Kinetic Isotope Effects of RuBisCO carbon, oxygen, and/or hydrogen, though currently only a significant isotope effect is seen for carbon isotope substitution. Isotopes are atoms that have the same number of protons but varying numbers of neutrons. "Lighter" isotopes (like the stable Carbon-12 isotope) have a smaller overall mass, and "heavier" isotopes (like the stable Carbon-13 isotope or radioactive Carbon-14 isotope) have a larger overall mass. Stable isotope geochemistry is concerned with how varying chemical and physical processes preferentially enrich or deplete stable isotopes. Enzymes like RuBisCO cause isotopic fractionation because molecules containing lighter isotopes have higher zero-point energies (ZPE), the lowest possible quantum energy
Kinetic isotope effect "k"/"k", of 1.0133±0.0002 along with a hydrogen kinetic isotope effect of 3.2 at the leaving hydrogen. Similarly, combining nitrogen and hydrogen isotope effects was used to show that syn eliminations of simple ammonium salts also follow a concerted mechanism, which was a question of debate before. In the following two reactions of 2-phenylcyclopentyltrimethylammonium ion with ethoxide, both of which yield 1-phenylcyclopentene, both isomers exhibited a nitrogen isotope effect "k"/"k" at 60C. Although the reaction of the trans isomer, which follows syn elimination, has a smaller nitrogen kinetic isotope effect (1.0064) compared to the cis isomer which undergoes anti elimination (1.0108),