Question

Calculate (show the calculations) the pI for histidine, and aspartic acid.

Answer 1

PI or isoelectronic point

The isoelectronic point or isoionic point is the pH at which the amino acid does not migrate in an electric field.

This means it is the pH at which the amino acid is neutral, i.e. the zwitterion form is dominant.

Amino acids with neutral side chains are characterised by two pKas : pKa1 and pKa2 for the carboxylic acid and the amine respectively.

The isoelectronic point will be halfway between,these two pKas,

pI = 1/2 (pKa₁ + pKa₂).

Amino acids with acidic side chains the neutral form exists under more acidic conditions

pI = 1/2 (pKa₁ + pKa₃)

For Aspartic acid pKa1 = 1.88, pKa2 = 9.60 and pKa3 = 3.65

so pI = 1/2 (pKa₁ + pKa₃) = 1/2 (1.88 + 3.65) = 2.765 or 2.77

Amino acids with basic side chains pI will be at a higher pH and the neutral form exists under more basic conditions

pI = 1/2 (pKa₂ + pKa₃)

For Histidine pKa₁= 1.82, pKa₂= 9.17 and pKa3 = 6.00

so pI = 1/2 (pKa₂ + pKa₃) = 1/2 (9.17 + 6.00) = 7.585 or 7.59