Low-energy (0.1 eV) electron attachment S-S bond cleavage assisted by coulomb stabilization

Electron capture by the ion H3C-S-S-CH2-CH2-NH3+ at either the -NH3+ site (to form the Rydberg radical H3C-S-S-CH2-CH2-NH3) or into the S-S antibonding sigma* orbital is shown to be able to produce the same S-S bond fragmentation products H3C-S and HS-CH2-CH2-NH2, albeit by very different pathways. Capture into the S-S sigma* orbital is, in the absence of the nearby positive site, enclothermic by approximately 0.9 eV and leads to an electronically metastable anion that can undergo dissociation or autodetachment. In contrast, in the presence of the stabilizing Coulomb potential provided by the nearby NH3+ site, electron attachment into the S-S sigma* orbital is rendered exothermic. As a result, as we have shown in this paper, the effective cross sections for forming the H3C-S and HS-CH2-CH2-NH2 products via attachment at the -NH3 and S-S sigma* sites are predicted to be comparable for our model compound. Moreover, we predict that the sigma* site will become more amenable to electron attachment compared with the -NH3+ site for compounds in which the distance between the S-S bond and the protonated amine is larger than in our cation. These findings and insights should be of substantial value to workers studying bond cleavage rates and fragmentation patterns in gaseous positively charged samples of peptides and proteins. (c) 2005 Wiley Periodicals, Inc.