Electron capture and loss by protonated peptides and proteins in collisions with C₆₀ and Na

Electron capture and loss cross-sections have been measured for collisions between fast multiply charged peptide and protein ions [M+nH]ⁿ⁺ and Na or C₆₀. The ions were produced in an electrospray ion source (ESI) and accelerated to an energy of n × 50 keV. We find that the size of the cross-sections depend strongly on molecule size, ionization energy and projectile charge state n. For multiply charged ubiquitin projectiles, the cross-sections for electron capture are found to be several times larger with a Na target than with a C₆₀ target. This observation is qualitatively explained by means of an over-the-barrier model for electron transfer using metal-sphere descriptions for the electronic responses of the collision partners.