Calculating Amino Acid Isoelectric Point (pI)
The isoelectric point (pI) is the pH value at which an amino acid carries no net electrical charge. For amino acids with non-ionizable side chains, the pI is the average of the $pK_a$ values of the alpha-carboxylic acid group and the alpha-amino group.
Key Information
- $pK_a$ of carboxylic group ($pK_{a1}$) = 2.17
- $pK_a$ of amino group ($pK_{a2}$) = 9.13
- Side chain is non-ionizable.
Isoelectric Point Calculation
The formula for calculating the pI is:
$ pI = \frac{pK_{a1} + pK_{a2}}{2} $
Substitute the given values into the formula:
$ pI = \frac{2.17 + 9.13}{2} $
$ pI = \frac{11.30}{2} $
$ pI = 5.65 $
The calculated isoelectric point is 5.65. Rounding to two decimal places, the value remains 5.65.
Important Questions from Chemical Biology {p}K_a Isoelectric Point
- Consider the two $pK_a$ values of valine as 2.32 and 9.62. The isoelectric point (pI) of this amino acid is __________. (rounded off to two decimal places)
- Consider the given peptide, Ala-Glu-Val-Asn-Ile-Asp-Pro-Asp-Gln-Gly-Asp. The net charge on the peptide at pH 1.0 will be ________
- The C-terminal carboxyl group and the N-terminal amino group in amino acids have a dissociation constant ($pK_a$) of 2.2 and 9.2, respectively. The $pK_a$ of side chain carboxyl group in glutamic acid is 4.2 and side chain amino group in lysine is 10.2. The difference in isoelectric point ($pI$) of lysine and glutamic acid (rounded off to two decimal places) is ________.
- Considering that the three pKas of histidine are pK$_1$=1.8, pK$_2$=9.2 and pK$_R$=6.0, its isoelectric point will be _____ (rounded off to one decimal place).
- An amino-acid has one proton donating group in the side chain (R). The $pK_{COOH}$, $pK_{NH2}$ and $pK_R$ values for this amino-acid are 2.19, 9.67 and 4.25, respectively. Which one of the following statements about this amino-acid is CORRECT?
