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 ow the rate of electron transfer from a Rydberg orbital on the amine site to the S S * orbital depends upon the distance between these two rbitals. These simulations are relevant to both electron-capture and electron-transfer dissociation mass spectrometry where protonated peptide or rotein samples are assumed to capture electrons in Rydberg orbitals of their protonated sites subsequent to which other bonds (especially S S and C ) are cleaved. By examining the dependence of three diabatic potential energy surfaces (one with an electron in the ground-state Rydberg rbital 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 he S S * orbital) on the S S bond length, critical geometries are identified at which resonant through-bond electron transfer (from either of the ydberg sites to the S S * orbital) can occur. Landau–Zener theory is used to estimate these electron transfer rates for three model compounds hat differ in the distance between the protonated amine and S S bond sites. Once the electron reaches the S S * orbital, cleavage of the S S ond occurs, so it is important to characterize these electron transfer rates because they may be rate-limiting in at least some peptide or protein ragmentations. It is found that the Hamiltonian coupling matrix elements connecting each of the two Rydberg-attached states to the *-attached tate decay exponentially with the distance between the Rydberg and * orbitals, so it is now possible to estimate the electron transfer rates for ther similar systems. 2006 Elsevier B.V. All rights reserved.