Identify Amino Acid with Aromatic R Group

Amino acids are classified based on their R (side) groups. Aromatic R groups contain a benzene ring or similar ring structure with delocalized electrons.

R Group Analysis

Let's examine the R groups of the given amino acids:

• Leucine: Has an aliphatic, branched R group ($-\text{CH}_2-\text{CH(CH}_3)_2$). Not aromatic.
• Arginine: Has a basic guanidinium group in its R group ($-\text{(CH}_2)_3-\text{NH}-\text{C(NH}_2)=\text{NH}_2^+$). Not aromatic.
• Tyrosine: Contains a phenyl group ($-\text{CH}_2-\text{C}_6\text{H}_4-\text{OH}$). The phenyl ring ($\text{C}_6\text{H}_4$) is aromatic.
• Threonine: Has a polar R group with a hydroxyl group ($-\text{CH(OH)-\text{CH}_3}$). Not aromatic.

Conclusion

Based on the R group structures, Tyrosine is the amino acid with an aromatic R group due to the presence of the phenyl ring.