Translational cooling and storage of protonated proteins in an ion trap at subkelvin temperatures

Gas-phase multiply charged proteins have been sympathetically cooled to translational temperatures below $1\mathrm{K}$ by Coulomb interaction with laser-cooled barium ions in a linear ion trap. In one case, an ensemble of 53 cytochrome c molecules (mass $\simeq 12390\mathrm{amu}$, charge $+17e$) was cooled by $\simeq 160$ laser-cooled barium ions to less than $0.75\mathrm{K}$. Storage times of more than $20\min$ have been observed and could easily be extended to more than an hour. The technique is applicable to a wide variety of complex molecules.

Figure 1

(Color online) Schematic overview of our setup. Singly or multiply protonated molecular ions from an ESI ion source are selected by a quadrupole mass filter and transferred via a rf octopole ion guide to a linear quadrupole trap in an UHV chamber. (a) CCD image of a typical trapped barium ion Coulomb crystal used as coolant for the molecular ions. (b) Secondary structure and ESI mass spectrum of cytochrome c showing the distribution of the different grades of protonation.

Figure 2

Radial excitation spectra of barium-ion crystals containing differently protonated cytochrome ions. The upper $\left({\mathrm{Cyt}}^{12+}\right)$ and lower $\left({\mathrm{Cyt}}^{17+}\right)$ curves were acquired at the same trap parameters and yield the same barium frequency of $95.4\mathrm{kHz}$ used for calibration. The obtained cytochrome frequencies are well confirmed by their theoretical values.

Figure 3

Ion extraction mass spectra of ${\mathrm{Ba}}^{+}∕{\mathrm{Cyt}}^{17+}$ ensembles at different temperatures. In the noncooled case (gray) the mass spectrum is broadened compared to the laser-cooled case (black). Here, not only the peak of the laser-cooled ${\mathrm{Ba}}^{+}$ ions, but also that of the simultaneously trapped ${\mathrm{Cyt}}^{17+}$ ions is narrower, which is a proof of their sympathetic cooling.

Figure 4

(Color online) Experimental and simulated CCD images of a ${\mathrm{Ba}}^{+}∕{\mathrm{Cyt}}^{17+}$ and a ${\mathrm{Ba}}^{+}∕{\mathrm{Cyt}}^{12+}$ ion crystal. (a), (d) Simulated images in radial (left) and axial view (right), as they would appear if all ions would fluoresce. The cooling lasers propagate to the left, separating the ${}^{138}\mathrm{Ba}^{+}$ ions (blue) and the barium isotopes (red) axially due to the light pressure force. The cytochrome ions (green) form a sheath around the barium ion subensemble. (b), (e) Experimental image of the barium-cytochrome-ion crystal and its simulation, showing only the fluorescing ${}^{138}\mathrm{Ba}^{+}$ ions. (c), (f) Crystals after the removal of the cytochrome ions. Here, the barium-ion ensemble is no longer deformed nor heated by the molecular ions.