AB-INITIO CALCULATIONS ON ARGININE-DISULFIDE COMPLEXES MODELING THE ONE-ELECTRON REDUCTION OF LYSOZYME - COMPARISON TO AN EXPERIMENTAL REINVESTIGATION

The one-electron reduction of hen egg white lysozyme has been reinvest igated by gamma-radiolysis using CO2.- free radicals as reductants. We show that the reaction is specific toward one out of the four disulfi de bridges, i.e. the 6-127 one. This bond is in interaction with the c harged end of arginine 5. The reduction leads to thiol functions and t o a lesser extent to fragmentation of the polypeptide chain, which can only come from electron migration from disulfide. To get a better ins ight into the mechanism induced by electron transfer to the protein, t he 6-127 disulfide bridge and the charged end of arginine 5 in lysozym e were modelized by R2S2 (R = H, CH3) and C(NH2)(3)(+), and ab-initio calculations were performed. All separate molecular and radical entiti es resulting from the electron addition were optimized with two basis sets (6-31G and 6-31+G*) and at the MP2 correlation level. The format ion of complexes was studied and four zwitterionic and two neutral rad ical complexes involved in charge transfer reaction were characterized at the MP2 level. The influence of the environment was taken into acc ount by using the Onsager reaction field method (SCRF) for the isolate d species as well as the complexes.