7.1.3 Introduction to Optical Isomers
1. Introduction to Optical Isomers
Optical isomers are a type of stereoisomer that are non-superimposable mirror images of each other. They are also known as enantiomers. For more details on the definition and criteria of optical isomerism, refer to Optical Isomerism→.
Similar Physical and Chemical Properties
Optical isomers of the same compound generally exhibit identical physical properties such as density, melting point, and boiling point. Their chemical properties are also very similar when reacting with achiral reagents.
Key Differentiating Property: Optical Activity
The primary way to distinguish between optical isomers is their interaction with plane-polarized light. Enantiomers rotate the plane of plane-polarized light in opposite directions. One enantiomer rotates the light clockwise (dextrorotatory, (+) form), and the other rotates it counter-clockwise (levorotatory, (-) form).
Biological Systems and Chirality
A crucial difference for optical isomers lies in their physiological effects within the human body and other biological systems. Biological sensors and biochemical reactions are highly dependent on the precise three-dimensional shapes of molecules.
2. Examples of Biological Effects of Enantiomers
The body's ability to distinguish between enantiomers is evident in various biological responses:
Asparagine
One enantiomer of the amino acid asparagine has a bitter taste, while its counterpart imparts a sweet taste.
Limonene
The molecule limonene exhibits chirality, and its enantiomers have distinct sensory profiles:
- (+)-limonene: Found in lemons and oranges, typically associated with a citrusy smell and taste.
- (-)-limonene: Present in pine needles, peppermint, and spearmint, possessing a different smell or taste compared to the (+) isomer.
3. Chirality and Preparation of Drugs
Chirality plays a critical role in the pharmaceutical industry due to the varied biological activities of enantiomers.
3.1. Pharmacokinetics and Pharmacodynamics
Therapeutic drugs often exhibit optical isomerism because many biologically active molecules are chiral. The different enantiomers of a drug can have distinct effects on the body, which are studied in two main areas:
Pharmacokinetics: This field investigates how the body handles a drug. It covers absorption (how the drug enters the bloodstream), distribution (where the drug travels within the body), metabolism (how the drug is broken down), and excretion (how the drug is removed from the body).
Pharmacodynamics: This field focuses on how the drug affects the body. It includes the actions of the administered drug on various bodily systems and the specific way the drug binds to its target site such as a receptor protein.
3.2. Industrial Significance in Drug Development
Designing, developing, and supplying pharmaceutical drugs is a major industrial concern with significant financial and therapeutic implications.
Shape-Dependent Biological Activity: The shapes of biological systems like receptors and enzymes are highly specific, and they interact differently with chiral molecules. Consequently, one enantiomer of a chiral drug molecule can have vastly different biological activity compared to its mirror image.
Examples of Chiral Drugs with Different Enantiomeric Effects:
Propoxyphene: One enantiomer of propoxyphene acts as an analgesic (pain-relieving), while the other possesses anti-coughing properties. Despite their differing actions, their commercial names might reflect their mirror-image relationship.
Thalidomide: This drug was famously prescribed as a sedative and to treat morning sickness in pregnant women. However, it led to severe birth defects. Subsequent research revealed the critical difference between its enantiomers:
- (+)-thalidomide: Acts as an effective sedative with no known side effects.
- (-)-thalidomide: Is teratogenic, meaning it causes birth defects.
For information on the separation techniques used to isolate these enantiomers, see Separation of Racemic Mixture of Thalidomide→.
Thalidomide Interconversion: A tragic aspect of the thalidomide case was the discovery that the two enantiomers readily interconvert within the body. This means that even if a pure (+)-thalidomide (sedative) was administered, it would convert into the harmful (-)-thalidomide (teratogenic) in vivo, rendering the administration of a single enantiomer ineffective in preventing the birth defects.