​A circular dichroism spectrum in the far-UV region informs on the kind and content of secondary structures in a protein. Near-UV and tryptophan emission spectra inform on the tertiary structure. Shown in the panels above are (A) Intrinsic fluorescence emission spectra of protein 'X', (B) Far-UV CD spectra of protein 'X', (C) Near-UV CD spectra of protein 'X' recorded under different conditions.
Curves represent the spectra of protein 'X' at pH 7.0 (black), pH 3.0 (green), and pH 7.0 in the presence of 6.0 M guanidine hydrochloride (red).
What does the experiment report?​

Correct option is A

Explanation-

Experimental Setup Summary:
The three conditions being compared for protein 'X':
                               1.   Black: pH 7.0 → normal condition
                               2.   Green: pH 3.0 → acidic
                               3.   Red: pH 7.0 + 6 M guanidine hydrochloride → denaturing condition

Panel A: Intrinsic Fluorescence (Tertiary Structure)
Fluorescence emission max for tryptophan:
                    327 nm (black) = buried in hydrophobic core → compact/folded
                    340 nm (green) = partial exposure → partially unfolded (molten globule)
                        350 nm (red) = fully exposed → unfolded
Interpretation: At pH 7.0, protein is folded.
                        At pH 3.0, partial unfolding → molten globule state.
                        With guanidine, fully unfolded.
Panel B: Far-UV CD (Secondary Structure)
          Strong signals at pH 7.0 (black) → high secondary structure content.
          Moderate signal at pH 3.0 (green) → partial structure.
          Very low/flat signal with guanidine (red) → loss of secondary structure.
Interpretation:
          Secondary structure intact at pH 7.0 , reduced at pH 3.0, destroyed with guanidine.
Panel C: Near-UV CD (Tertiary Structure)
           Black (pH 7.0) → clear signal = well-packed tertiary structure.
           Green (pH 3.0) → flat = molten globule, tertiary structure lost.
           Red (guanidine) → flat = fully unfolded.
Interpretation:
           Tertiary structure intact at pH 7.0 , disrupted at pH 3.0 and with guanidine.

Correct Answer: Option a - “Protein is fully folded at pH 7.0, acid-induced molten globule at pH 3.0 and unfolded in 6M guanidine hydrochloride.
  Fluorescence: 327 → 340 → 350 nm (increasing exposure)
  Far-UV CD: high → reduced → minimal (secondary structure loss)
  Near-UV CD: intact → lost → lost (tertiary structure loss)

Incorrect options-
Option b: Says secondary structure is reduced at pH 7.0 — false, it's highest at pH 7.0.
Option c: Talks about hydrodynamic radius — not measured in this experiment.
Option d: Says extensive denaturation at both pH 3.0 and guanidine — wrong, pH 3.0 causes partial unfolding, not full.