Week 27 - biological molecules

Question 1

Carbohydrates and classification

Answer 1

The main functions are:
-Storage of energy (glucose)
-Recognition and interaction (blood types)
-Cellular structures/membranes (cell wall)
Molecular formula of Cn(H2O)m
->A modern definition of carbohydrates includes polyhydroxy- aldehydes, polyhydroxy- ketones and compounds that are easily hydrolysed to them

Question 2

Monosaccharides

Answer 2

Known as simple sugars: they cannot be hydrolysed to more simple compounds - they
are single poly-hydroxy aldehydes (aldoses) or ketones (ketoses)
Empirical formula - Cn(H2O)m
->Glucose and fructose have the same chemical formula but different arrangement of
carbon atoms and the bonds between them (structural isomers)

Question 3

Fisher projections

Answer 3

Asymmetric carbons (*) with 4 different substituents (hashed and wedged bonds) -> each C with a specific stereochemistry (R and S)
->Fisher projection is a convention used to depict a stereoformula in two
dimension without destroying the stereochemical information, i.e., absolute configuration, at chiral centres

Question 4

L and D configurations

Answer 4

-Monosaccharides come in two stereochemical forms: L and D sugars
-Naturally occurring sugars are in their D configuration
->L and D configuration derives from the stereochemical configuration of the highest
numbered chiral carbon (the asymmetric carbon farthest from the carbonyl group –
aldehyde for aldose sugars and ketone for the ketose sugars)
On a Fisher projection of glucose, C1 is the aldehyde, C5 is the highest numbered chiral carbon
D sugar: OH group of the right side of the projection (D-right)
L sugar: OH group of the left side of the projection (L-left)
D-glucose is the enantiomer of L-glucose, they differ for the
stereochemistry of the final chiral carbon C5

Question 5

Epimers

Answer 5

Three criteria guide the classification of monosaccharides:
1) Whether the sugar contains a ketone or an aldehyde group
(aldoses and ketoses) – both sugars are aldoses
2) The stereochemical configuration of the asymmetric carbon atom farthest from the carbonyl group (D or L sugar) – both sugars have D configuration (OH on the right side of the Fisher projection)
he number of carbon atoms in 3) The carbon chain - triose (three carbons); tetrose (four carbons); pentose (five carbons);
hexose (six carbons); heptose (seven carbons) - both sugars
are hexose

Question 6

Epimers

Answer 6

Sugars that differ only by the stereochemistry at a single carbon are called epimers and the carbon atom where they differ is generally stated (C2 epimers)

Question 7

Reactions of carbohydrates

Answer 7

Hemiacetal formation: functional group interconversion of aldehyde to hemiacetal
->Hemiacetal contains one –
OH and one –OR group