What is the length of a single transmembrane α-helix in lipid bilayers if it spans ~3 nm?
Geometrical constraints of fluid phospholipid bilayer dictate minimal span length for alpha-helical membrane proteins. Hydrophobic thickness between carbonyl regions of liquid-crystalline phosphatidylcholine bilayer averages three point zero nanometers. An alpha helix advances zero point fifteen nanometers per residue along axis with three point six residues per turn, pitch zero point five four nanometers stabilized by intramolecular hydrogen bonds between carbonyl i and amide i plus four. Dividing three nanometers by zero point fifteen per residue yields about twenty residues required for per
Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 10, Transmembrane Alpha-Helix Length and Bilayer Thickness.
