The pKa's of arginine's α-Carboxyl group,α-Amino group and side chain are 1.8,9.0 and 12.5,respectively.Calculate the isoelectric point.
A)7.8
B)7.2
C)10.8
D)5.4

Question

Quizplus · Accepted Answer

Isoelectric point (pI) is the pH at which the net charge on the molecule is zero. For arginine, the side chain is the most basic group, so the pI can be estimated by averaging the pKa values of its α-carboxyl and α-amino groups with the pKa of the side chain nitrogen:

pI = (pKa of α-COOH + pKa of α-NH3+) / 2

pI = (1.8 + 9.0) / 2 = 5.4 + 3.6 = 9.0

Therefore, the isoelectric point of arginine is approximately 9.0. However, the side chain can also contribute to the charge at the isoelectric point, so the pI value can be slightly different. In this case, the pKa of the side chain is closer to the basic side of the neutral pH range, so the pI value will be higher than 9.0. The closest answer choice is C (10.8), which is the correct choice.

The PKa's of Arginine's α-Carboxyl Group,α-Amino Group and Side Chain