Final answer:
To find the pI of an amino acid with 3 pKa values (including the side chain), compare the pKa values to determine the pI as the average of the two values closest to each other.
Step-by-step explanation:
The isoelectric point (pI) of an amino acid is the pH at which the amino acid exists as a zwitterion, meaning it has both a positive and negative charge, resulting in a neutral overall molecule. To find the pI of an amino acid, you need to consider the pKa values of the acidic and basic groups in the amino acid side chain. In this case, if an amino group has 3 pKa values (including the side chain), you would need to compare these pKa values to determine the pI.
For example, let's say the pKa values of the amino group are pKa1, pKa2, and pKa3. The pI would be the average of the two pKa values that are closest to each other. So if pKa1, pKa2, and pKa3 are 2.0, 3.0, and 4.0 respectively, the pI would be (3.0 + 4.0) ÷ 2 = 3.5.