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 SS σ* 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 SS and NCα) 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 SS σ* orbital) on the SS bond length, critical geometries are identified at which resonant through-bond electron transfer (from either of the Rydberg sites to the SS σ* 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 SS bond sites. Once the electron reaches the SS σ* orbital, cleavage of the SS 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 σ*-attached state decay exponentially with the distance between the Rydberg and σ* orbitals, so it is now possible to estimate the electron transfer rates for other similar systems.