the alpha helix
/ AL-fuh HEE-licks /
Picture taking the polypeptide chain and winding it up into a tight, right-handed spiral, like the coils of an old telephone cord or the thread of a screw. That coiled shape is the alpha helix, one of the two most common patterns a protein backbone falls into.
What holds the spiral together is a regular ladder of hydrogen bonds running up its length. The carbonyl oxygen (the C=O) of each residue reaches forward and hydrogen-bonds to the amide hydrogen (the N-H) of the residue four positions further along the chain. Repeat this all the way up and you get a stable coil that turns once roughly every 3.6 residues. The backbone forms the inner core of the spiral; the side chains all point outward, like bristles on a bottle brush, so they are free to interact with the protein's surroundings. Because of this geometry, a helix can be water-loving on one side and greasy on the other — an amphipathic helix — which is exactly what is needed to sit at the boundary between water and oil.
Alpha helices are everywhere. They are the rods that span cell membranes in many receptors, the recognition fingers many proteins poke into the grooves of DNA, and the springy filaments of hair, nails, and wool (keratin). A practical caveat: not every sequence will form a helix — proline, whose ring kinks the backbone, and glycine, which is too floppy, both tend to interrupt helices, which is why they often mark the boundaries of helical segments.
Stretch a hair gently and it springs back: you are stretching and re-coiling millions of alpha helices of keratin. Stretch it too far, the hydrogen bonds break, and the helix unwinds — the hair stays longer until the bonds reform.
The springiness of hair is the springiness of countless alpha helices.
The hydrogen bonds run along the backbone (i to i+4), not through the side chains, and the helix is specifically right-handed. Proline and glycine are common helix-breakers.