Alphahelix

An alpha helix is a common secondary structure in proteins, characterized by a right-handed coiled backbone. In this motif, the carbonyl oxygen of residue i forms a hydrogen bond with the amide hydrogen of residue i+4, stabilizing a repeating turn. A complete turn spans 5.4 angstroms and comprises about 3.6 amino acids, giving the helix a diameter of roughly 10 to 12 angstroms. Side chains protrude outward from the helix axis, minimizing steric clashes with the backbone.

Alpha helices can be amphipathic, with one face enriched in hydrophobic residues and the opposite face in polar or charged residues. The propensity of amino acids to adopt an alpha-helix varies; proline disrupts helices due to its rigid ring and lack of backbone amide hydrogen, whereas glycine allows unusual flexibility. Stability is influenced by helix capping motifs at the termini and by the broader protein environment. In Ramachandran space, typical phi and psi angles for helices cluster near -57° and -47°, respectively.

Alpha helices contribute to a protein's three-dimensional architecture and function. They commonly form the core of globular proteins, participate in coiled-coil assemblies, and serve as transmembrane helices that span lipid bilayers. Helices can pair to create helical bundles, or align in arrays that mediate protein–protein interactions. The arrangement and length of helices influence folding, stability, and dynamics of the protein.

Historically, the alpha helix was proposed in 1951 by Linus Pauling, Robert Corey, and Herman Branson as part of a three-dimensional model of protein structure. It is typically studied by X-ray crystallography and nuclear magnetic resonance, and its presence is often inferred from circular dichroism spectroscopy. The concept remains a central element of protein structure and bioinformatics.