Question

The electrophoresis of a mixture of histidine (His), aspartic acid (Asp) and phenylalanine (Phe) is carried out at a pH of 5.91. Match the amino acid to its predicted behaviour under electrophoresis at that pH.

1. This amino acid will bear no net charge at pH 5.91 and so will remain at the origin

2. This amino acid will carry a net negative charge at pH 5.91 and so will migrate towards the positive electrode

3. This amino acid will carry a net positive charge at pH 5.91 and so will migrate to the negative electrode

I need to match each amino acid to either 1, 2 or 3 but am not quite sure where to start on the question

Answer 1

I know that the pI (isoelectric points) for each amino acid are as follows:

His = pI 7.64
Asp = pI 2.98
Phe = pI 5.91

So I think that Phe will bear no net charge at 5.91 and will remain at the origin because it is at the pH where it is likely to be attracted to a positive or negative electrode equally. I am not sure what would happen with His and Asp though

Answer 2

I think that:

Phe = remain at origin (1)
Asp = migrate towards positive electrode (2)
His = migrate towards negative electrode (3)

Is this correct?