Discussion
Calculate the pI value of aspartic acid from the given pK values. (Given : pK1 = 1.99, pK2 = 9.90 and pKR = 3.90)
Answer: A
The pI of aspartic acid is 2.945.
The isoelectric point is the average of the two pK values on either side of the form with no net charge. Aspartic acid has two acidic groups, the alpha-carboxyl (pK1 = 1.99) and the side-chain carboxyl (pKR = 3.90), so the neutral form lies between them:
\[ \begin{aligned} pI &= \frac{1.99 + 3.90}{2} \\ &= 2.945 \end{aligned} \]
5.945 averages pK1 and pK2, the rule for an amino acid with no ionisable side chain. 6.9 averages pKR and pK2, and 7.895 is half the sum of all three.
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