Since its introduction, pulse radiolysis has been an important technique for examining the properties of organic and inorganic radicals, and for enumerating those reactions responsible for cellular damage by ionizing radiation. Biochemists, and biophysicists outside the area of radiation biology appear, perhaps for historical reasons, to have an incomplete appreciation of the technique's potential. Protein chemists in particular, have been only dimly aware of the numerous reports of, and the significant results obtained from pulse radiolysis studies of proteins. Our purpose here is to bring some of these results together in order to emphasize the power and usefulness of pulse radiolysis experiments both for elucidating enzyme reaction mechanisms, and for gaining information on the structure of proteins in aqueous solutions. Reviews containing related, or in part the same material to be covered here have appeared previously; for example, Land (1970), Adams et al. (1972a), Shafferman & Stein (1975), Adams & Wardman (1977). This review updates these earlier works, but more importantly approaches the topic of protein pulse radiolysis with a different emphasis.

It is generally assumed that the mechanism of enzyme-catalysed reactions would be defined if all the intermediates, complexes and conformational states of each enzyme could be characterized, and the rate-constants for their inter-conversion recorded. In spite of the introduction during the last decades of methods for rapid data acquisition, which permit detection of the number and sequence of intermediates and complexes, measurement of rate-constants, identification of the types of catalysis involved, etc., at best a semi-quantitative understanding of the mechanism of enzyme-catalysis is obtained. The definition of the exact chemical nature of intermediates and complexes is missing because techniques establishing the structures are restricted to the study of transient states which are stable for periods that exceed the half-life of most of typical intermediates. In such conditions, while conformational changes are obviously an essential feature of enzyme activity, the conformational basis of such activity cannot be understood at the molecular level, and enzyme catalysis is still termed a ‘miracle’ compared to the rate-enhancements and specificity of ordinary chemical catalysts.