Electron capture and deprotonation processes observed in collisions between Xe8+and multiply protonated cytochrome-C

Abstract

Electron-transfer processes in interaction between highly charged ions and multiply protonated proteins have been studied. Collisions between Xe${}^{8+}$ at 96 keV and protonated cytochrome-C at selected charge state ($q$ from 15+ to 19+) result in mass spectra composed mainly of intact molecular ions. From the spectra, single and double electron capture processes by Xe${}^{8+}$ from the protonated molecular ions were identified and the relative cross sections were measured. An unexpected process, the deprotonation process, was also observed. It is tentatively attributed to the loss of a proton induced by the strong electric field carried by the projectile ion in long-distance collisions. Upon charge variation of the molecular target from 15 to 19, the single and double electron capture cross sections remain nearly constant, while the relative cross section of the deprotonation process increases dramatically from 0.8% (\ifmmode\pm\else\textpm\fi{}0.1%) to 17% (\ifmmode\pm\else\textpm\fi{}1%). This strong charge dependency is explained by the decrease of the proton affinities with the charge. This proton removal process has not been observed previously. It seems to be specific to the long-distance Coulomb interactions between protons bound along the protein chain and the highly charged atomic ions.