How many amino acid residues does an ahelical peptide need i

How many amino acid residues does an a-helical peptide need in order to cross a 2nm membrane bilayer once?

Solution

According to the geometry of alpha helix, there occurs atleast 3.5-3.6 amino acid per turn of the right-handed biological form of alpha helix. Also, each helix is nearly 1.5 angstroms or 0.15 nm in linear length. Thus, this information can be used to identify the number of amino acids required to span the 2nm membrane bilayer once as below:

Number of amino acids in 0.15 nm length = 3.6

Number of amino acids in 1 nm length = 3.6/0.15

Thus, number of amino acids required fro 2 nm length = (3.6 * 2)/0.15 or 48

Thus, it can be seen that atleast 48 amino acids are required to completely span the 2nm membrane bilayer once by the alpha helix peptide.