Question

In an \(\alpha\) -helix, A. side-chain groups can align to give a polar face. B. cach peptide bond forms two hydrogen bonds. C. there are 3.6 amino acids per turn. D. all of the above. E. none of the above.

Short answer

Question: Examine the following statements about an α-helix and choose the correct answer: A. Side-chain groups can align to give a polar face. B. Each peptide bond forms two hydrogen bonds. C. There are 3.6 amino acids per turn. D. All of the above. Answer: D. All of the above. Explanation: Statements A and C are true, while statement B is false. However, since the true statements are included in the "all of the above" option, the correct answer is D.

Step-by-step solution

Examine Statement A

We start by examining the statement A, "side-chain groups can align to give a polar face." In an α-helix, the side-chain groups are oriented outward, away from the helical axis. Therefore, it is possible for the side-chain groups to align and form a polar face on one side of the helix, while the other side can be more hydrophobic. This statement is true.

Examine Statement B

Next, we examine statement B, "each peptide bond forms two hydrogen bonds." In an α-helix, each amino acid participates in two hydrogen bonds - one with the carbonyl group of the amino acid four residues away and the other with the amide group of the amino acid also four residues away. However, this means that each amino acid forms two hydrogen bonds rather than each peptide bond. Therefore, this statement is false.

Examine Statement C

Now, let's look at statement C, "there are 3.6 amino acids per turn." The α-helix has a specific geometry, with 3.6 amino acids per turn, resulting in a helical pitch (the length of one complete turn along the axis) of 5.4 Å. This statement is true.

Determine the correct answer

Since statements A and C are true, the correct answer for this exercise is D. All of the above.