How many complete turns are there in an ideal α-helix that contains 15 amino acids and has a pitch of 0.54 nm and a rise of 0.15 nm?
A)1
B)4
C)8
D)59

Question

Quizplus · Accepted Answer

The formula for calculating the number of turns in an α-helix is:
Number of turns = (length of the helix) / (height of one turn)
Length of the helix = number of amino acids × pitch
So, for this α-helix:
Length of the helix = 15 amino acids × 0.54 nm/turn = 8.1 nm
Height of one turn = rise per amino acid × number of amino acids per turn = 0.15 nm × 3.6 amino acids = 0.54 nm/turn
Number of turns = 8.1 nm / 0.54 nm/turn = 15 turns
However, we need to give the answer in terms of complete turns. As one complete turn corresponds to 3.6 amino acids, the total number of complete turns in this α-helix will be:
Number of complete turns = 15 amino acids / 3.6 amino acids/turn = 4 turns. 
Therefore, the correct answer is B) 4.

How Many Complete Turns Are There in an Ideal α-Helix