Radiation-Induced Oxidation of Protein in Aqueous Solution

Development of a comprehensive chemistry of aqueous protein systems has been limited by a number of experimental factors. One of the major problems arises from the fact that the complex chemical composition of protein offers a multiplicity of loci for radiation-induced reaction. The magnitude of the analytical problems involved in characterizing these reactions is indicated most directly by studies of the amino acid composition of hydrolyzates prepared from irradiated protein solutions. Although the aromatic and heterocyclic constituents and, in particular, the sulfur moieties appear to be the most susceptible to indirect action, the evidence is that the over-all reaction of protein involves a diversity of amino acid residues even at the lowest irradiation levels (1-3). In fact, quite recent studies of the radiation-induced inactivation of enzymes containing a maximal number of free -SH groups show that an appreciable fraction of the absorbed energy is not accounted for simply in terms of the destruction of sulfhydryl function (4, 5). Although such studies of the loss of components and functions of protein structure have provided empirical estimates of the radiation sensitivity of amino acid residues, still, most concepts regarding the nature of the products and the mechanism of the reactions involved are largely based on various types of indirect evidence (6). The present paper describes a direct approach to the study and measurement of radiation-induced modifications in the covalent structure of protein. The reactions to be considered are largely those of the N-C bond which, in its various chemical forms, represent the single most characteristic covalent linkage of protein structure.