A helical structure consists of repeating units that lie on the wall of a cylinder such that the structure is super- imposable upon itself if moved along the cylinder axis. * A helix looks like a spiral or a screw. A zig-zag is a degenerate helix. * Helices can be right-handed or left handed. The difference between the two is that: - Right- handed helices or screws advance (move away) if turned clockwise. Examples: standard screw, bolt, jar lid. - Left-handed helices or screws advance (move away) if turned counterclockwise. Example: some automobile lug nuts. * Helical organization is an example of secondary structure. These helical conformations of macromolecules persist in solution only if they are stabilized.
Helices in carbohydrates
Carbohydrates with long sequences of alpha (1 -> 4) links have a weak tendency to form helices. Starch (amylose) exemplifies this structure. The starch helix is not very stable in the absence of other interactions (iodine, which forms a purple complex with starch, stabilized the starch helix), and it commonly adopts a random coil conformation in solution.
Helices in nucleic acids
Single chains of nucleic acids tend to from helices stabilized by base stacking. The purine and pyrimidine bases of the nucleic acids are aromatic rings. These rings tend to stack like pancakes, but slightly offset so as to follow the helix. The stacks of bases are in turn stabilized by hydrophobic interactions and by van der Waals forces between the pi-clouds of electrons above and below the aromatic rings. In these helices the bases are oriented inward, toward the helix axis, and the sugar phosphates are oriented outward, away from the helix axis.
Helices in proteins
Properties of the peptide bond dominate the structures of proteins. Properties of the peptide bond include: 1. The peptide bond has partial double character. Partial double character is conferred by the electronegative carbonyl oxygen, which draws the unshared electron pair from the amide hydrogen. 2. The peptide bond is that the atoms of the peptide bond can form hydrogen bonds.