Questions on Stereochemistry in biomolecules

1. (a) Draw 3-D representations of the two enantiomers of cysteine that show clearly that they are mirror images of each other.

(b) Identify, with a reason, which of the two configurations you have drawn represents the only one found in proteins.

(c) List all the 2-amino acids in Section 33 of the data booklet that either contain no chiral carbon atoms or more than one chiral carbon atom.

2. Cis-retinal is produced in the retina from vitamin A. The structures of vitamin A and cis-retinal are given in Section 35 of the data booklet.

(a) State the two types of reaction that occur when vitamin A is converted into cis-retinal.

(b) Explain why both vitamin A and cis-retinal absorb light in the visible region of the spectrum but only cis-retinal absorbs at approximately 1700-1750 cm^-1 in the infrared region of the spectrum.

(c) Light can convert cis-retinal to trans-retinal. Discuss the significance of this reaction in the visual cycle.

3. (a) Starch exists in two different forms, amylose and amylopectin. Discuss how both are formed from glucose and state how they differ structurally.

(b) Cellulose is another polysaccharide formed from glucose. Discuss how cellulose is formed from glucose and how its structure differs from amylose.

(c) State one important function of cellulose in plants and one important function of cellulose in human diet.

4. (a) The straight chain structure of fructose is given in Section 34 of the data booklet.
Explain whether the structure given is for D-fructose or L-fructose.

(b) In solution, fructose exists mainly as an equilibrium mixture consisting of a six-membered ring called fructopyranose and a five-membered ring called fructofuranose.

Both of the structures shown are the β form. Draw the Haworth projections for the α forms of D-fructopyranose and D-fructofuranose.

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