Question

Part A - Zwitterionic Form

Two structures of alanine can be drawn that have a net charge of zero, but the predominant form of alanine at its pI is zwitterionic. Why is alanine predominantly zwitterionic rather than completely uncharged at its pI? What percent (to one significant figure) of alanine molecules would be in the completely uncharged form at the pI? Hint: pK₁ of alanine is 2.35 and pK₂ is 9.87.

Answer should be in % form.

Answer 1

The pI of alanine occurs at a pH well above the pKa of the carboxyl group and well below the pKa of the amino group of alanine so both groups are present predominantly zwitterionic rather than completely uncharged.

The pI of alanine is 6.11 midway between two pKa values Applying Henderson -Hasselbach equation

pH= pKa + log ([A-]/[HA])

pH- pKa = log ([A-]/[HA])

log ([A-]/[HA]) = 3.76

[A-]/[HA] = 10^-3.76 = 1 / 5.75 x 10³

1 molecule in 5750 is still in the form -COOH similarly at pH = pI 1 molecule in 5750 is in the form -NH2

So fraction of molecules with both group uncharged is 1 in 5750 x 5750 or 1 in 3.3 x 10⁷

1 in 3.3 x 10⁷ so X in 100

so X = 3.03 x 10^-6 %

Ans: 3 x 10^-6 %