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 10.3em0exK by Coulomb interaction with laser-cooled barium ions in a linear ion trap. In one case, an ensemble of 53 cytochrome c molecules (mass 120.3em0ex3900.3em0examu, charge $+17e$) was cooled by 160 laser-cooled barium ions to less than 0.750.3em0exK. Storage times of more than 200.3em0exmin have been observed and could easily be extended to more than an hour. The technique is applicable to a wide variety of complex molecules.