Organic analysis — Question 5

5a)

Spot P is glutamic acid (Glu), Q is cysteine (Cys) and R is arginine (Arg). At pH 5, Glu is negatively charged and so would move towards the positively charged electrode. This is because the buffer’s pH is higher than the pH of Glu’s isoelectric point. At pH 5, Arg is positively charged and so would move to the negative electrode. This is because the buffer’s pH is much less than the pH of Arg’s isoelectric point. Cys has little charge due to the pH of the buffer and isoelectric point being similar. This means that the molecule would not move very far towards the negative electrode.

• determines that spot P is Glu, spot Q is Cys and spot R is Arg [1 mark]

• explains link between the buffer’s pH and the pH of the isoelectric point of Arg and Glu [1 mark]

• explains link between the charge of Arg or Glu and their positions in the gel [1 mark]

• explains link between the buffer’s pH and the pH of the isoelectric point of Cys and its position in the gel [1 mark]

5b)

Spot X is between spot Q and R, so it must be from an amino acid with an isoelectric point between 5 and 10.7. Spot X is closer to spot R; therefore its isoelectric point is close to 10.7. Therefore, spot X is lysine, with a pH of 9.7 at its isoelectric point.

• identifies spot X as Lys [1 mark]

• explains link beyween the pH of the isoelectric point of the amino acid chosen with reference to the isoelectric point of Cys or Glu or Arg [1 mark]