Polyalanine is an amino acid polymer that can be produced as multiple isomers. The repeating sequence found in polyalanine is -NH-CH(CH3)-CO. The L isomer of polyalanine is the one most often studied because it is the isomer that occurs naturally in proteins. Depending on the method used to prepare the L isomer of polyalanine, the substance may take one of two substantially different crystalline forms. These forms consist of a right-handed alpha helix in which the chain is somewhat compressed with intrachain hydrogen bonds and a beta sheet wherein the chain is comparatively long with interchain hydrogen bonds. The structure of the alpha helix of polyalanine and other amino acid polymers were first described in 1951.