10. t5 13 C as an Indicator of Carboxylation Reactions

The nomenclature b 13C describes quantitatively the phenomenon of fractionation of carbon isotopes. Discrimination between isotopes emphasizes the differences in thermodynamic and kinetic properties of the isotopic species and generally assumes the chemical behavior of the species is similar. UREY (1947) advanced the theory of isotope fractionation in a paper on the thermodynamic properties of isotopic substances, and this work predicted that fractionation would occur between the two stable isotopes of carbon, 12C and 13C. Atmospheric CO 2 is a mixture of the stable isotopes 12C160160, 13C160160 and 12C180180 and fractionation between the stable carbon isotopes can be expected during the assimi­ lation of CO 2 in photosynthesis. As reviewed in this chapter, the carboxylation reactions of photosynthesis are the major sites of carbon isotope fractionation in higher plants. This phenomenon can be used to identify particular carboxylation reactions in plants and to study heavy-atom isotope effects associated with enzymes. The results of these studies have now been extended to disciplines as diverse as paleoecology, animal physiol­ ogy, agronomy and medicine.