Question: The central rod domain of keratin protein is 300 amino acids in length. What is the approximate length (in Å) of the rod domain if the peptide consists of:

• i) Distorted α-helix
• ii) True α-helix
• iii) Anti-parallel β-sheet

Explanation:

• Distorted α-helix: In a distorted α-helix, the rise per residue is slightly reduced compared to a true α-helix. Let's assume a rise of 1.4 Å per residue. Therefore, the total length would be:

  300 residues * 1.4 Å/residue = 420 Å

• True α-helix: In a true α-helix, the rise per residue is approximately 1.5 Å. So, the total length would be:

  300 residues * 1.5 Å/residue = 450 Å

• Anti-parallel β-sheet: In an anti-parallel β-sheet, the rise per residue is about 3.5 Å. Thus, the total length would be:

  300 residues * 3.5 Å/residue = 1050 Å

Answer: The closest option to the calculated values is (1) (i) 425 Å; (ii) 450 Å; (iii) 1041 Å.