Organic chemistry's D and L configurations indicate the orientations of pairs of optical isomers, that is, molecules that are mirror images of each other but that cannot be superimposed. These capital letter types are related to, but do not always match, the lowercase d and l configurations of optical isomers, which indicate which direction a solution of these molecules bend light. The capital versions are based on a standard.
Organic molecules are often tremendously complex, and this complexity comes not just in the number of different atoms bound together, but in the ways they are bound together. For most molecules, these configurations include two or more optical isomers, configurations which are mirror images of each other, according to Elmhurst College. This property of molecules is known as chirality. Despite being identical in terms of mass and most chemical reactions, in biological systems, they are treated very differently. In the majority of cases, only one configuration is biologically active, with the other either totally inactive, harmful or subject to conversion before use.
Because of this, the biological mechanisms of organisms always create molecules with the same chirality. In inorganic natural systems, chemical reactions that produce chiral molecules tend to create equal numbers of each isomer.