Answer to Question #272559 in Organic Chemistry for rythm

Question

If on one amino acid, there are two amine groups and two COOH attached, and all of their 4 pka values are given how do you find the isoelectric point?

Answer 1

Since the pI is the pH at which the amino acid has no overall net charge, you need to average the pKa

values relevant to the protonation/deprotonation of the form with no net charge. The pI for amino acids (the pH of the isoelectric point) is exactly halfway between the pKa values of the two functional groups. Note that the aspartame has the carboxylic acid functionality of phenylalanine converted to an ester and the carboxylic acid of aspartic acid and linked as an amide to the amine group of phenylalanine. This leaves the aspartic acid amine group and the side chain carboxylic acid functional group of aspartic acid. Therefore aspartame will have an isoelectric point at a pH between the pKa values of these functional groups. Find the values for these two groups of aspartic acid and calculate a predicted pI value for aspartame.