​The circular dichroism spectra for near –UV and fa-UV regions of a polypeptide chain are given below:​

​Which one of the following options represents a correct inference about the polypeptide fold based on the above data?​

Correct option is D

To interpret the polypeptide fold from the circular dichroism (CD) spectra, we need to examine both:

• Far-UV CD spectrum (left graph): Provides information on secondary structure.

• Near-UV CD spectrum (right graph): Provides information on tertiary structure and environment of aromatic residues.

Far-UV CD (195–250 nm):

• Shows a broad negative band near 218 nm, and a positive band near 195 nm.

• This pattern is typical of β-sheet structure.

• α-helix would show double minima at ~208 nm and ~222 nm — not observed here.

 So the secondary structure is predominantly β-sheet.

Near-UV CD (250–320 nm):

• Shows distinct signals, especially between 270–290 nm, indicating:

  • Well-packed tertiary structure

  • Aromatic side chains (like Tyr, Trp, Phe) are in an ordered, asymmetric environment

• This suggests a stable 3D fold, often seen in proteins with both α and β domains, or complex tertiary folds.

Interpretation of options:

1. It contains only β-sheets
    Too strict. Near-UV data implies more complex folding.

2. It has to be an alternate α/β fold
    No α-helix evidence in far-UV. This needs alternating α-helix and β-sheet — not supported here.

3. It has to be a mixed α+β fold
    No α-helix in the far-UV spectrum — again, ruled out.

4. It belongs to either alternate α/β or mixed α+β fold
   This is the most inclusive and correct answer, considering:

   • Far-UV: Strong β-sheet signal

   • Near-UV: Structured tertiary fold, suggesting potential for both α and β regions

   • Could be a mixed fold or α/β arrangement that’s not fully evident in far-UV due to weak α content