Amino Acids (Cambridge (CIE) A Level Chemistry): Exam Questions

Much research has been carried out in recent years investigating the exact structure of silk. The silk of a spider's web is at least five times as strong as steel, and twice as elastic as nylon. The silk fibre is composed of many identical protein chains, which are mainly made from the amino acids glycine, alanine and serine, shown in Fig. 1.1, with smaller amounts of four other amino acids.

Fig. 1.1

Amino acids can exist as zwitterions. Draw the zwitterion structure for glycine.

Amino acids can act as acids or bases. Write equations to show:

i) the reaction between alanine and HCl (aq),

[1]

ii) the reaction between serine and NaOH (aq).

[1]

i) Draw the structural formula of a portion of the silk protein, showing three main acid residues. Label a peptide bond on your structure.

[3]

ii) State the polymer type of silk protein.

[1]

The M_r of a silk protein molecule is about 600,000. Assuming it is made from equal amounts of the three amino acids in Fig. 1.1, calculate the average number of amino acid residues in the protein chain. Show your working.

[M_r (glycine) = 75;   M_r (alanine) = 89;     M_r (serine) = 105]

number of residues .............................................

A mixture of amino acids may be separated using electrophoresis. A typical practical set-up is shown in Fig. 2.1.

Fig. 2.1

When the power supply is switched on, some amino acids may not move, but remain stationary. Suggest an explanation for this observation.

The amino acid glycine has the formula H₂NCH₂CO₂H. Identify the species formed on the filter paper if glycine moves to the left (positive) end of the filter paper.

The result shown in Fig. 2.2 was obtained from another electrophoresis. 

Fig. 2.2

What can be deduced about the relative sizes of, and charges on, the amino acid species A, B and C?

A tripeptide was formed from three different amino acids, alanine, serine and valine. The structural formulae of the amino acids are shown in Table 2.1.

Table 2.1

i) How many different tripeptides can be made using one molecule of each of the amino acids in Table 2.1?

[2]

ii) Draw the tripeptide, showing the peptide bonds in displayed form.

[1]

Fig. 3.1 shows the structures of the amino acids, alanine and valine.

Fig. 3.1

i) Give the systematic name for valine.

[1]

ii) Draw the species formed when alanine is in a solution of high pH.

[1]

Alanine reacts with propanol, CH₃CH₂CH₂OH in the presence of concentrated sulfuric acid, H₂SO₄. 

Name the type of reaction and draw the fully displayed structure of the product.

The isoelectric point of alanine is at pH 6.0.

i) What is meant by the term isoelectric point.

[1]

ii) Draw the structure of alanine at pH 6.0.

[1]

Valine exists as a pair of stereoisomers.

Explain the meaning of the term stereoisomers and how the stereoisomers of valine can be distinguished from one another. 

The amino acid alanine is shown below.

alanine

Alanine will react with excess chloromethane. Draw the structure of the product formed in this reaction.

Alanine can also react with serine to produce two different dipeptides. The structure of serine is shown below.

serine

Draw both dipeptides formed in this reaction.

Explain the conditions in which alanine exists as a solid and draw the structure of the amino acid in these conditions. 

The isoelectric point is the pH at which an amino acid exists as a zwitterion. The isoelectric point of valine is 6.0 and of lysine is 9.8 and their structures are shown below.

valine                                                           lysine

A mixture of the dipeptide, val-lys, and its two constituent amino acids, valine and lysine, was analysed by electrophoresis using a buffer at pH 6.0.

Draw and label three spots on the diagram in Fig. 1.1 of the electrophoresis paper to indicate the likely position of each of these species after electrophoresis. Explain your answer.

Fig. 1.1

Tryptophan is a naturally occurring α-amino acid. Its structure is shown below.

tryptophan

Draw a circle around the R group in this amino acid

Deduce the molecular formula for tryptophan and write a balanced equation for the reaction of tryptophan with dilute nitric acid.

Draw the structures for the organic product of the reaction of tryptophan and the following substances:

i) NaOH (aq)

[1]

ii) C₂H₅OH and concentrated sulfuric acid.

[2]

Two more naturally occurring amino acids are leucine and isoleucine shown below:

State how many optical isomers there are for leucine and isoleucine.