When the Imidazole Ring Is Protonated?

Properties of the imidazole side chain. The conjugate acid (protonated form) of the imidazole side chain in histidine has a pK_a of approximately 6.0. Thus, below a pH of 6, the imidazole ring is mostly protonated (as described by the Henderson–Hasselbalch equation).

When the imidazole ring of histidine is protonated?

We know that the resonance makes a compound stable. Therefore, protonation occurs before α nitrogen than β nitrogen. So, the order of protonation is γ>α>β. So, the correct answer is Option C.

Can imidazole be protonated?

Protonation of gaseous imidazole is predicted to occur exclusively at the N-3 imine nitrogen to yield a stable aromatic cation.