Kinetic Isotope Effect

Synonyms

Isotope effect; Isotopic effect

Definition

A kinetic isotope effect (KIE) is an effect observed due to differences in the chemical reaction rates of a molecule with a heavier isotope (k _H) relative to one containing a lighter isotope (k _L):

$$ KIE={k}_{\mathrm{L}}/{k}_{\mathrm{H}}. $$

Molecules with different isotopic compositions generally react with other molecules in the same manner, but their reaction rates show differences due to their mass differences and sometimes also due to the lower natural abundance of certain isotopes. For example, ¹³C¹⁶O₂ is heavier than ¹²C¹⁶O₂ by about 2.3 %, and the abundance ratio of ¹³C¹⁶O₂/¹²C¹⁶O₂ is about 1.1 %. In photosynthesis by plants, the fixation rate of ¹³C¹⁶O₂ is slower than that of ¹²C¹⁶O₂, which causes a decrease of the ¹³C/¹²C ratio in biosynthesized glucose. Thus, the isotopic composition of natural samples is an important parameter in studying their origins. For example, the isotopic composition of organic carbon can be used...