isomer
Meaning of isomer: Same atoms in the same numbers, assembled differently, and often with a completely different effect inside the body.
Definition of isomer
An isomer is one of two or more compounds built from exactly the same atoms in the same numbers but joined or oriented differently. Because shape decides how a molecule fits the proteins it acts on, isomers of one formula can differ in melting point, solubility, smell, taste and biological activity. Two isomers may have opposite effects, or one may be active while the other is inert.
What kinds are there?
The differences fall into a few recognized patterns, running from gross rearrangement to a subtle difference in handedness.
| Kind | What differs | Everyday illustration |
|---|---|---|
| Structural | Which atom is bonded to which | Straight chain against a branched chain |
| Geometric | Arrangement across a rigid double bond | The cis and trans forms of a fat |
| Optical | Mirror image handedness | A left hand against a right hand |
Why does handedness matter in medicine?
A carbon atom carrying four different groups can be built two ways, and the two versions are mirror images that cannot be superimposed, in the way a left glove will not fit a right hand. The receptors and enzymes of the body are themselves handed, so they grip one form well and the other poorly. The consequences are practical. Many drugs are sold as a mixture of both forms, in which only one does the work while the other is carried along. Some are purified to the active form alone, which is why a drug name occasionally begins with a letter such as S or with the prefix levo or dextro. In a few well documented instances the inactive form is not harmless but toxic, a lesson that reshaped how medicines are licensed.
Where else do isomers show up?
Sugars are a familiar case. Glucose, fructose and galactose share the formula for six carbons, twelve hydrogens and six oxygens, yet the body handles each differently and they taste different. Vitamin A relatives, steroid hormones and fats all exist in multiple forms whose activity depends on shape. In laboratory work an isomer explains why one chemical analysis can report a single formula while the sample contains several distinct substances, so techniques that separate by shape are used before a compound is identified with confidence. Isotopes are a different idea altogether and are easily confused with isomers because the words look alike. An isotope is one element whose atoms carry different numbers of neutrons, which changes the weight of the atom rather than the arrangement of a molecule.
Used in a sentence
The compound was resolved into its two optical isomers before biological testing.
Good to know
Isomers of one formula can differ sharply in potency and in toxicity, so a drug purified to a single optical form is not interchangeable with the mixture of both.
Word origin
Where “isomer” comes from
From the Greek isos, meaning equal, plus meros, meaning part, so a compound made of equal parts.