The polypeptide backbone of an α helix is twisted by an equal amount about each α-carbon with a phi angle of approximately −57° and a psi angle of approximately −47°. A complete turn of the helix contains an average of 3.6 aminoacyl residues, and its pitch or rise per turn is 0.54 nm (Figure 1). The R groups of each aminoacyl residue in an α helix face outward (Figure 2). Since proteins are comprised solely of _L-amino acids, the only stable α helices they can form are right-handed. Schematic diagrams of proteins often represent α helices as coils or cylinders.

fig1. Orientation of the main chain atoms of a pep tide about the axis of an α helix.

fig2. View down the axis of a polypeptide α helix .The side chains (R) are on the outside of the helix. The van der Waals radii of the atoms are larger than shown here; hence, there is almost no free space inside the helix.

The stability of an α helix arises primarily from hydrogen bonds formed between the oxygen of the peptide bond carbonyl and the hydrogen atom of the peptide bond nitrogen of the fourth residue down the polypeptide chain (Figure 3). The ability to form the maximum number of hydrogen bonds, supplemented by van der Waals interactions in the core of this tightly packed structure, provides the thermodynamic driving force for the formation of an α helix. Since the peptide bond nitrogen of proline lacks a hydrogen atom, it is incapable of forming a hydrogen bond with a carbonyl oxygen. Consequently, proline can only be stably accommodated within the first turn of an α helix. When present elsewhere, proline disrupts the conformation of the helix, producing a bend. Because it possesses such a small R group, glycine can disrupt packing, that may introduce a bend within an α helix.

fig3. Hydrogen bonds (dotted lines) formed between H and O atoms stabilize a polypeptide in an α-helical conformation.

Many α helices have predominantly hydrophobic R groups projecting from one side along their central axis and predominantly hydrophilic R groups projecting from the other side. These amphipathic helices are well adapted to the formation of interfaces between polar and nonpolar regions such as the hydrophobic interior of a protein and its aqueous environment. Clusters of amphipathic helices can create channels, or pores, through hydrophobic cell membranes that permit specific polar molecules to pass.