A-LevelChemistryCarboxylic acids and derivatives9701/42 · Oct/Nov 2018 · Q95 Marks

(a) Use information from the Data Booklet to draw the structure of the dipeptide glu-cys. The isoelectric point is the pH at which an amino acid exists as a zwitterion. The isoelectric point of glutamic acid is 3.2 and of cysteine is 5.0. A mixture of the dipeptide glu-cys and its two constituent amino acids, glutamic acid and cysteine, was analysed by electrophoresis using a buffer at pH 5.2. The results obtained are shown. [Figure 9.0]

(a)Use information from the Data Booklet to draw the structure of the dipeptide glu-cys.[2]
(b)Suggest identities for the species responsible for spots E, F and G. Explain your answers. spot | identity --- | --- E | F | G | explanation[3]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2018). The mark or mark type is shown in brackets.

[1] M1 peptide link
[1] M2 rest of the structure
[1] M1 correct table
[1] M2 Explanation of why Cys moves the least - because it exists as a zwitterion / it is almost neutral
[1] M3 Explanation of why Glu-Cys moves a smaller distance than Glu – a comparitive statement that Glu-Cys has a greater Mr than Glu

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