Diastereomers are optically active isomers that are not enantiomers. They are characterized by having more than one chiral center. A diastereomer with n chiral centers has 2n possible stereoisomers. Thus, for a diastereomer with two chiral centers, there are four possible stereoisomers: a pair of diastereomers with two pairs of enantiomers. One example is the diastereomers l-isoleucine and d-alloisoleucine. For isoleucine, the enantiomers are l- and d-isoleucine and for alloisoleucine the enantiomers are l- and d-alloisoleucine (see Fig. 3 in Amino Acid).
Diastereomers have different physical properties (different melting points, boiling points, solubilities, etc.). Their chemical properties also differ in that they react with reagents at different rates. Thus, they can be directly separated by chromatographic methods without the need for chiral reagents.