The backbone of a polypeptide chain forms a linear group, if its dihedral angles Up = - 60° and \|/ = — 50°) are repeated. An a-helix with 3.6 amino acid residues per turn has a rise/residue ratio of 0.15 nm. The C = O group of each residue at position i is hydrogen bonded to the NH group at position i + 4. The polypeptide backbone forms a cylinder of 0.23 nm radius in which the backbone atoms are close-packed in van der Waals contact with each other inside the helix. The hydrogen-bonding pattern generates a loop of connectivity through covalent and hydrogen bonds that includes 13 atoms in four turns of the helix (a 413 helix).

In addition to the a-helix, a shorter helix, a 310 helix, is sometimes observed in protein molecules. Its energetically disadvantageous geometry restricts frequent occurrence of the 310 helix in proteins; usually pieces of about one turn are observed. These pieces tend to be at the N and C termini of a-helixes.

A third helical structure is the polyproline II helix found in collagen (gelatin). This is an extended, left-handed helical structure with 3.3 residues per turn generated by dihedral angles of <p = - 80° and y = 4-150°. Stability of this structure relies on the specific conformational constraints of the proline residues by locking into <p = — 80° and not by hydrogen bonds.

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