30. The circular dichroism spectra of three proteins P, Q, and R are given below:
Choose the correct combination.
(a) P: α–helix, Q: β–sheet, and R: Random coil
(b) P: β–sheet, Q: α–helix, and R: Random coil
(c) P: α–helix, R: Random coil and R: β–sheet
(d) P: Random coil, Q: α–helix, and R: β–sheet
Introduction
Circular Dichroism (CD) spectroscopy is a powerful tool used to determine protein secondary structure.
Different folding patterns—α-helix, β-sheet, and random coil—produce distinct molar ellipticity signatures in the far-UV range (190–250 nm).
This question presents three CD spectra (P, Q, and R). Using known spectral fingerprints, we can assign the correct structure to each protein.
CD Spectral Fingerprints You MUST Know
| Structure | CD Signature |
|---|---|
| α-helix | Two negative minima at 208 & 222 nm, strong positive peak near 190 nm |
| β-sheet | One negative band near 215–218 nm, positive band near ~195 nm |
| Random coil (disordered) | Negative minimum near 200 nm, little/no structure beyond 210 nm |
Keep this table for future exam questions!
Analysis of Spectra
Protein P
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Shows two strong negative minima (~208 nm and ~222 nm)
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Classic double-dip signature
👉 Conclusion: α-helix
Protein Q
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One broad negative band near ~217 nm
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Slight positive hump around 195 nm
👉 Conclusion: β-sheet
Protein R
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Single dip near 200 nm
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Flat region afterward (no ordered secondary structure)
👉 Conclusion: Random coil
Correct Structural Assignment
✔ P: α-helix, Q: β-sheet, R: Random coil
Option-by-Option Explanation
(a) P: α-helix, Q: β-sheet, R: Random coil
✔ Correct – matches CD signatures perfectly.
(b) P: β-sheet, Q: α-helix, R: Random coil
❌ Incorrect
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Swaps P and Q
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P clearly shows 208/222 double minima → α-helix
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Q only has single 217 nm minimum → β-sheet
(c) P: α-helix, R: Random coil, Q: β-sheet
❌ Incorrect
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Correct assignments, but order mismatched with options
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R is random coil but Q is β-sheet, not in this combination
(d) P: Random coil, Q: α-helix, R: β-sheet
❌ Incorrect
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Completely wrong assignments for all three proteins
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Does not match any CD features given
Final Answer
🎯 (a) P = α-helix, Q = β-sheet, R = Random coil
Why This Matters
CD spectroscopy:
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Helps infer protein folding
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Indicates protein denaturation or folding changes
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Supports structural biology alongside X-ray crystallography and NMR
