The crystal structures of polymorphisms of free amino acids have been reviewed. The polymorphisms were classified into three types: The first, including DL-methionine, DL-norleucine and DL-a-amino-butyric acid, has polymorphs of different packing arrangement of molecular double layers which is rigidly bound by hydrogen bonds. In this type, the polymorphs can interconvert in solid state by temperature. The second type, glycine, L-cystine and L-histidine, has polymorphs in which hydrogen bonding schemes are different although molecular conformations are similar. The third type, L-cystein, L-aspartic acid and L-glutamic acid, has polymorphs with different hydrogen bonding schemes and different molecular conformations. In this type, metastable form converts irreversibly to stable form by solvent-mediated transformation mechanism.
