Electron capture and deprotonation processes observed in collisions between Xe${}^{8+}$ and multiply protonated cytochrome-C

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% (±0.1%) to 17% (±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.

Figure 1

Experimental setup.

Figure 2

Time diagram of the experiment.

Figure 3

Mass spectra of trapped protonated cyt-C [$M$+18H]¹⁸⁺ without (a) and with Xe${}^{8+}$ ion irradiation (b). SEC, DEC, and DP stand for single electron capture, double electron capture, and deprotonation, respectively. The upper scale indicates the charge of intact molecular ions.

Figure 4

Mass spectra (a)–(e) in collisions between Xe${}^{8+}$ and protonated cyt-C, [$M$ + $q$H]${}^q$${}^{+}$ ($q$ = 15–19). The upper scale indicates the charge of intact molecular ions.

Figure 5

(a) One-letter sequence of bovine cytochrome-C. Alanine (A), arginine (R), asparagine (N), aspartic acid (D), cysteine (C), glutamic acid (E), glutamine (Q), glycine (G), histidine (H), isolucine (I), leucine (L), lysine (K), methionine (M), phenylalanine (F), proline (P), serine (S), threonine (T), tryptophan (W), tyrosine (Y), valine (V). Arginine (R) and lysine (K) basic residues are labeled in red. The numbers indicate the position of residues on the backbone of cyt-C protein from the N terminus. (b) Model structures of cytochrome-C for extended structure assuming dihedral angles of 180° for each peptide bond, followed by structural relaxation using the Assisted Model Building with Energy Refinment force field. (c) Native structure as obtained from x-ray diffraction, taken from the protein database pdb (1CYC).

Figure 6

Scheme of geometrical collision con-figuration.

Figure 7

Cross sections for the single (diamonds) and double (squares) capture and deprotonation (triangle) processes versus the charge of protonated cyt-C, [$M$ + $q$H]${}^q$${}^{+}$. The lines are to guide the eye.

Figure 8

Calculated apparent proton affinity as a function of the charge of protonated cyt-C, [$M$ + $q$H]${}^q$${}^{+}$.