Simple carbohydrates

Question 1

What are the classes of carbohydrates (4)

Answer 1

1. Simple - e.g. sugar
2. Complex - e.g. starch
3. Aldose - CHO (carbon, hydrogen, oxygen) group, e.g. ribose → aldopentose (5 carbon)
4. Ketose - CO (carbon, oxygen) group, e.g. fructose → ketohexose (6 carbon)

Question 2

What are the structural aspects of simple carbohydrates (3)

Answer 2

1. Chrial → mirror image, cannot be superimposed (placed over each other and aligned exactly)
2. D-sugars (dextro-sugars) → hydroxyl group at the bottom chiral centre pointing to the RIGHT
3. L-sugars (levo-sugars) → hydroxyl group at the bottom chiral centre pointing to the LEFT

Question 3

How do cyclic structures of monosaccharides form hemiacetals (3)

Answer 3

1. Alcohol + aldehyde → hemiacetal (double bond breaks, H binds to O, OR2 binds to central carbon)
2. Alcohol + ketone → hemiketal (double bond breaks, H binds to O, OR2 binds to central carbon)
3. If the Carbonyl (C=O) and hydroxyl (OH) groups are in the same molecule, intramolecular nucleophilic addition is needed to form a cyclic hemiacetal.

Question 4

what is mutarotation in monosaccharide anomers and an example (3)

Answer 4

1. Optical Rotation is the ability of a chiral molecule to rotate the plane of polarised light.
2. Mutarotation is the change in optical rotation observed when a solution of a pure single sugar anomer equilibrates to a mixture of anomers.
3. e.g. Alpha-glucose at 112º decreases to 52° at eq’m. Beta-glucose at 19º increases to 52° at eq’m

Question 5

What are the important aspects of disaccharides (5)

Answer 5

1. The reaction of a monosaccharide with an alcohol yields a glycoside.
2. If the alcohol is itself a sugar, the glycosidic product is a disaccharide.
3. Disaccharides contain a glycosidic bond between the anomeric carbon (the carbonyl carbon) of one sugar and an -OH group at any position on the other sugar.
4. Glycosidic bonds between the first carbon of the first sugar and the -OH at the 4th carbon of the second sugar are very common. This is a 1-4’ link.
5. Glycosidic bonds can be either alpha (pointing down) or beta (pointing up)

Question 6

What is ABBA (2)

Answer 6

1. Alpha - below
2. Beta - Above