Provide the structure of histidine at a physiological pH.

Structure of histidine at physiological pH, showing its amino and carboxyl groups.

Structure of histidine with a positive charge on the amino group, indicating its protonated form.

Structure of histidine with a positive charge on the amino group and a neutral imidazole ring.

Structure of histidine with a positive charge on both amino groups, showing its fully protonated form.