,6-phosphate
H
O
OH
CH2OP(O)(OH)2
Question
• What is the relationship between the structures
shown?
Reactions of Sugars
• A) constitutional isomers B) meso forms
• C) enantiomers
D) epimers
• E) diastereomeric disaccharides
Monosaccharides
Oxidation-Reduction Products
H S OH
OH
O
R
OH
OH
Redox Reactions of Monosaccharides
R
OH
R
CHO
H
HO
H
H
OH
H
CO2H
+
Ag
OH
OH
HO
H
H
OH
CH2OH
H
OH
H
NaBH4
OH
CH2OH
Sorbitol
+
Ag
Oxidation
Ketones and alcohols cannot be oxidized by Br2
Questions to Consider:
Reactions of Monosaccharides
Oxidation - Reduction
OH
H
OH
The carbon chain of an aldose can be increased by one
carbon in a Kiliani–Fischer synthesis
[O]
OH
O
OH
[O] = Ag+, NH3, H2O
or Br 2 / H2O
OH
Aldoses are "reducing" sugars, that is, they are oxidized
under very mild conditions. (Only the aldehyde is oxidized.)
H
OH
OH
[H]
OH
O
OH
OH
[H] = ?
1) Identify the starting sugar as D- or L-.
2) Draw Fisher projections for the products.
3) Draw a Haworth projection of the beta pyrano anomer.
The Ruff degradation shortens an aldose chain by one
carbon
Preparation of the Calcium D-Gluconate for the
Ruff Degradation
Periodic Acid Oxidation
Also Cleaved by HIO4
α-Hydroxy carbonyl compounds
Vicinal diols are cleaved by HIO4.
R
O
C
HO
HIO4
C
C
O + O
C
HO
C
OH
C
O + O
C
HO
OH
• Cleavage of a vicinal diol consumes 1
mol of HIO4.
R2 C
RC
HIO4
• Cleavage of an α-hydroxy carbonyl
compound consumes 1 mol of HIO4.
One of the products is a carboxylic acid.
Also Cleaved by HIO4
Structure Determination Using HIO4
Compounds that contain three contiguous
carbons bearing OH groups
O
CH CR'2
HIO4
R2 C O + HCOH
OH OH
Distinguish between furanose and pyranose forms
of methyl arabinoside
+ R'2C
HOCH2
O
• 2 mol of HIO4 are consumed. 1 mole of
formic acid is produced.
O
OH
O
HO
OCH3
HO
OCH3
HO
2 vicinal OH groups;
consumes 1 mol of HIO4
Question
• How many moles of formic acid and formaldehyde,
respectively, would be produced when D-arabinitol
is oxidized with periodic acid (HIO4)?
•
A)
0 moles of formic acid: 5 moles of
formaldehyde
•
B)
1 moles of formic acid: 4 moles of
formaldehyde
•
C)
2 moles of formic acid: 3 moles of
formaldehyde
•
D)
3 moles of formic acid: 2 moles of
formaldehyde
OH
3 vicinal OH groups;
consumes 2 mol of HIO4
Reactions of Monosaccharides
Glycoside (acetal) formation
H
OH
OH
O
CH3OH
OH
O
OH
H
OH
HO
+
HO
CH3OH
H
+
H
OH
HOCH2CH
HO
O
+
OCH3
OH
OCH3
OH
OH
Question
• When glucose is allowed to react with
methanol in the presence of a catalytic
amount of acid, to which carbon of the ring
will the -OCH3 be attached?
Formation of Glycosides
The acetal (or ketal) of a sugar is called a glycoside
•
•
•
A)
C)
1 only
B) 1 and 5
1, 2, 3, and 4 D) 2, 3, 4 and 5
Mechanism of Glycoside Formation
The formation of a glycoside favors the α-glucoside
product: the anomeric effect
Formation of an N-Glycoside
Reducing Sugars
A sugar with an aldehyde, a ketone, a
hemiacetal, or a hemiacetal group is a
reducing sugar
Examples of Reducing Sugars
Examples of Reducing Sugars
• Aldoses: because they have an aldehyde
function in their open-chain form.
• Ketoses: because enolization establishes an
equilibrium with an aldose.
O
CH2OH
CHOH
CH
C
C
CHOH
O
R
OH
R
• Disaccharides that have a free hemiacetal
function.
HOCH2
HOCH2
O
O
Maltose
OH
O
HO
HO
OH
HO
OH
R
oxidized by Cu2+
Examples of Reducing Sugars
Glycosides are not reducing sugars
• Disaccharides that have a free hemiacetal
function.
HOCH2
HOCH2
O
Maltose
OH
CH
O
HO
HO
HOCH2
HO
HO
OH
HO
OH
O
OH
oxidized by Cu2+
O
OCH3
Methyl α-D-glucopyranoside lacks a free
hemiacetal function; cannot be in equilibrium
with a species having an aldehyde function
Question
The most common disaccharide sucrose:
Sucrose is not a reducing sugar
• The disaccharide below is maltose.
Maltose is a reducing sugar.
A)
True
B) False
Questions to Consider
• Given a Haworth structure: a) draw the Fisher
structure of the open straight chain, identify it as: eg.
aldose, pentose, reducing, etc. b) draw a Haworth
structure for the opposite anomer of the given
structure, c) draw the most stable chair conformer, is
it the α- or β-?
• Identify an isomer: D- or L- from a structure. How
many possible stereoisomers could the structure
possibly have?
• Give an example of mutarotation.
• Identify/name structures: saccharides, glycosides,
glycosamine, etc.
Carbohydrate Structure
Determination
Carbohydrate Structure
Determination
Monosaccharides
D-glucose beta anomer
O
CHO
H
HO
OH
HO
H
H
OH
H
OH
OH
H
O
OH
OH
OH
all groups are equatorial
or 1,2 - trans to each other
CH2OH
HOCH2
HO
O
HO
OH
OH
•Spectroscopy
•X-Ray Crystallography
– Chemical Tests
once used extensively; now superceded
by spectroscopic methods and x-ray
crystallography
reactions of carbohydrates can involve
either open-chain form, furanose, or
pyranose form
Monosaccharides
Identification: 1H NMR couplings
Examples
1,2-disubstituted cyclohexanes
Coupling Constants
Dihedral angles
Using Coupling Values
• A mixture of anomers of D-glucose was
separated and analyzed by nmr.
• The coupling constants of the epimeric
protons were 3.5 Hz for anomer X and
8.6 Hz for anomer Y.
• Identify the alpha and beta anomer.
Dihedral Angles
The larger coupling constant corresponds to the
larger dihedral angle. Therefore X is alpha.