Question 23.13: A number of amino acids can be categorized as aromatic amino......
A number of amino acids can be categorized as aromatic amino acids because they have aromatic groups in their side chains. Can the amino acid histidine be categorized as an aromatic amino acid?
STRATEGY
Determine if there is a ring system in which all the atoms are sp² hybridized and the π electrons part of a conjugated system. Next count the number of π electrons in the ring to evaluate if the 4n + 2 rule is satisfied.
The side chain in histidine has a five-member ring with three carbon and two nitrogen atoms, all of which are sp² hybridized. Two double bonds contribute four π electrons to the π system. Now evaluate lone pairs of electrons. Include lone pairs that are in p orbitals (these are the ones that are conjugated); don’t include lone pairs in sp² hybrid orbitals because they are not in the π system!
The electrons on the NH group are adjacent to a double bond and conjugated. Therefore, they are in a p orbital and part of the π system. The other lone pair on the N atom in the ring is in a sp² hybrid orbital, and not part of the π system. Counting the four electrons in the π bonds and the two electrons in the lone pair of the NH group, there are a total of six electrons, making this amino acid aromatic.
