Q.41 Which of the following peptides do/does NOT absorb ultraviolet light above
250 𝑛𝑚 wavelength?
(A) MQRTVWG
(B) YDEIGVL
(C) PLASNGK
(D) GSQTKRL
*Peptides lacking aromatic amino acids do not absorb ultraviolet light above 250 nm, as this absorption arises from π-π transitions in Trp (W), Tyr (Y), and Phe (F) side chains.**​
Amino Acid UV Absorption Basics
Peptide bonds absorb below 230 nm, but wavelengths above 250 nm require aromatic residues like tryptophan (strongest at ~280 nm), tyrosine (~275 nm), and phenylalanine (~260 nm). Non-aromatic residues show negligible absorbance in this range under standard conditions.​
Option Analysis
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(A) MQRTVWG: Contains W (tryptophan), which absorbs strongly above 250 nm due to its indole ring.​
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(B) YDEIGVL: Contains Y (tyrosine), which absorbs via its phenolic ring around 275-280 nm.​
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(C) PLASNGK: Lacks W, Y, or F; all residues (P, L, A, S, N, G, K) are non-aromatic, so no absorption above 250 nm.​
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(D) GSQTKRL: Lacks W, Y, or F; residues (G, S, Q, T, K, R, L) contribute only peptide bond absorption below 230 nm.​
Correct Answer
Options (C) and (D) do not absorb UV light above 250 nm.​
Introduction to Peptides UV Absorption Above 250 nm
Peptides UV absorption above 250 nm wavelength depends on aromatic amino acids like tryptophan (W), tyrosine (Y), and phenylalanine (F), crucial for CSIR NET Life Sciences and GATE exams. This guide analyzes sequences MQRTVWG, YDEIGVL, PLASNGK, GSQTKRL to identify non-absorbers.​
Why Aromatic Amino Acids Drive UV Absorption
Proteins and peptides absorb UV light above 250 nm primarily from π-π* transitions in aromatic side chains: Trp (~280 nm, ε=5500), Tyr (~275 nm, ε=1400), Phe (~260 nm, ε=200). Peptide bonds absorb <230 nm; charged/non-aromatic residues show weak or no signal above 250 nm in standard assays.​
Detailed Breakdown of Each Peptide Sequence
| Option | Sequence | Aromatic Residues | Absorbs >250 nm? | Reason |
|---|---|---|---|---|
| (A) | MQRTVWG | W (Trp) | Yes | Indole ring strong absorber ​ |
| (B) | YDEIGVL | Y (Tyr) | Yes | Phenolic ring peak ~275 nm ​ |
| (C) | PLASNGK | None (P,L,A,S,N,G,K) | No | All non-aromatic ​ |
| (D) | GSQTKRL | None (G,S,Q,T,K,R,L) | No | No W/Y/F present ​ |
Key Takeaway: PLASNGK and GSQTKRL lack aromatic residues, confirming no absorption above 250 nm.​
Exam Relevance for CSIR NET/GATE
This question tests biochemistry fundamentals on chromophores in biomolecules, common in competitive exams. Practice tip: Scan sequences for W, Y, F single-letter codes first.​​
Practical Applications in Protein Quantification
UV absorbance at 280 nm quantifies proteins via aromatic content; peptides without these are “UV-silent” above 250 nm, aiding spectroscopy assays. Exceptions like high-charge effects are minor and exam-irrelevant here.​
