Principles of Isotope Exchange

Study of the initial rates of bisubstrate and trisubstrate reactions in both directions, and in the presence and absence of products, wOl usually eliminate many possible reaction pathways and give a reasonably good idea of the main features of the mechanism, but it will not usually reved the existence of minor alternative pathways. Even if a clear mechanism does emerge from the initial rate and product inhibition studies, it is valuable to confirm its validity independently. The important technique of isotope exchange can often satisfy these requirements. 

This technique was initially developed by Boyer (1959) and subsequently considerably extended to involve rate measurement of loss, or exchange, of essentially il possible atoms or functional groups, the determination of kinetic isotope effects, the definition of stereochemical processes, and investigation of activators and inhibitors (Wedler Shalango, 1982 Wedler, 1995 Purich AUison, 2000). 

AH of the studies described in earlier chapters were concerned with the observed net velocity of a chemical reaction. In die steady state, the net velocity is a difference between the absolute forward and reverse velocity of any step. The unidirectional velocity of a step may be considerably faster than the observed net velocity. Isotope exch ge is based on the simple fact that, in a chemical reaction, even if it is at equilibrium, when its net rate is zero by definition, the unidirectional rates through steps or groups of steps can be measured by means of isotopic tracers. Therefore, the isotope exchange studies provide a way of measuring the unidirectional rates of individual steps within a reaction sequence. 

The introduction of a labeled substrate or product may be utilized to trace the course of the reaction quantitatively. Depending upon the position of the isotopic atom(s) in the labeled substrate or product, various exchanges may be possible. Various types of exchanges may be illustrated by considering a bisubstrate reaction in the following form  

Isotope Half-life Isotope weight abundance (%) 

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