9 - CARBOHYDRATES I Flashcards
Carbohydrate
Carbohydrates have the basic formula (CH2O)n
Saccharides is another name for carbohydrates
Monosaccharides
The simplest carbohydrates are molecules with one aldehyde or ketone group and multiple hydroxyl groups
Monosaccharide Terminology
They can be classified in two ways:
i) Depending on whether they have a aldehyde or ketone group = aldoses and ketoses, respectively
ii) Depending on the number of carbon atoms they contain: 3 = triose; 4 = tetrose; 5 = pentose; 6 = hexose 7 = heptose
E.g. Glucose is an aldose (aldehyde group) and a hexose (6 carbons), so is an aldohexose
Monosaccharides
The simplest monosaccharides are 3 carbon trioses:
- glyceraldehyde : aldotriose
- dihydroxyacetone: ketotriose
Optical Isomers
Glyceraldehyde has one chiral carbon atom which means there are two optical isomers or enantiomers
• Most naturally occurring monosaccharides are in the D-form
• A molecule with n asymmetric centres (chiral atoms) and no plane of symmetry has 2^n isomers
Cyclisation
In solution, pentoses and hexoses cyclise to form rings
Aldohexoses usually form PYRANOSE rings
The cyclisation forms a new asymmetric carbon at C-1 called the ANOMERIC carbon.
2 different configurations can exist, a and b.
The a / b isomers are called ANOMERS and, in solution, can interconvert via the open-chain form in a process called MUTAROTATION
What type of rings do ketohexoses form?
FURANOSE rings
For ketoses, the anomeric carbon is always C-2
Mutarotation – The Proof
Plane polarised light passing through 10cm of a 1g/ml solution of beta-D-glucopyranose is rotated through +18.7 degrees
The alpha anomer rotates the light through +112 degrees
This is called OPTICAL ROTATION
Conformation of Monosaccharides
Pyranose rings adopt CHAIR and BOAT conformations.
Furanose rings adopt ENVELOPE conformations.
