Final answer:
Biologically active molecules and their enantiomers behave differently due to the specific three-dimensional shapes that dictate their binding interactions with other chiral molecules, like receptors, leading to different biological effects.
Step-by-step explanation:
The reason why a biologically active molecule and its enantiomer do not behave the same way in the body, especially when binding to a receptor, is primarily due to molecular shape and orientation. Enantiomers, also referred to as optical isomers, are a type of stereoisomers that consist of molecules which are mirror images of each other and cannot be superimposed. This chiral nature implies that although the enantiomers share the same chemical and physical properties in a non-chiral environment, their interactions with other chiral molecules, such as receptors or enzymes, will be notably different.
Receptors and enzymes are often chiral as well, meaning their binding sites will preferentially accommodate one enantiomer over the other, similar to how a left-hand glove will only fit comfortably on the left hand. This specificity in binding translates to different biological functions or effects within the body, as seen with enantiomers such as dextromethorphan and levomethorphan, which each cause different pharmacological effects despite being enantiomers of the same compound methorphan.
It's also important to note that the three-dimensional chiral arrangement of atoms in enantiomers affects how the molecules are able to interact with light—rotating polarized light in opposite directions, termed dextrorotatory (d-) or levorotatory (l-), which can be measured in a polarimeter. The dynamic nature of molecular interactions and the influence of chiral centers combine to ensure enantiomers behave differently when involved in biochemical reactions.