If the length of a single continuous α-helical polypeptide is 108 Å, which one of the following statements is true?

Correct option is A

The correct answer is: (1) The α-helix contains 72 amino acids without Proline(s) and Glycine(s)

Explanation:
The typical length of an α-helix is about 5.4 Å per amino acid residue. Given the total length of the α-helix is 108 Å, we can divide the total length by the length of a single amino acid in the helix to estimate the number of amino acids.

​$$\text{Number of amino acids} = \frac{\text{Length of }\alpha\text{-helix (in Å)}}{\text{Length per amino acid in an }\alpha\text{-helix (in Å)}} = \frac{108 \, \text{Å}}{1.5 \, \text{Å}} = 72 \, \text{amino acids}$$

Thus, an α-helix of 108 Å contains approximately 72 amino acids. Since Proline and Glycine are known to disrupt the α-helical structure, the given statement (1) assumes that the α-helix does not contain Proline or Glycine residues, which is consistent with this estimation.

Information Booster:

1. α-Helix Structure: The α-helix is a coiled structure that stabilizes through hydrogen bonds, with each amino acid contributing about 1.5 Å to the total length.

2. Proline and Glycine Disruptions: Proline is a helix breaker due to its rigid ring structure, while Glycine, due to its small size and flexibility, is also often excluded from the α-helix.

3. Number of Amino Acids Calculation: The typical α-helix with 108 Å would span about 72 amino acids, assuming no disruptions in the helical structure.

4. Length per Residue: The 1.5 Å per residue is a general rule for α-helices in polypeptides.

5. Proline's Role: Proline residues, due to their side chain structure, induce kinks in the helix, making them incompatible with the smooth helical form.

6. Amino Acid Properties: The amino acids most likely to disrupt an α-helix are bulky or highly flexible residues such as Proline and Glycine.

Additional Knowledge:

• Option 2: The α-helix containing 76 amino acids without Alanine and Valine would not fit within a 108 Å length because those residues would not contribute to the necessary structure as efficiently.

• Option 3: An α-helix with 80 amino acids without Lysine and Aspartic Acid would exceed the 108 Å length limit, making this statement unlikely.

• Option 4: The α-helix would not contain 74 amino acids if it avoids Arginine and Histidine. These residues are less likely to significantly disrupt the helix compared to Proline and Glycine, and the number of amino acids would be higher for the given length.

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