Radiation-induced electron capture by proteins containing disulphide linkages: an electron spin resonance study

Abstract

Exposure of dilute solutions of a wide range of proteins, enzymes, or polypeptides containing disulphide bonds to ⁶⁰Co γ-rays at 77 K gave readily detectable yields of (RSSR)^–σ* radicals, characterised by their e.s.r. spectra. The results suggest that electron capture at RSSR groups is remarkably selective, in the absence of other groups with higher electron affinities. In the absence of RSSR groups electron capture gave organic radicals (g ca. 2.002) presumably formed by dissociative electron capture at amide linkages. It is suggested that this method could be developed into a sensitive test for RSSR groups. These results imply that electrons added to proteins can move efficiently within the protein until they encounter S–S bonds. In the absence of such bonds and of other groups with high electron affinity they remain in the protein until they are trapped at amide linkages. In the particular case of the Cu^II form of superoxide dismutase, preferential electron capture occurred at Cu^II. However, after conversion to Cu^I, the expected (RSSR)^– radicals were detected.