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CAIE A-Level Chemistry A2 34.4 Amino Acids Questions

Practise amino-acid charge forms, peptide structures and electrophoresis predictions from pH and pI.

Syllabus
2028–2030
Course
Chemistry 9701
Level
A2

Exam points

  • draw protonated, zwitterionic or deprotonated amino-acid structures from the stated pH
  • form peptide bonds in the named residue order and display every –C(=O)–NH– link
  • predict electrophoresis direction from net charge and relative distance from charge magnitude and Mr

Covered syllabus points

34.4.1—Acid / base properties of amino acids34.4.2—Peptide bond formation34.4.3—And predict the results of electrophoresis

Question 1

[Maximum number: 6]

Amino acids are molecules that contain −NH2-\mathrm{NH}_{2} and -COOH functional groups.
Glycine, H2NCH2COOH\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{COOH}, is the simplest stable amino acid.

Question (a)

(a)

The isoelectric point of glycine is 6.2.

[ 2 ]

Question (i)

(i)

Define isoelectric point.

[ 1 ]

Question (ii)

(ii)

Draw the structure of glycine at pH 4 .

[ 1 ]

Question (b)

(b)

Fig. 6.1 shows two syntheses starting with glycine.

Fig. 6.1

Fig. 6.1

[ 4 ]

Question (i)

(i)

State the essential conditions for reaction 1.

[ 1 ]

Question (ii)

(ii)

Identify the reagent used in reaction 2 .

[ 1 ]

Question (iii)

(iii)

A molecule of phenylalanine, R, can react with a molecule of glycine to form two dipeptides, S and T.

S and T are structural isomers.

Figure for Question (iii) — CAIE A-Level Chemistry A2

Draw the structures of these dipeptides. The peptide bond formed should be shown fully displayed.
S
T

[ 2 ]

Question 2

[Maximum number: 7]

Question (a)

(a)

The isoelectric point of asparagine, asn, is at pH 5.4 .

[ 2 ]

Question (i)

(i)

Describe the meaning of the term isoelectric point.

[ 1 ]

Question (ii)

(ii)

Draw the structure of asparagine at pH 1.0.

[ 1 ]

Question (b)

(b)

Asparagine can polymerise to form poly(asparagine).

Draw the structure of poly(asparagine), showing two repeat units. The peptide linkage should be shown displayed.

[ 2 ]

Question (c)

(c)

The isoelectric point of lysine, lys, is at pH 9.8.
lysine

Fig. 6.2

Fig. 6.2

A mixture of the dipeptide lys-asn and its two constituent amino acids, asparagine and lysine, is analysed by electrophoresis using a buffer at pH 5.0 . The results obtained are shown in Fig. 6.3.

Fig. 6.3

Fig. 6.3

Suggest identities for the species responsible for spots E, F and G. Explain your answers.

Table for Question (c) — CAIE A-Level Chemistry A2
[ 3 ]

Question 3

[Maximum number: 11]

Table 4.1 shows the structures of sections of three polymers, X, Y and Z.
Each polymer is made from only one type of monomer.

Table 4.1

Table 4.1

Question (a)

(a)

Amino acids can act as monomers.

State what is meant by the isoelectric point of an amino acid.

[ 1 ]

Question (b)

(b)

Electrophoresis can be used to separate and identify amino acids.

Table 4.3 shows information about the three amino acids glycine, lysine and glutamic acid.

Table 4.3

Table 4.3

[ 10 ]

Question (i)

(i)

A mixture containing these three amino acids is analysed in a buffer solution of pH 6.0 .

Draw and label three spots on Fig. 4.1 to indicate the predicted position of each of these amino acids, gly, lys and glu, after electrophoresis.

Fig. 4.1

Fig. 4.1

[ 2 ]

Question (ii)

(ii)

Electrophoresis is repeated using a buffer solution of pH 11 .

Predict how the position of glycine will change, if at all, after electrophoresis.

[ 8 ]

Question 4

[Maximum number: 3]

The isoelectric point of lysine, lys, is at pH 9.8.
lysine

Fig. 6.2

Fig. 6.2

A mixture of the dipeptide lys-asn and its two constituent amino acids, asparagine and lysine, is analysed by electrophoresis using a buffer at pH 5.0 . The results obtained are shown in Fig. 6.3.

Fig. 6.3

Fig. 6.3

Suggest identities for the species responsible for spots E, F and G. Explain your answers.

Table for Question 4 — CAIE A-Level Chemistry A2
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