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CAIE A-Level Chemistry 34.4 Amino Acids

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

34.4 Amino acids 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 ]

34.4 Amino acids 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 ]
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