(also called diastereomers), stereoisomers with several asymmetrical centers in which the configuration of some of the asymmetrical atoms coincides and that of others is opposite. In contrast to optical antipodes, in which the configuration of all asymmetrical centers is opposite and which differ from each other only in the sign of optical rotation (with complete coincidence of all other properties) diastereoisomers differ not only in the extent of optical rotation but also in other physical properties, such as melting point and solubility, in some chemical properties (for example, reaction rates). For example, mesotartaric acid is a disastereoisomer with respect both to (-) -tartaric and (+)-tartaric acid, although the two are optical antipodes. The method of splitting racemic compounds into optically active substances is based on the difference in the physical properties of disastereoisomers. For example, if an optically active base acts on racemic acid, the resultant salts will be disastereoisomers and can be separated by ordinary methods such as crystallization, after which optically active acids are regenerated from the salts. Racemic bases are separated in a similar manner.