Chapter #10: Carbohydrates Flashcards
What are carbohydrates?
- They are aldehydes or ketones with multiple hydroxyl groups.
- The general formula is (CH2O)n, i.e. C3H6O3, C6H12O6
Describe the Fischer projections of carbohydrates
Horizontal bonds to an asymmetric carbon are in front of the page
Vertical bonds to an asymmetric carbon are behind the page
Numbering starts from the end of the molecule with the carbonyl group.
What do D and L designations refer to on a Fischer proection?
D and L designations refer to the absolute configuration of the asymmetric carbon farthest from the carbonyl group (also known as the chiral carbon, determining the D or L designation).
NOTE: Simply switching the configuration at the chiral carbon of a D sugar does not create the L version of the named sugar. By definition ALL the chiral centers have to be flipped to create a mirror image (i.e. D- and L-glucose). Switching the configuration at the chiral carbon alone does create an L-sugar, but it is some other L-sugar.
What are isomers?
Isomers: Molecules with the same chemical formula in which the atoms are arranged differently.
Constitutional, or structural, isomers differ in the order of attachment/connectivity of atoms
What are stereoisomers?
Isomeric molecules whose atomic connectivity is the same but whose atomic arrangement in space is different.
What are enantiomers?
Stereoisomers that are non-superimposable, complete mirror images of each other
What are diastereoisomers?
Stereoisomers that are not mirror images of each other.
What are epimers?
A stereoisomer that has a different configuration at only one of several chiral carbon centers.
What is the difference between ribose and deoxyribose?
They do not have the same chemical formula, thus, they are not isomers. But know their structures, as we will study them later.
What are tautomers?
structural isomers that differ in the position of the protons and electrons.
Why are carbohydrates not usually open chains?
*The predominant forms of many sugars are not open chains, by cyclic ring structures which are energetically more stable.
*An aldehyde can react with an alcohol to form a hemiacetal.
*A ketone can react with an alcohol to form a hemiketal.
How is an intramolecular hemiacetal made?
The C-5 hydroxyl attacks the C-1 (aldehyde) to form an intramolecular hemiacetal.
How is an intramolecular hemiketal made?
- The C-5 hydroxyl attacks the C-2 (ketone) to form an intramolecular hemiketal.
Summarize how intramolecular hemiacetal or hemiketal are made cyclical, respectively.
The C-5 hydroxyl attacks the C-1 (aldehyde) or C-2 (ketone) to form an intramolecular hemiacetal or hemiketal respectively.
How do you switch from Fischer to Haworth projections
If a substituents points to the right in the Fisher structure, it points down in the Haworth.
If it points left in the Fisher structure, it points up in the Haworth.
The OH on the α-anomer points down (ants down), while on the β-anomer it points up (butterflys up).
What are anomers?
isomers that differ at a new asymmetric C atom formed on ring closure (define α and β).
Summarize the Haworth Projection
- Carbon atoms in the ring are not shown
- Plane of the ring is roughly perpendicular to the plane of the paper
- Heavy line on the ring projects toward the reader
- Thin line on the ring projects away from the reader
- Hydrogen atoms might not be shown
Describe an example of glucose cyclization?
How are disaccharides made?
- Monosaccharides can be joined by O-glycosidic bonds, very commonly between the C-1 and C-4 carbon atoms.
- If the sugar on the C-1 side is in the α configuration (bond is below plane of the ring), it is an α-1,4 glycosidic bond, other option is β-1,4 glycosidic bond.
- Combining two sugars together creates a disaccharide.
Describe sucrose
α −D-Glucopyranosyl-(1 -> 2)-β-D-fructofuranose
*Table sugar
*Not a reducing sugar
*Most important sugar in plants
Describe lactose
β −D-Galactopyranosyl-(1 -> 4)-α-D-glucopyranose
*Milk disaccharide
*Glycosidic bond = β-1,4 *Cleaved by:
* lactase (humans)
* β-galactosidase (bacteria)
Describe Maltose
α−D-Glucopyranosyl-(1 -> 4)-α-D-glucopyranose
*Produced by the hydrolysis of starch
*Cleaved by maltase
*Obtained by malting of grains (used in brewing).
What are polysaccharides?
- Combining several monosaccharides together creates an oligosaccharide, and beyond that length they are typically called polysaccharides.
What are glycogen and starch?
Glycogen and starch are all homopolymers of glucose.
How are most glucose units connected?
Most of the glucose units are connected by α-1,4-glycosidic bonds, and branches are formed by α-1,6-glycosidic bonds.
What is glycogen?
Glycogen is the major homopolymer used for storage of glucose in animal cells, with branch points occurring at approximately every 10 glucose units.
What is starch?
Starch, cleaved by α-amylase, is the nutritional reservoir of carbohydrates in plants, and the source of the majority of human carbohydrates. Starch is composed of amylose & amylopectin. Amylose has no branching, while amylopectin has branch points occurring at approximately every 30 glucose units
What is cellulose?
- Cellulose is an unbranched form of glucose that serves a structural role in plants.
- Unlike glycogen and starch, cellulose contains β-1,4-glycosidic bonds – a linkage optimized for high strength which forms very long (tens of thousands of glucose units in length) straight chains.
- Fibrils are formed by parallel chains that interact through hydrogen bonding.
What is the take home message carbohydrates?
- Carbohydrates share a specific structure that can be summarized with the general formula (CH2O)n.
- Carbohydrates with the same formula are isomers.
- Carbohydrates are mainly present in cells in a cyclic form.
- Carbohydrates like glucose or galactose are monomers, that can form more complex carbohydrate polymers, via the generation of glycosidic bonds.
- Polysaccharides play a central role as the storage mechanism of carbohydrates
