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In my laboratory workbook, it has been mentioned that the zwitterionic form of the amino acid is the dominant form of the amino acid at pI and no movement of the molecules takes place under an external electric field. I am a bit confused by this statement.

It is clearly not possible for all the molecues to exist in the zwitterionic form. Some of them must exist in any of the other charged forms, even at pI. This should lead to these few charged molecules, moving in an electric field.

My question is:

  • Is it true that all molecules are not zwitterions at pI?
  • When we say no migration of ions takes place at pI, do we actually mean no "NET" migration of ions occurs i.e. whatever number of ions move towards one side, causes an equal number of ions to move towards the other?
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  • $\begingroup$ Try to think longer about it. It would lead to less questions. What happens to trace cation/anion forms if they migrate to regions with less than equilibrium concentration of the zwitterion? $\endgroup$
    – Poutnik
    Jul 4, 2022 at 5:54
  • $\begingroup$ @Poutnik They get converted to the neutral zwitterions. But still, in some other part of the solution, when zwitterions tend to be at an excess, the reverse reaction will dominate. Then charged molecules should form? $\endgroup$ Jul 4, 2022 at 6:09
  • $\begingroup$ Check typical respective pKa and concentration ratios for used pH. Involve imagination of molecular dynamics It is quite analogical to chromatography and dynamic redistribution between both phases. (zwitterion like in stationary one and other forms like in mobile one). But here direction of "mobile phase" goes both ways. $\endgroup$
    – Poutnik
    Jul 4, 2022 at 6:21

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