why-do-amino-acids-with-non-ionizable-r-groups-have-only-two-pka-values

Question

Why do amino acids with non-ionizable R groups have only two pKa values?

Accepted Answer

Because their R groups do not participate in acid-base reactions.

Answer

Because they are always charged at physiological pH.

Answer

Because they have an additional pKa for the R group.

Answer

Because they can form ionic bonds.