Question

The \( pK_a \) values of the carboxylic and amino groups of an amino acid with a non-ionizable side chain are 2.17 and 9.13, respectively. The isoelectric point (rounded off to two places decimals) of this amino acid is ________.

Hint:

The isoelectric point of an amino acid is the average of the \( pK_a \) values of the amino and carboxyl groups. For amino acids with a non-ionizable side chain, this formula gives the isoelectric point.

Answer 1

Correct Answer: 5.65

The isoelectric point \( pI \) of an amino acid is the average of the \( pK_a \) values of the carboxylic and amino groups. It is given by: \[ pI = \frac{pK_a (\text{carboxyl}) + pK_a (\text{amino})}{2}. \] Given:
- \( pK_a (\text{carboxyl}) = 2.17 \),
- \( pK_a (\text{amino}) = 9.13 \).
Substituting the values: \[ pI = \frac{2.17 + 9.13}{2} = \frac{11.30}{2} = 5.65. \] Thus, the isoelectric point of the amino acid is \( \boxed{5.65} \).