alphahelices

An alpha helix is a common type of secondary structure in proteins. It is a right-handed coil in which the backbone NH groups of residue i form hydrogen bonds with the carbonyl oxygen of residue i+4. This pattern stabilizes the structure and gives it a regular geometry. An alpha helix typically contains about 3.6 amino acid residues per turn, a pitch of about 5.4 Å, and extends outward with side chains projecting away from the helix axis.

The helix has characteristic dihedral angles with the most favored region around phi ≈ -57°, psi ≈ -47°. Right-handed helices are far more common in natural proteins; left-handed helices are rare due to steric constraints and are generally found only under specific conditions or in particular proteins.

Variations include distorted helices and amphipathic helices, in which hydrophobic and polar side chains segregate onto opposite faces. Proline and glycine influence helices; proline often breaks helices due to its rigid ring and lack of amide hydrogen, while glycine, though flexible, can destabilize regular helices in some contexts. Helices can be capped at their ends by specific residues to stabilize termini. In membrane proteins, alpha helices may span the lipid bilayer and form helices with hydrophobic surfaces.

Alpha helices are detected by X-ray crystallography or nuclear magnetic resonance and are commonly analyzed by circular dichroism spectroscopy, which shows characteristic signals with negative bands near 208 and 222 nm. They account for a substantial fraction of residues in many globular proteins, contributing to overall folding and function.