Which from following amino acids has a unique structure so that the side chain connects to the back bone of amino acid at two points ?

Step-by-step solution

Proline is a unique amino acid because its side chain is bonded to both the -carbon and the amino group nitrogen, forming a cyclic pyrrolidine ring. Due to this cyclic structure, the side chain connects to the backbone of the amino acid at two points. Other given amino acids (Histidine, Tryptophan, Tyrosine) have side chains that connect only to the -carbon. Answer: Proline