Distance dependence of through-bond electron transfer rates in electron-capture and electron-transfer dissociation

Ab initio electronic structure calculations on model cations containing a disulfide linkage and a protonated amine site are carried out to examine how the rate of electron transfer from a Rydberg orbital on the amine site to the S-S sigma* orbital depends upon the distance between these two orbitals. These simulations are relevant to both electron-capture and electron-transfer dissociation mass spectrometry where protonated peptide or protein samples are assumed to capture electrons in Rydberg orbitals of their protonated sites subsequent to which other bonds (especially S-S and N-C-alpha) are cleaved. By examining the dependence of three diabatic potential energy surfaces (one with an electron in the ground-state Rydberg orbital of the protonated amine, one with the electron in an excited Rydberg orbital on this same site, and the third with the electron attached to the S-S sigma* orbital) on the S-S bond length, critical geometries are identified at which resonant through-bond electron transfer (from either of the Rydberg sites to the S-S sigma* orbital) can occur. Landau-Zener theory is used to estimate these electron transfer rates for three model compounds that differ in the distance between the protonated amine and S-S bond sites. Once the electron reaches the S-S sigma* orbital, cleavage of the S-S bond occurs, so it is important to characterize these electron transfer rates because they may be rate-limiting in at least some peptide or protein fragmentations. It is found that the Hamiltonian coupling matrix elements connecting each of the two Rydberg-attached states to the sigma*-attached state decay exponentially with the distance between the Rydberg and sigma* orbitals, so it is now possible to estimate the electron transfer rates for other similar systems. (C) 2006 Elsevier B.V. All rights reserved.