Syllabus Edition
First teaching 2023
First exams 2025
This question is about amino acids.
State the general formula for an amino acid.
The amino acid alanine is shown in Fig. 1.1
Draw the two structures of alanine in acidic and basic conditions.
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Electrophoresis is an analytical technique which separates ions by placing them in an electrical field.
Describe briefly how a sample of an amino acid can be separated using this technique.
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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.
| Dipeptide 1 |
Dipeptide 2 |
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
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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
Draw the structures for the organic product of the reaction of tryptophan and the following substances:
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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:
[1]
[1]
[3]
[1]
3 marksThe Mr 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.
[Mr (glycine) = 75; Mr (alanine) = 89; Mr (serine) = 105]
number of residues .............................................
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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?
| amino acid | relative size | charge |
| A | ||
| B | ||
| 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.
| amino acid | structural formulae |
| alanine | H2NCH(CH3)CO2H |
| serine | H2NCH(CH2OH)CO2H |
| valine | H2NCH(CH(CH3)2)CO2H |
[1]
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Fig. 3.1 shows the structures of the amino acids, alanine and valine.
Fig. 3.1
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.
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.
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