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Molecule that shares a molecular formula with another molecule but which does not possess identical atom-to-atom chemical bonds in identical places.
Structural isomers are common among hydrocarbons and their derivatives. Isopropanol and Propanol, both three-carbon alcohols, for example, are structural isomers. That is, the carbon that is bonded to the hydroxyl group is directly bound to only a single other carbon atom in propanol whereas in isopropanol the carbon that is bound to the hydroxyl group is bound to two different carbon atoms. Note that structural isomers, especially for more complex molecules, are not limited to just two alternative forms.
While structural isomers share a molecular formula but possess different bonding patterns between atoms, for geometric isomers this is more subtly the case. For enantiomers the subtleties between isomeric forms are greater still. Nevertheless, all of these are examples of isomers, i.e., which share molecular formulas but which differ in the arrangement of atoms.
Structural isomers tend to have at least subtlety distinct chemical properties. The specificity of enzymes, however, assures that their structural differences result in substantial metabolic differences since very often an enzyme that can recognize one structural isomer will be unable to recognize an alternative isomer. As a result, the ability of a compound to participate in an organism's physiology – or the type of participation involved – typically will be dependent upon its isomeric form.
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