How do the pKa values of an ionizable side chain compare when the amino acid is free versus when it is in a polypeptide chain?
A)The pKa of the side chain is independent of whether the amino acid is in a polypeptide chain or is free.
B)The pKa of the side chain is always lower for the free amino acid.
C)The pKa of the side chain may be lower or higher in a polypeptide chain due to weaker inductive effects and differences in their microenvironments.
D)The pKa of the side chain is usually higher in a polypeptide chain due to stabilization from nearby residues in the three-dimensional structure.

Question

Quizplus · Accepted Answer

The pKa of an ionizable side chain can be affected by the microenvironment in a polypeptide chain, such as weaker inductive effects and potential electrostatic interactions with nearby residues. Therefore, the pKa of the side chain may be either higher or lower in a polypeptide chain compared to when it is in a free amino acid form. However, it is not always lower for the free amino acid (choice B) or always higher in a polypeptide chain (choice D). Choice A is incorrect because the pKa of an ionizable side chain can be affected by its environment.

How Do the PKa Values of an Ionizable Side Chain