Isoelectric point alanine, calculation

Each leg of the titration curve is calculated separately. The first leg, from pH 1 to 6, corresponds to the dissociation of protonated alanine, H2A the second leg, from pH 6 to 11, corresponds to the dissociation of zwitterionic alanine, HA. Exactly halfway between the two legs is the isoelectric point at 6.01. In essence, it s as if we started with H2A at low pH and then titrated with NaOH. When 0.5 equiv of NaOH is added, the deprotonation of H2A is 50% done when 1.0 equiv of NaOH is added, the deprotonation of HgA" is complete and HA predominates (the isoelectric point) when... 

For each amino acid, there is a specific pH at which the concentration of the zwitterionic form reaches its maximum value. This pH is called the isoelectric point (pi), and each amino acid has its own unique pi. For amino acids that lack an acidic or basic side chain, the pi is simply the average of the two piT values. The following example shows the calculation for the pi of alanine. 

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