Question

The pKa values for the carboxy and ammonium protons of phenylalanine are 2.58 and 9.24, respectively. What is the isoelectric point of phenylalanine? Draw the structure of phenylalanine at its isoelectric point.

Answer 1

Answer:The isoelctric point of phenylalanine is 5.91. Kindly refer the attachment for structure.

Explanation:

Isoeectronic point is the pH at which the amino acid do not migrate in presence of an electric field.Hence at isoelectric point the molecule exists as a neutral molecule.

So for a molecule to not migrate in electric field it must be neutral. At isoelectric point the net charge on the amino acid molecule is zero.

The amino acid at isoelectric point exists as a zwitter ion which has a equal positive and equal negative charge making the net charge of the amino acid to be zero.

since the amino acid molecule has two kinds of functional group present which are an amine group and a carboxylic acid group.

So to calculate the isoelectric point we must know the pKa of these functional groups.

The formula for calculating the isoelectic point (pI) is :

$pI=[pKa1+pKa2]\times 1/2$

pI=Isoelectric point

pKa1=pKa of carboxy group

pKa2=pKa of amine group

we have the pKa1 and pKa2 given in the question so putting these values in above formula we can calculate the isoelectric point.

pKa1=2.58

pKa2=9.24

pI=1/2×[pKa1+pKa2]

pI=1/2×[2.58+9.24]

pI=5.91

The isoelectric point is 5.91 hence at a pH of 5.91 phenylalanine would exist as a zwitterion and would not migrate in presence of a electric field.