Lecture Nine - Carbohydrates I Flashcards
What is the definition of a carbohydrate?
A polyhydroxyaldehyde or a polyhydroxyketone, or a substance that gives these compounds upon hydrolysis.
General formula = Cn(H2O)m - BUT not all carbohydrates have this general formula.
E.g. Glucose = C6H12O6 or C6(H2O)6
E.g. Sucrose C12H22O11 or C12 (H2O)11
Monosaccharide:
A source of energy in metabolic processes.
A carbohydrate that cannot be hydrolysed to a simpler carbohydrate.
They have a general formula CnH2nOn. Where n varies from 3 to 8.
Aldose = A monosaccharide containing an aldehyde group.
Ketone = A monosaccharide containing a ketone group.
Monosaccharides are name according to how many carbons they contain.
E.g. Triose = 3 carbons, tetrose = 4 carbons, pentose = 5 carbons.
PICTURE.
How many trioses are there?
There are only two trioses.
Often aldo- and keto- are omitted and these compounds are referred to simply as trioses, although this designation does not reveal the nature of the carbonyl group, it does reveal the number of carbons involved in the structure.
PICTURE.
What is Fischer projection?
Fischer projection: A two dimensional, open chain representation for showing the configuration of tetrahedral stereo-centres.
Horizontal lines represent bonds projecting forwards.
Vertical lines represent bonds projecting into the page.
The most oxidised part points up.
Longest carbon chain is placed at the bottom, vertically.
PICTURE.
What is the D, L convention?
According to the conventions proposed by Fischer:
D monosaccharide - A monosaccharide that, when written as a Fischer projection, has the -OH on its penultimate (second last) carbon on the right.
The D monosaccharides are the naturally occurring carbohydrates, as opposed to amino acids, where the L amino acids are the naturally occurring ones.
L monosaccharide - A monosaccharide that, when written as a Fischer projection, has the -OH on its penultimate carbon on the left.
PICTURE.
What is the formula that can be used to work out how many stereoisomers a molecule with a given number of stereo-centres has?
2^n, where n is the number of stereo-centres.
PICTURE.
What are hemiacetals?
A molecule containing an -OH and an -OR or an -OAr group bonded to the same carbon atom.
Compounds derived from aldehydes and keytones.
The open form of a sugar can be in equilibrium with one or more ring forms.
C6 monosaccharides have hydroxyl and carbonyl groups in the same molecule and exist almost entirely as five- and six-membered cyclic hemiacetals.
Five- and six-membered hemiacetals are represented as planar pentagons or hexagons, as the caee may be, viewed through the edge.
Most commonly drawn with the anomeric carbon on the right and the hemiacetal oxygen to the back right.
PICTURE.
What are beta and alpha molecules?
The designated beta means that the -OH on the anomeric carbon is cis to the terminal -CH2OH, alpha means that it is trans.
Anomeric carbon - the new stereo centre resulting from cyclic hemiacetal formation.
Anomers - carbohydrates that differ in configuration at their anomeric carbons.
The molecule is more stable in the beta position, as in the alpha position, the OH interacts with the OH attached to the second carbon in the ring.
PICTURE.
