Polymers and biomolecules — Question 6

6a)

Peptide bond Water is formed

• identifies peptide bond [1 mark]

• identifies water is formed [1 mark]

6b)

6 tripeptides can be formed: Gly-His-Lys and Gly-Lys-His

• determines 6 tripeptides can be formed [1 mark]

• describes one tripeptide [1 mark]

• describes a second tripeptide [1 mark]

6c)

The isoelectric point of His is 7.6. If the pH of the buffer solution is 7.6 histidine will not migrate towards either the anode or cathode because it remains neutral. The isoelectric point Gly is 6, which is less than 7.6. Therefore, in a buffer solution of 7.6 Gly will form a negative ion and migrate towards the anode. Similarly, the isoelectric point for Lys is 9.7, therefore, Lys will form a positive ion and migrate towards the cathode.

• explains when buffer solution is pH 7.6 - histidine is neutral and will not migrate [1 mark] - Gly forms a negative ion and migrates towards anode (positive terminal) [1 mark] - Lys forms a positive ion and migrates towards the cathode (negative terminal) [1 mark]