To calculate the pI value, we take the average of the pKa values from the 2 groups that result in a net charge of 0.

For that to happen, the pKa values should be at the midpoint of titration where the charge is either +0.5 or -0.5.

Since pKa can also be calculated from -log(ka), but the pKa value from that calculation doesn't necessary mean the group is half titrated (+/-0.5 charge). So are the published pKa values for amino acids all at half titrated position (pH=pKa -> +/-0.5 charge).

  • $\begingroup$ pKa Is related to half mean charge of the functional group, not the whole molecule. Particularly for amino acids without an extra COOH or NH2 group, the molecule mean charge at pH=pKa would be +0.5, if all amino groups are protonized. If not, it would be lower. $\endgroup$ – Poutnik Oct 23 '19 at 11:38

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