Accessible Rate Constants Using Rapid Reaction Methods

In general, photolysis induces substitutional and redox-related changes, whereas pulse radiolysis primarily promotes redox chemistry. Indeed one of the unique features of the latter method is to induce unambiguous one electron reduction of multi-reducible centers. Metalloproteins can be rapidly reduced to metastable conformational states and subsequent changes monitored. 

Monitoring of events following perturbations can be achieved in much shorter times by photolysis. A variety of monitoring techniques have been linked to both methods (Table 3.7). It is valuable to obtain kinetic data by more than one method, when possible. The measurement of spin-change rates have, for example, been carried out by a variety of rapid-reaction techniques, including temperature-jump, ultrasonics and laser photolysis with consistent results (Sec. 7.3). 

In this method, the reaction under study competes with another fast process, which may be spin relaxation (nmr and epr), fluorescence or diffusion towards an electrode. Monitoring of the competition is generally internal, making use of the characteristics of the fast process itself. This approach will be treated in some of the next sections. 

7 Accessible Rate Constants Using Rapid Reaction Methods 

It is the half-life of a reaction that will govern the choice of the initiation method (Fig. 3.1) and it is the character of the reaction that will dictate the monitoring procedure (Sec. 3.8 on). 

---