Carbohydrates Flashcards
Describe saccharides?
They are the most abundant biological molecules
(CH2O)n where n>3
Monomer = monosaccharides
Polymer = polysaccharide
What are Fischer projections?
All vertical bonds point into the plane of the paper
All horizontal bonds point out of the plane of the paper
The assignment of D- or L- is based on the absolute configuration of the asymmetric centre farthest from their carbonyl group
D-sugars are the most biologically abundant form
What are monosaccharides?
Aldehyde or ketone derivatives - containing at least 3 carbon atoms
If the C=O group is an aldehyde, the sugar = aldose
If the C=O group is a ketone, the sugar = ketose
What are the names given to carbohydrates per number of carbons in the chain?
3C = trioses 4C = tetroses 5C = pentoses 6C = hexoses 7C = heptoses
Describe aldoses
For aldohexose sugars all but two of its carbons are chiral
= 16 possible stereoisomers (2^4)
e.g. D-Glucose, D-Mannose and D-Galactose
Describe ketoses?
For ketohexose sugars three of its carbons are chiral
= 8 possible stereoisomers (2^3)
e.g. D-Fructose
What are epimers?
Differ only in the configuration around one carbon and as bonds need to be broken Interconversion can’t take place spontaneously
How can monosaccharides be altered in a reaction?
They can react with alcohols
Alcohol + Aldehyde = Hemiacetal
Alcohol + Ketone = Hemiketal
How can the alcohol reactions with monosaccharides also take place?
The hydroxyl group OH, within the monosaccharide can react with it’s own C=O group (intramolecularly) to form cyclic hemiacetal and hemiketals
How are cyclic hemiacetals and hemiketals represented?
Haworth projections
The heavier ring bonds project in front of the plane of the paper
The lighter ring bonds project behind it
What are the names given to the cyclic molecules formed after intramolecular reaction with OH?
Five-membered rings = furanoses
e.g. D-Fructofuranose
Six-membered rings = pyranoses
e.g. D-Glucopyranose
What is an anomer?
Within a cyclised monosaccharide, the carbonyl carbon = anomeric carbon, is now the chiral centre with 2 configurations
The 2 stereoisomers differing at the anomeric carbon are anomers
What are the 2 anomers?
alpha - OH groups:
Down, up, down, down
beta - OH groups:
Down, up, down, up
What is significant about the two anomers of D-glucose?
They are diastereoisomers
They have slightly different physical and chemical properties (including optical rotations)
The anomers freely interconvert in aqueous solution = mutarotation
Describe the structure of the rings?
Tetrahedrally (sp3) hybridised
Chair conformation (not planar)
The most stable form is the bulky substituent groups (OH and CH2OH) occupying equitorial positions - extending alternately above and below the ring
How can sugars be modified and linked further?
- Aldoses are oxidised, the aldehyde is oxidised to a carboxylic acid = aldonic acid (-onic acid at the end) e.g. D-Gluconic acid
- Oxidation of primary alcohol group of aldoses = -uronic acids e.g. D-Glucuronic acid
- Aldoses and ketoses can be reduced under mild conditions (e.g. NaBH4) yeilds polyhydroxy alcohols = alditols (-itol) e.g. ribitol
- Monosaccharide units with one OH group replaced by H are known as deoxysugars
- In amino sugars one or more OH groups can be replaced with NH2 - often then acetylated
e. g. D-Glucosamine
How are sugars linked to other compounds?
Glycosidic bonds - link the anomeric carbon to other compounds
The anomeric group can condense with an alcohol to form alpha or beta glycosides and releasing H2O
Give an example of a glycosidic bond where a sugar is linked to another compound?
Formation of an N-glycosidic bond between the anomeric carbon and an amine
The link of D-ribose to purines and pyrimidines in nucleic acids
Describe glycosidic bonds?
They hydrolyse very slowly under physiological conditions but fast at a low pH
If an anomeric carbon is involved in a glycosidic bond it cannot convert between a and b forms via linear form = aldehyde group is not available to react = non-reducing sugar
Saccharides with anomeric carbons, that have not formed glycosides = reducing sugars
Describe polysaccharides?
Consist of monosaccharides linked by glycosidic bonds
AKA Glycans
Classified as homopolysaccharides and heteropolysaccharides, consisting of one type or more than one type of monosaccharide
Form branched and linear polymers
What are disaccharides?
The simplest polysaccharides - many occuring as the hydrolysis of larger molcules
Examples:
b-galactose and glucose form lactose (found naturally in milk)
a-glucose and b-fructose form sucrose (highly abundant in plants, table salt)
How do we systematically name sugars?
For Lactose:
- Name the two component monosaccharides: galacto- gluco-
- Describe the ring types: pyranosyl, pyranose (the glucose is still a reducing sugar = ose at the end)
- What are the anomeric forms: b (galactose), glucose can be a or b so leave out
- How are they linked: 1→4
Therefore = O-b-D-galactopyranosyl-(1→4)-D-glucopyranose
What are some polysaccharides?
Structural:
Cellulose and Chitin
Storage:
Starch and Glycogen
Describe the structure of cellulose?
Linear polymer of up to 15,000 D-glucose residues linked by b(1->4) glycosidic bonds
Cellulose fibres are chains of flat ribbons in which successive glucose rings are flipped 180 ° relative to the previous glucose ring - allowing H2O removal
C3-OH of each glucose forms a H-bond with the ring oxygen (O5) of the next glucose
Parallel cellulose chains form sheets with interchain H-bonds - microfibril
Contains lignin – a plastic-like phenolic polymer, to form a matrix of the cell wall = withstand large stress
