Step by Step
F
Phenylalanine
Contains a benzene ring side chain and is classified as nonpolar.
W
Tryptophan
Contains an indole ring side chain, is classified as nonpolar, and is the largest of all the standard amino acids.
Y
Tyrosine
Contains a hydroxyl group attached to a benzene ring, making it polar (unlike the other two aromatic amino acids) — and also, as covered in an earlier lesson, a phosphorylation site.
UV
UV absorbance and its practical application
Proteins absorb UV light at 280 nm specifically because of these three aromatic residues, with Tryptophan being the most UV-absorbent of the three. This property is used practically to measure protein concentration in a solution, via absorbance at 280 nm (A₂₈₀).
Applied Walkthrough
1
Phenylalanine's benzene ring and Tryptophan's indole ring both give these two amino acids their aromatic character while remaining nonpolar overall — with Tryptophan additionally standing out as the largest amino acid among all twenty standard types.
2
Tyrosine also carries an aromatic ring, but with a hydroxyl group attached, making it polar rather than nonpolar — a notable exception among the three aromatic amino acids, and consistent with its earlier-covered role as a phosphorylation site.
3
Because these three amino acids share aromatic ring structures, any protein containing them absorbs UV light specifically at 280 nm, with Tryptophan contributing the most to this absorbance.
4
This UV absorbance property has a genuinely practical laboratory application: measuring a solution's absorbance at 280 nm (A₂₈₀) provides a standard, widely-used method for determining protein concentration, relying entirely on the aromatic side chains covered in this lesson.
Exam Application
Exams test whether you can name the three aromatic amino acids (Phenylalanine, Tryptophan, Tyrosine), whether you know Tyrosine is polar (unlike the other two), and whether you understand the practical application of UV absorbance at 280 nm for measuring protein concentration.
⚠ Common Trap
The most common trap is assuming all three aromatic amino acids share identical polarity — Phenylalanine and Tryptophan are nonpolar, but Tyrosine, due to its hydroxyl group, is polar, an exception worth specifically remembering within this otherwise nonpolar-dominated group.
✓ Quick Self-Check
1. What are the three aromatic amino acids?
Phenylalanine, Tryptophan, Tyrosine.
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2. Which aromatic amino acid is the largest of all standard amino acids?
Tryptophan.
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3. Which aromatic amino acid is polar, unlike the other two?
Tyrosine (due to its hydroxyl group).
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4. At what wavelength do proteins absorb UV light due to aromatic residues?
280 nm.
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5. What practical laboratory application relies on this UV absorbance property?
Measuring protein concentration via A₂₈₀ absorbance.
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