Relationship between the time-dependence of a transient-state kinetic isotope effect and the location of complexes in a reaction sequence.

We prove here a new transient-state kinetic rule which states that the ratios of the first derivatives of kinetic isotope effect time courses, extrapolated to zero time, provide integral values which specify the precise step number in the reaction sequence in single isotope substituted reactions. This rule defines such absolute numbers even where the steps involved are too fast to provide measurable concentrations of intermediates and when the full reaction sequence is unknown.