If the backbone of polypeptide is twisted by equal amount about each α-carbon, it forms a coil or helix. Difference types of helices, formed by imparting differing extent and direction of twist, are described by the number (n ) of aminoacyl residues, per turn and the pitch (P) or distance per turn that the helix rises along its axis. The α-Helices of protein contain anywhere from 4 to 50 residues (average, about 12 residues). The relevant parameters of an α-helix are n=3.6 residues per turn and p=0.54nm (5.4A). The distance along the helix axis that separates equivalent main-chain atoms of adjacent residues is 0.15nm (1.5A).

The aminoacyl R groups are directed outward from the helix axis minimizing mutual stearic interference. α-Helixes are stabilized by hydrogen bonding between the carboxy oxygen and the amino hydrogen 4 amino acids further along the chain. The α-helix is the lowest energy and most stable conformation for a polypeptide chain and it is formed spontaneously. These hydrogen bonds have an essentially optimal N to O distance of 28nm. Van der Waals interaction also confers additional stability. The tightly packed atoms at the core of an α-helix are in Van der Waals contact with one another across the axis of the α-helix.