Which from following amino acids contains heterocyclic ring at a side chain ( R ) group ?

Step-by-step solution

The presence of a heterocyclic ring in an amino acid side chain requires examining structural features beyond aliphatic or simple functional groups. Threonine's side chain -CH(OH)CH₃ is purely aliphatic with hydroxyl substitution. Cysteine's -CH₂SH contains a thiol group without cyclic structure. Valine's -CH(CH₃)₂ is a branched alkyl chain. Histidine possesses an imidazole ring -CH₂ -imidazole as its side chain, featuring a five-membered aromatic heterocycle with two nitrogen atoms. Only histidine contains a heter