What is the pI of arginine? The structure and pKa values are shown below.

Question

Quizplus · Accepted Answer

The pI of a molecule is the pH at which it has no net charge. To determine the pI of arginine, we need to find the pH at which it has equal amounts of positive and negative charge. Arginine has three ionizable groups: the α-amino group (pKa ~9.0), the guanidinium group (pKa ~12.5), and the carboxyl group (pKa ~2.2). At very low pH, all three groups are protonated, giving a net charge of +3. As the pH increases, the carboxyl group loses its proton first, followed by the α-amino group. At very high pH, all three groups are deprotonated, giving a net charge of +1. To calculate the pI, we average the two pKa values that bracket the pH at which the net charge is zero. In this case, the pKa values that bracket the pH are ~9.0 and ~12.5, so the pI of arginine is (9.0 + 12.5) / 2 = 10.76. Therefore, the best choice is A.

What Is the PI of Arginine? the Structure and PKa