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The one-electron reduction of hen egg white lysozyme has been reinvestigated by γ-radiolysis using CO2•- free radicals as reductants. We show that the reaction is specific toward one out of the four disulfide bridges, i.e. the 6−127 one. This bond is in interaction with the charged end of arginine 5. The reduction leads to thiol functions and to a lesser extent to fragmentation of the polypeptide chain, which can only come from electron migration from disulfide. To get a better insight into the mechanism induced by electron transfer to the protein, the 6−127 disulfide bridge and the charged end of arginine 5 in lysozyme were modelized by R2S2 (R = H, CH3) and C(NH2)3+, and ab-initio calculations were performed. All separate molecular and radical entities resulting from the electron addition were optimized with two basis sets (6-31G* and 6-31+G*) and at the MP2 correlation level. The formation of complexes was studied and four zwitterionic and two neutral radical complexes involved in charge transfer react...