For a specific amino acid, the successive acid dissociation constants are strictly (for the group), (for the group), and (for an additional basic side chain). At what precise pH will the molecule exis

Question

For a specific amino acid, the successive acid dissociation constants are strictly $pK_{a1} = 2.18$ (for the $-COOH$ group), $pK_{a2} = 8.96$ (for the $-NH_3^+$ group), and $pK_{a3} = 10.53$ (for an additional basic side chain). At what precise pH will the molecule exist primarily as an electrically neutral zwitterion (the isoelectric point)?

Prashant Kotian · Accepted Answer

Correct answer: B. The isoelectric point () implies the net charge on the species must be exactly zero. Considering it as a triprotic acid, the fully neutral zwitterionic form is stabilized strictly between the second and third ionizations (where the basic side group and amino group are balanced). Thus, .

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