Amino Acids (Part 4 of 5) - Titrations and pI (Isoelectric pH)

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Here, I discuss amino acid titration curves and how to find / calculate / determine the isoelectric point (the isoelectric pH), otherwise known as the pI, which is the pH at which an amino acid will most likely have an overall charge of zero.

I chose to use the basic amino acid histidine as the example for how to find isoelectric pH (pI) because it has three titratable groups: 1) the alpha carboxylic acid group, 2) the alpha amino group, and 3) the side chain's imidazole group. Most amino acids have only two titratable (or ionizable) groups, the alpha carboxylic acid group and the alpha amino group. So, finding the pI for most amino acids is super easy - you just have to take the average of the pKa values of the two ionizable groups, the alpha carboxylic acid group and the alpha amino group. For amino acids with three ionizable groups [the acidic amino acids (aspartic acid and glutamic acid) and the basic amino acids (histidine, arginine, and lysine)], solving for the pI is slightly more complex. You still must average out two pKa values, but which two? The answer is: the two pKa values that are on either side of the zero-charged species.

So, this video is a walkthrough for the titration of histidine specifically and how to determine which two pKa values must be averaged out to determine the pI and WHY that is.

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