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Protonated states of the imidazole ring of histidine side-chain. The top panel represents the singly protonated states with Nδ1 protonated in the left figure and Nε2 protonated in the right. The bottom panel represents the resonance forms of the doubly protonated state. All the protonated nitrogen atoms are colored in red. The figure was prepared using ChemDraw. The doubly deprotonated state is not physiologically relevant due to a pKa of 14.5, hence not included in these structures.
