HL Chemistry - Option B: Human Biochemistry. Carbohydrates. “The Discovery of Honey” by Piero de Cosimo (1462). Part 1. Overview of Carbohydrates. General Characteristics . The term carbohydrate is derived from the French: “hydrate de carbone”
“The Discovery of Honey” by Piero de Cosimo (1462)
Overview of Carbohydrates
Carbohydrates can be:
Carbohydrates can be classified by size:
Aldoses (e.g. glucose) have an aldo (aldehyde) group at one end
Ketoses (e.g. fructose) have a keto (ketone) group (usually at C2)
D & L designations are based on the configuration about the single asymmetric C in glyceraldehyde
The lower diagrams are Fischer Projections.
For sugars with more than one chiral center, D or Lrefers to the asymmetric C farthest from the aldehyde or keto group (in yellow)
Most naturally occurring sugars are D isomers
images of one another
They have the same root
name (but a different
have unique names,
(e.g. glucose, mannose,
The number of stereoisomers is 2n, where n is the number of asymmetric (chiral) centers
The 6-C aldoses have 4 asymmetric centers. Thus there are 16 stereoisomers (8 D-sugars and 8 L-sugars).
Biomolecules (in this case sugars) can be represented in three main ways (visualized in the following slides):
Glucose forms an intra-molecular hemiacetal, as the C1 aldehyde & C5 OH react, to form a 6-member pyranose ring, named after pyran
The representations of the cyclic sugars (bottom) are called Haworth Projections
Haworth projections represent the cyclic sugars as having essentially planar rings, with the OH at the anomeric C1:
The representation above reflects the chair configuration of the glucopyranose ring more accurately than the Haworth projection
Invert sugar -19.8
1. The nature of the compound
2. The length of the tube (cell or sample container) usually expressed in decimeters (dm)
3. The wavelength of the light source employed; usually either sodium D line at 589.3 nm or mercury vapor lamp at 546.1 nm
4. Temperature of sample
5. Concentration of carbohydrate in grams per 100 ml
and selected derivatives
The anomeric hydroxyl and a hydroxyl of another sugar or some other compound can join together, splitting out water to form a glycosidic bond:
R-OH+ HO-R'R-O-R' + H2O
e.g. methanol reacts with the anomeric OH on glucose to form methyl glucoside (methyl-glucopyranose).
Maltose, a cleavage product of starch (i.e. amylose), is a disaccharide with an a(1®4) glycosidiclink between the C1 - C4 OH’s of 2 glucoses. It is the a anomer (C1 O points down)
Cellobiose, a product of cellulose breakdown, is the otherwise equivalent banomer (O on C1 points up).
The b(1®4) glycosidic linkage is represented as a zig-zag, but one glucose is actually flipped over relative to the other
Because the configuration at the anomeric C of glucose is a (O points down from ring), the linkage is a(12)
The full name of sucrose is a-D-glucopyranosyl-(12)-b-D-fructopyranose.)
Another view of amylose and amylopectin, the two forms of starch. Amylopectin
is a highly branched structure, with branches occurring every 12
to 30 residues
Glycogen, the glucose storage polymer in animals, is similar in structure to amylopectin, but glycogen has more a(16)branches
Cellulose, a major constituent of plant cell walls, consists of long linear chains of glucose with b(1®4) linkages.
(chitin is found in the exoskeleton of insects, crayfish, etc]
(these are the two most abundant polysaccharides in nature)
(Notice the presence of “sheets” that can be pealed away. Think about a piece of celery and how you can strip off the fibers)
in water adopt a helical
Iodine (I2) can insert in
the middle of the amylose
helix to give a blue color
that is characteristic and
diagnostic for starch
(b) The structure of cellulose– shows b linkages
Oligosaccharides that are covalently attached to proteins or to membrane lipids may be linear or branched chains
O-linked oligosaccharidechains of glycoproteins vary in complexity.
O-linked glycoproteins are found in the blood of Arctic and Antarctic fish, enabling them to live at sub-zero water temperatures
N-acetylglucosamine (GlcNAc) is a common O-linked glycosylation product of serine or threonine residues
N-linked oligosaccharides of glycoproteins tend to be complex and branched. First N-acetylglucosamine is linked to a protein via the side-chain N of an asparagine residue in a particular 3-amino acid sequence.