Like most free amino acids valine is a diprotic acid that can undergo two dissoc

Question

Like most free amino acids valine is a diprotic acid that can undergo two dissociation reactions (Carboxyl pKa = 2.32; Amine pKa = 9.62).

a. Draw the structure of the zwitterion of valine

b. Draw the chemical structures of the fully protonated form and the fully deprotonated forms of valine.

c. Indicate in the previous drawings which would be present at the highest concentration in solutions of the following pH: i)1.3; ii) 5.9; iii) 7.54; and iv) 12.0. Explain answers in terms of pH relative to the two pKa values

Explanation / Answer

c) AT pH = 1.3

The pH is less than pKa 1 (2.32). Thus the predominat form is fully protonated form

At pH = 5.9

The pH is closer to the pI = 5.97= (pKa1 +pka2)/2

thus the predominant form is zwitterionic form.(nearly 100%)

At pH = 7.54

This pH is less than pKa2. Thus the most acidic proton of -COOH is lost , but the NH3+ proton is still there.

So the predominant form is zwitterionic form .

AT pH = 12.0

It is much above the pKa2,thus all the acidic protons are lost .Thus the predominant form is fully deprotonated form.

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