76 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



ISOTOPE EFFECTS 



The term isotope effect is used to denote the influence upon reactions 

 due to the presence of a radioisotope label or of an abnormal mixture of 

 stable isotopes. Effects resulting from the physiologic response of tissues 

 to the radiation are not considered in this category. Since isotopes of the 

 same element differ in mass and thus in mobility, it is expected that there 

 would be some differences in physical and chemical behavior. As the 

 extreme case, the differences between normal hydrogen (H^, deuterium 

 (H^ or D), and tritium (H^ or T) are large and well recognized, because 

 the actual mass difference represents an appreciable fraction of the atomic 

 weight of the element. Thus one scarcely considers H2O, D2O, and T2O 

 as the same substance from any point of view. In fact, optical activity 

 has been detected in molecules with no centers of asymmetry except those 

 created by the substitution of deuterium for hydrogen (39). In general, 

 the higher the atomic weight of the element, the less will be the fractional 

 mass difference between isotopes, and the magnitude of the isotope effect 

 will be correspondingly decreased. From the standpoint of tracer exper- 

 iments, it is important to anticipate the occurrence and magnitude of 

 these effects. 



The mass differences may exert their effect either upon the equilibrium 

 constant of a reaction or upon the rate of a reaction. An example of the 

 former is as follows : 



Ci^N- + HC12X ;^ C12N- + HC^^N (2-7) 



gas solution gas solution 



If there were no isotope effect, the equilibrium constant for this reaction 

 would be unity, whereas it is actually 1.012 at 25°C (52). This means 

 that the amount of C^* in gaseous HCN is slightly greater than that in the 

 ion CN~, with which it is in equilibrium. It is thus shown that the 

 mass-difference effect is not the same for different chemical combinations 

 of the isotope. However, the effect on reaction rates is perhaps of more 

 importance and will be illustrated later. 



It should be noted that many elements occur naturally as mixtures of 

 isotopes and that the radioisotope is often of intermediate mass. For 

 example, stable copper consists of two isotopes of mass numbers 63 and 

 65 with relative abundances of 70.1 and 29.9 per cent, respectively. The 

 experimental behavior of radiocopper of mass 64 in a biological system 

 would not be expected to be greatly different from the over-all behavior 

 of the naturally occurring mixture, so far as mass effects are concerned. 

 In accurate measurements of some processes such as diffusion, however, 

 differential behavior at this level of mass difference may be observed. 

 Thus, after 16 hr of contact between solid copper metal and silver sulfide 



