Why should isoelectric point for Aspartic acid (2.98) be so much lower than that of leucine?

Explanation

This may be explained by considering following ion equilibria

It is apparent that ions (A) and (B) are neutral, while (C ) is a cation and (D) is dianion. In species (D), the anion is derived from the second `-COOH` group present in aspartic acid and is not possible in leucine. At neutral pH a significant concentration of (D), will be present in aqueous solution. It will therefore, be necessary to decrease the pH of such a solution if the formation of (D) is to be suppressed to a stage where anions and cations are present in equal concentration (the isoelectric point).