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

Question

Quizplus · Accepted Answer

To find the pI of arginine, we need to identify the pH at which the molecule has a net charge of zero. This can be done by averaging the pKa values of the two ionizable groups that are most likely to be protonated or deprotonated at physiological pH, which are the α-amino group (pKa=7.5) and the guanidine group (pKa=12.5). 
pI = (pKa1 + pKa2)/2
pI = (7.5 + 12.5)/2
pI = 10.76 
Therefore, the correct answer is A.

What Is the PI of Arginine? the Structure and PKa