alphahelical

An alpha helical structure, or alpha helix, is a common right-handed coiled conformation that polypeptide chains can adopt as part of a protein’s secondary structure. In an alpha helix, the backbone forms a regular pattern of hydrogen bonds between the carbonyl oxygen of residue i and the amide hydrogen of residue i+4, stabilizing a helical axis. The helix typically contains about 3.6 residues per turn and a rise of 5.4 Å per turn. Individual helices are usually 10 to 15 residues long, though longer helices exist in some proteins.

The stability and appearance of an alpha helix depend on the amino acid sequence. Residues such as alanine, leucine, methionine, and glutamate favor helix formation, while proline and glycine tend to disrupt helices. The side chains extend outward from the helical axis, producing patterns of hydrophobic and hydrophilic faces that influence folding, interactions, and localization. Alpha helices are widespread in globular proteins and frequently form transmembrane segments, where hydrophobic faces interact with lipid environments.

In many proteins, alpha helices assemble into larger motifs, such as coiled-coils, where two or more helices wrap around each other to create elongated, rod-like structures. Helices can also participate in active sites, binding surfaces, and signal transduction pathways, illustrating the versatility of this secondary structure.

The concept of the alpha helix was established in the early 1950s by Linus Pauling, Robert Corey, and Herman Branson, based on X-ray crystallography and later confirmed by additional structural methods. Today, alpha helices remain a foundational element in protein structure analysis, engineering, and comparative biology.