Chapter 26.3, Problem 4P

Interpretation Introduction

Interpretation:

The isoelectric point $\left(\text{pI}\right)$ of cysteine is to be determined on the basis of the ${\text{pK}}_{\text{a}}$ values of the three acid groups in it.

Concept Introduction:

Isoelectric point of an amino acid is the pH at which it has zero net charge.

It is the average of the two ${\text{pK}}_{\text{a}}$ values of that species when the amino acid contains only two functional groups that can behave as acids/bases.

If an amino acid contains more than two functional groups that can behave as acids, then the isoelectric point is generally the average of the two groups that are not charged in the acid form, or the groups that are most similar acids.