Definition: A phenomenon in which a chemical compound exists in two forms of different structure (isomers) in equilibrium, the two forms differing, usually, in the position of a hydrogen atom; keto-enol tautomerism, R–CH2–C(O)–R' ? R–CH=C(OH)–R'.
[G. tautos, the same, + meros, part]
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Examples: glitazone, GI cocktail, etc.