Ab-Initio Calculations on Arginine−Disulfide Complexes Modeling the One-Electron Reduction of Lysozyme. Comparison to an Experimental Reinvestigation

Abstract

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 reaction were characterized at the MP2 level. The influence of the environment was taken into account by using the Onsager reaction field method (SCRF) for the isolated species as well as the complexes.

Excess Electron Attachment to Disulfide-Bridged l,l-Cystine. An ab Initio Study

“…8 Even though the number of publications devoted to cysteinecontaining compounds seems large, it is surprising that, to the best of our knowledge, there have been no papers dealing with the possibility of an excess electron binding to cystine published in the literature thus far. The only attempt to study the nondissociative electron capture by the species which mimic the disulfide-bridged cystine containing one S-S bond is that reported by Carles et al and investigating the nondissociative electron capture by a series of saturated disulfides given by the R-S-S-R formula (where R stands for CH 3 , C 2 H 5 , and C 3 H 7 ). 9 The neutral model systems RS-SR were examined using Rydberg electron-transfer spectroscopy (RETS), negative ion photoelectron spectroscopy, and computational techniques (MP2 and DFT methods).…”

Single electron localisation on the cystine/cysteine couple: sulphur or carbon?

“…Experimental results demonstrated that the reactivity of the disulphide bonds could vary. In some proteins, like immunoglobulins, all disulphide bridges could be reduced [8], whereas, e.g., in hen egg white lysozyme, only one disulphide bridge was reactive [17]. This selectivity could not be related to the accessibility since non-reactive bonds were also located on the surface.…”

“…This is well known for the thiyl radicals [15,16] (Scheme 1). We could show that it can also happen with disulphides [17]. Indeed, electrons can also localise Scheme 1 Electron transfer in the disulphide:dithiol redox couple on the carbonyl functions in amino acids [18] or in peptides [19].…”

“…Our choice is justified by the very different reactivities of these two cystines: that of lysozyme (Fig. 1A) cannot be reduced by radiolysis and has an ''extended'' geometry [17]. That of thioredoxin (Fig.…”

Electron Attachment Step in Electron Capture Dissociation (ECD) and Electron Transfer Dissociation (ETD)

“…0.9 eV). In the study of ref 13, it was observed that an electron could bind either to the (NH 2 ) 3 C + site of the (NH 2 ) 3 C + ‚‚‚HSSH complex or to the HSSH site. The latter they viewed as a zwitterion having the character (NH 2 ) 3 C + ‚‚‚HSSH -, but they did not make the observation that the electron was bound in the S-S σ*-orbital.…”