Chiral Centre

All the monosaccharides except dihydroxyacetone contain one or more asymmetric (chiral) carbon atoms and thus occur in optically active isomeric forms. The simples aldose, glyceraldehyde, contains a chiral centre and therefore has two different optical isomers, or enantiomers. By convention, one of these two forms is designated the D isomer of glyceraldehyde; the other is the L isomer.