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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).

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