Q.33 Molar absorption spectra labeled (i), (ii) and (iii) for three different amino acids are shown below.
Which one of the following is the correct combination of spectral assignments?
(A) (i) – tryptophan, (ii) – tyrosine, (iii) – phenylalanine
(B) (i) – phenylalanine, (ii) – tryptophan, (iii) – tyrosine
(C) (i) – proline, (ii) – tyrosine, (iii) – tryptophan
(D) (i) – tryptophan, (ii) – proline, (iii) – phenylalanine

Question Overview

The molar absorption spectra labeled (i), (ii), and (iii) correspond to three different amino acids. Based on their UV–Vis absorption characteristics, the correct combination of spectral assignments must be identified.

The amino acids involved are:

• Tryptophan

• Tyrosine

• Phenylalanine

Key Concept: UV Absorption of Aromatic Amino Acids

Aromatic amino acids absorb UV light due to π → π* electronic transitions in their aromatic rings. Their relative molar absorptivities follow this order:

Tryptophan > Tyrosine > Phenylalanine

Analysis of the Given Spectra

Spectrum (i)

• Shows the highest molar absorbance

• Broad and intense peak near 280 nm

• Characteristic of tryptophan

Spectrum (ii)

• Moderate absorbance

• Peak slightly below 280 nm

• Typical of tyrosine

Spectrum (iii)

• Very weak absorbance

• Peak near 258 nm

• Matches phenylalanine

Correct Answer

✅ Option (A)

(i) – tryptophan, (ii) – tyrosine, (iii) – phenylalanine

Explanation of All Options

Option (A): Correct ✅

• Matches the known UV–Vis absorption intensities

• Correct wavelength positions and relative peak heights

• Fully consistent with aromatic amino acid spectroscopy

Option (B): Incorrect ❌

• Assigns the strongest absorbance to phenylalanine

• Phenylalanine has the weakest UV absorption

• Contradicts experimental data

Option (C): Incorrect ❌

• Proline is non-aromatic

• Proline does not show strong UV absorption in this region

• Spectrum (i) cannot be proline

Option (D): Incorrect ❌

• Proline again incorrectly assigned UV absorption

• Phenylalanine cannot show the strongest peak

• Violates known absorption order

Final Conclusion

The molar absorption spectra clearly follow the known UV absorption trend of aromatic amino acids. The correct identification is:

(i) Tryptophan, (ii) Tyrosine, (iii) Phenylalanine

🎯 Correct Option: (A)