Zwitterion Behavior of Amino Acid

Biochemical Engineering


Problem Statement: At pH=7, what fraction of alanine has no net charge (i.e., electrically neutral)? Set up the equations.

Solution:

   An amino acid acts as a zwitterion, i.e., it can be either:
     1) positively charged in an acid solution
     2) negatively charged in an alkaline solution, or
     3) neutral at the isoelectric point.
   Zwitterion Reaction:
     NH3+-CHR-COOH ----> NH3+-CHR-COO- + H+
     NH3+-CHR-COO- ----> NH2-CHR-COO-  + H+
   which can be abbreviated as:
     A+ ----> A  + H+
     A  ----> A- + H+
   The Dissociation constants for these steps are:
     Dissociation constant #1: pK1=2.34=-log([A][H+]/[A+])   eq. (1)
     Dissociation constant #2: pK2=9.69=-log([A-][H+]/[A])   eq. (2)
   Add one more equation to find the fraction:
     [A+] + [A] + [A-] = 1                                   eq. (3)
   From three eqs (1)--(3), solve for three unknowns ([A+], [A], [A-])
   at pH=7 (i.e., [H+]=10-7)

Actual numerical solution:

A closely related example with MATLAB, which was a problem solved in CHBE250.


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Biochemical Engineering -- Zwitterion Behavior of Amino Acid
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