Helices are chiral too. The a helix found in proteins is right handed, like a regular screw thread. Reflect a length of right-handed a helix in a mirror, and you will get a left-handed a helix composed of d amino acids. Because of the actual structures, it turns out that lamino acids fit nicely in a right-handed a helix, while d amino acids fit nicely in a left-handed a helix. (If you doubt the chirality of helices, try making wire models of the two shown in the accompanying figure and see if they can be superimposed.)

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