aromatic

Meaning: An aromatic group is a planar, cyclic, fully conjugated ring with 4n + 2 π electrons, which makes it unusually stable. The aromatic amino acids are phenylalanine (benzene ring), tyrosine (benzene ring with an OH), and tryptophan (indole). Histidine's imidazole ring is also aromatic, but histidine is usually grouped as a basic residue. Phe and Trp side chains are hydrophobic and tend to be buried in the folded core or in membrane-spanning regions. Tyr is less hydrophobic because its OH can hydrogen bond and deprotonate at high pH. Ring faces can also stack with each other (π-stacking).

Equation: A = εcl, where A is absorbance (unitless), ε is molar absorptivity (m²/mol in SI, often given as M⁻¹cm⁻¹), c is concentration (mol/m³, often mol/L), and l is path length (m, often cm).

On the MCAT: Protein concentration is estimated from absorbance at 280 nm, which comes mainly from Trp and Tyr. Nucleic acids absorb near 260 nm. Other questions ask you to sort amino acids into classes, predict where a residue sits in a folded protein, or judge how a mutation such as Phe → Ser affects stability.

Common confusion: Not every ring is aromatic. Proline's ring is saturated and has no conjugated π system, so proline is not aromatic. Tyrosine is aromatic even though some tables list it as polar because of its OH.