Why is it important to use pKa values specific to amino acid residues when calculating the isoelectric point of a peptide?

• A. Because pKa values are irrelevant for free amino acids.
• B. Because free amino acids do not have pKa values.
• C. Because peptide bonds alter the chemical structure of amino acids.
• D. Because the microenvironment in peptides can shift pKa values compared to free amino acids.

Answer: (D)