Question

How will I find the pI for a non-ionizable amino acid?

Answer 1

First, we need to know what is meant by pI.

pI refers to the isoelectric point of an amino acid. Isoelectric point of an amino acid is the pH at which the amino acid does not migrate in an electric field, i.e the pH at which it is neutral in charge.

Now, we need to differentiate between ionizable and non-ionizable amino acids while calculating iso-electric point. Ionizable amino acids are those which have readily ionizable side chain groups attached to it. These ionizable side chains can readily undergo proton exchange with the medium or with any other molecule. Thus the pK_a value of each side chain group determines the pH value at which there will be neutral charge on the amino acid, i.e it determines its isoelectric point.

Out of the 20 most common naturally occuring amino acids, 7 are usually considered ionizable. They are aspartic acid, glutamic acid, histidine, arginine, lysine, tyrosine and cysteine. The rest can all be considered as non-ionizable amino acids.

For non-ionizable amino acids, where the side chain does not contain any ionizable group, the pI is simply the average of the pK_a values of the amine and the carboxylic acid functional groups contained in that amino acid.

To calculate the pI, the amino acid needs to be considered in its zwitter-ionic form at the isoelectric point.

The methodical calculation of pI is illustrated in the image.