How many residues are there per 360-degree turn in an alpha helix?

In an alpha helix, the structure is characterized by a specific pattern of hydrogen bonding between the backbone amide and carbonyl groups of the polypeptide chain. This helical structure typically comprises about 3.6 amino acid residues per complete turn, corresponding to a rise of approximately 5.4 angstroms per turn.

This arrangement allows for optimal stability due to the regular pattern of hydrogen bonds that form between the NH group of one residue and the CO group of another residue four positions earlier in the sequence (the n and n+4 residues). The presence of 3.6 residues ensures that the helix is tightly packed and maintains a uniform diameter, contributing to the overall stability of the protein structure.

The number 3.6 is derived from the helix geometry; it reflects the concept that during one complete rotation (360 degrees), there are fractional and whole residues that fit into the helical structure. Specifically, each residue contributes to roughly 100 degrees of the turn, leading to this average of approximately 3.6 residues within one complete turn. This fundamental characteristic is essential to understanding protein structure and function.