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.

Sleeping Sanctuary

Sleeping Sanctuary

Salvation For The Sleep Deprived The Ultimate Guide To Sleeping, Napping, Resting And  Restoring Your Energy. Of the many things that we do just instinctively and do not give much  of a thought to, sleep is probably the most prominent one. Most of us sleep only because we have to. We sleep because we cannot stay awake all 24 hours in the day.

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