Irradiation of isolated collagen mimetic peptides by x rays and carbon ions at the Bragg-peak energy

We report on an experimental irradiation of isolated peptides by carbon ions at the Bragg-peak energy. Collagen mimetic peptides and their noncovalent complexes undergo nondissociative ionization, but also inter- and intramolecular fragmentation induced by internal energy transfer. We also detect nondissociative proton detachment, an ion-specific process. The average amount of energy transferred by ions is similar to the case of x-ray single photoabsorption, and our results indicate that it changes with the conformation and size of a given molecular system. Our work paves the way to experimental investigation of high-energy ion collisions involving proteins and DNA strands, to investigate the role of ion kinetic energy in internal energy deposition.

Figure 1

Structure of the collagen triple-helix peptidic model ${\left({\left(\mathrm{PPG}\right)}_{10}\right)}_3$ (Protein Data Bank ID: 1K6F [32]). Backbones are shown as ribbons, with one color for each peptide.

Figure 2

Scheme of the experimental setup.

Figure 3

Mass spectra of the ${\left[{\left(\mathrm{PHypG}\right)}_{10}+3\mathrm{H}\right]}^{3+}$ peptide after irradiation by an x-ray beam at 150 eV (top) and the ${\mathrm{C}}^{4+}$ ion beam at 0.98 MeV/u (bottom). Red dashes indicate the precursor ion ($m/z=898.0$); the corresponding peak appears negative as explained in the experimental section. NDI stands for nondissociative ionization, $M^{2+}$ is the doubly protonated peptide ion, and $b_{3n}/y_{3n}$ backbone fragment ions have been assigned. In the ${\mathrm{C}}^{4+}$ spectrum, no peaks appear below $m/z=100$, due to the higher trap RF voltage and thus $m/z$ cutoff.

Figure 4

Mass spectra of the ${\left[{\left({\left(\mathrm{PPG}\right)}_{10}\right)}_3+7\mathrm{H}\right]}^{7+}$ trimer after irradiation by x rays at 150 and 288 eV as well as by ${\mathrm{C}}^{4+}$ ions. The position of the precursor ion ($m/z=1085.7$) is represented by red dashes. Nondissociative ionization (NDI), monomers ($M^{n+}$) and the main backbone fragment ions ($b_{3n}/y_{3n}$) are indicated in red, blue, and green, respectively. In the ${\mathrm{C}}^{4+}$ spectrum, no peaks appear below $m/z=100$, due to the higher trap RF voltage and thus $m/z$ cutoff.

Figure 5

Relative yields of nondissociative ionization (NDI), sum of monomers in 2+, 3+, and 4+ charge states, and backbone fragments of the ${\left[{\left({\left(\mathrm{PPG}\right)}_{10}\right)}_3+7\mathrm{H}\right]}^{7+}$ trimer as a function of the absorbed x-ray photon energy. The evolution of the different charge states of the monomer is also shown in the bottom plot. Uncertainties come from the method used for data analysis.

Figure 6

Relative yields of nondissociative ionization (NDI) and double ionization (NDDI), sum of monomers in 3+ and 4+ charge states, and backbone fragments of the ${\left[{\left({\left(\mathrm{PPG}\right)}_{10}\right)}_3+6\mathrm{H}\right]}^{6+}$ trimer as a function of the absorbed x-ray photon energy between 288 and 300 eV. The doubly charged monomer is not included since it has the same $m/z$ as the precursor ion and thus cannot be detected.

Figure 7

Relative yields of NDI, fragments, monomers, and proton detachment after x-ray (150 eV for monomers, 150 and 288 eV for trimers) and carbon irradiation as a function of charge state. The results are highly similar for both ${\left(\mathrm{PHypG}\right)}_{10}$ and ${\left(\mathrm{PPG}\right)}_{10}$ monomers, we thus only show the data for ${\left(\mathrm{PHypG}\right)}_{10}$, extracted from the spectra of Fig. 3. For ${\left[{\left({\left(\mathrm{PPG}\right)}_{10}\right)}_3+5\mathrm{H}\right]}^{5+}$, NDI cannot be separated from $M^{2+}$; they both contribute to the sum of monomers. For ${\left[{\left({\left(\mathrm{PPG}\right)}_{10}\right)}_3+6\mathrm{H}\right]}^{6+}$, the $M^{2+}$ monomers formed by dissociation of the triple helix have the same $m/z$ as the precursor ion, and thus do not contribute to the sum of monomers. Proton detachment from ${\left[{\left({\left(\mathrm{PPG}\right)}_{10}\right)}_3+7\mathrm{H}\right]}^{7+}$ and $M^{2+}$ have the same $m/z$; they both contribute to the sum of monomers.