Chemistry Asked by Rahul Verma on October 5, 2021
The following amino acid is called lysine. I was asked to calculate its isoelectric point, with the given $mathrm pK_mathrm a$ values.
I’ve searched a lot, and the most helpful post that I found was How do I calculate the isoelectric point of amino acids with more than two pKa’s?
According to orthocresol’s answer:
Since the $mathrm{pI}$ is the $mathrm{pH}$ at which the amino acid has no overall net charge, you need to average the $mathrm pK_mathrm a$ values relevant to the protonation/deprotonation of the form with no net charge.
Let’s call the ends $e_1, e_2$ and $e_3$ (from left to right).
So, $mathrm pK_mathrm a$‘s of $e_1$ and $e_2$ are relevant.
$$Rightarrow mathrm{pI} = frac{10.53 + 8.95}{2} = 9.74$$
But, is there some limit to number of protonations/deprotonations or some procedures to follow?
For instance,
This time, $mathrm pK_mathrm a$‘s of $e_3$ and $e_1$ are relevant. But, the calculated $mathrm{pI}$ isn’t correct.
So, how can I validate the approaches?
Problem source: FIITJEE study material
$mathrm pK_mathrm a$ and $mathrm{pI}$ values table for amino acids: https://www.anaspec.com/html/pK_n_pl_Values_of_AminoAcids.html
Isoelectric point of an amino acid is the $mathrm{pH}$ at which the molecule carries no net charge[1]. It can be calculated by the average of the relevant $mathrm pK_mathrm a$ values as you have mentioned.
Your confusion seems to stem from choosing the relevant $mathrm pK_mathrm a$ values. For this we should refer to the titration curve of the amino acid.
For a neutral amino acid[2]:
From the curve we can infer that the $mathrm{pI}$ is simply the average of the two $mathrm pK_mathrm a$ values of the carboxylic acid and the amino group.
For a basic amino acid[2]:
From the curve we can infer that the $mathrm{pI}$ is simply the average of the two $mathrm pK_mathrm a$ values of the two amino groups. The $mathrm pK_mathrm a$ of the carboxylic acid group is not relevant.
For an acidic amino acid[3]:
From the curve we can infer that the $mathrm{pI}$ is simply the average of the two $mathrm pK_mathrm a$ values of the two carboxylic acid groups. The $mathrm pK_mathrm a$ of the amino group is not relevant.
Here are examples for all three cases:
References:
Correct answer by trinitrotoluene on October 5, 2021
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