Question

How do you determine the Isoelectric Point of a molecule given multiple pKa values?

I am working on a problem that gives me six pKa values, two of which share the same value. I've constructed a titration diagram already, but I am struggling to determine how to choose which pKa values to use to calculate the isoelectric point. I know that the IP is the point at which the overall charge of the molecule is neutral after levels of deprotonation, but I can't reason out how to determine which points to use. The atoms associated with the pKa values are virtually identical aside from a CO2 group that is clearly the most acidic. I don't have any concentrations available of either the base or the acid. Any tips, equations, or instructions on how to do this would be greatly appreciated.

Answer 1

The Henderson Hasselbach equation is shown below:

pH = pKa + log(a/1-a)

Here, 'a' is the degree of dissociation of the compound or the atom.

Suppose your compound is an amino acid, which consists of two dissociable groups, one is an amine group(NH₃⁺) and the other is the COOH group.

Now in order to obtain a neutral charge, the COOH group must be 50% dissociated into COO^- group which will give a -0.5 charge, and the NH₃⁺ must also be 50% dissociated into NH₂ group which will give a +0.5 charge, so the net charge becomes zero.

So first use the equation for the COOH group, put its pKa value and put a = 0.5, and calculate the corresponding pH.

Next do the same for amine group, and calculate the pH. Then take the average pH value, that will be the required pI value ( also called as the isoelectric point).

Hope this helps !