Calculating Valine Isoelectric Point (pI)
The isoelectric point (pI) represents the pH at which an amino acid carries no net electrical charge. For amino acids like valine, with a non-ionizable side chain, the pI is calculated as the average of the two relevant dissociation constants ($pK_a$) values: the one for the $\alpha$-carboxyl group and the one for the $\alpha$-amino group.
Valine pKa Values
- $pK_{a1}$ (carboxyl group) = 2.32
- $pK_{a2}$ (amino group) = 9.62
Isoelectric Point Calculation
The formula for calculating the pI is:
$ \text{pI} = \frac{pK_{a1} + pK_{a2}}{2} $
Substituting the given $pK_a$ values for valine:
$ \text{pI} = \frac{2.32 + 9.62}{2} $
$ \text{pI} = \frac{11.94}{2} $
$ \text{pI} = 5.97 $
Rounding off to two decimal places, the pI of valine is 5.97.
Important Questions from Chemical Biology {p}K_a Isoelectric Point
- 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 of decimals) of this amino acid is ________.
- 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?
