| Date | May 2017 | Marks available | 1 | Reference code | 17M.3.sl.TZ2.7 |
| Level | SL | Paper | 3 | Time zone | TZ2 |
| Command term | Deduce | Question number | 7 | Adapted from | N/A |
Question
The structures of the amino acids cysteine, glutamine and lysine are given in section 33 of the data booklet.
Deduce the structural formula of the dipeptide Cys-Lys.
Identify the type of bond between two cysteine residues in the tertiary structure of a protein.
Deduce the structural formula of the predominant form of cysteine at pH 1.0.
A mixture of the three amino acids, cysteine, glutamine and lysine, was placed in the centre of a square plate covered in polyacrylamide gel. The gel was saturated with a buffer solution of pH 6.0. Electrodes were connected to opposite sides of the gel and a potential difference was applied.
Sketch lines on the diagram to show the relative positions of the three amino acids after electrophoresis.
Markscheme
correct order
amide link
Accept CO–NH but not CO–HN for amide link.
Penalize incorrect bond linkages or missing hydrogens once only in 7 (a) and 7 (c).
[2 marks]
covalent
Accept “S-S/disulfide”.
[1 mark]
Penalize incorrect bond linkages or missing hydrogens once only in 7 (a) and 7 (c).
[1 mark]
Cys and Gln move to positive electrode AND Lys to negative electrode
Cys further to positive electrode than Gln
Do not penalize if lines are omitted or if different markings are given (eg, spots etc.), as long as relative positions are correctly indicated.
Accept Gln on original position indicated.
Award [1 max] for reverse order of amino acids.
[2 marks]
