  Gas-phase multiply charged proteins have been sympathetically cooled to
translational temperatures below 1 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 ~ 12390 amu, charge +17 e) was cooled by ~ 160 laser-cooled
barium ions to less than 0.75 K. Storage times of more than 20 minutes have
been observed and could easily be extended to more than an hour. The technique
is applicable to a wide variety of complex molecules.
