original_query: What is a kinetic isotope effect (KIE)?
query: What is a kinetic isotope effect (KIE)?
answer: A kinetic isotope effect (KIE) is the change in the reaction rate of a chemical reaction when one of the atoms in the reactants is replaced by one of its isotopes.  It's formally defined as the ratio of rate constants for reactions involving light and heavy isotopically substituted reactants.