| Abstract: | The equation of Northrop 1975, Biochemistry, 14, 2644] for calculating intrinsic isotope effects from observed deuterium and tritium isotope effects of V/K, in which hydrogen is the reference isotope, has been extended to experimental designs using either deuterium or tritium as a reference. Partial derivatives of the intrinsic equations allow calculation of the relative precision of the three referenced isotope effects and these favor the order deuterium > tritium > hydrogen. In comparisons of observed and calculated isotope effects when hydrogen tunneling is present, both the precision and the magnitude of the difference was greater for intrinsic calculations than for exponentiations based upon a breakdown in the Swain-Schaad relationship. |
