What amino acid has an aromatic ring in its side chain?
1 Answer
Well... more than one, that's for sure. You have Tyrosine, Phenylalanine, and Tryptophan.
Notice how all three of those have conjugated pi orbital systems in their sidechains. The benzene ring is a good hint as to whether or not their sidechains have an aromatic ring---benzene is the most basic model for aromaticity!
From there, you might ask yourself, "is the rest of the sidechain conjugated?" Try drawing the resonance structure of Tryptophan, starting at the NH lone pair, going counterclockwise. You should end up on the left carbon on the upper-right double bond on benzene.
CHALLENGE: Does Tryptophan have any conservative mutations? That is, are there any amino acids similar enough to take its place and perform its function? Assume the mutation is only between the known 20 amino acids (Ala, Cys, Asp, Glu, Phe, Gly, His, Ile, Lys, Leu, Met, Asn, Pro, Gln, Arg, Ser, Thr, Val, Trp, Tyr).
