How many amino acid residues in a-helical conformation

Recall that an a-helix has 3.6 amino acid residues per turn, and a pitch of 0.54 nm/turn. The thickness of the hydrophobic core of the lipid bilayer in biological membranes is about 3.0 nm. How many amino acid residues in a-helical conformation would be required to cross a membrane?

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