Catching Proteins in Liquid Helium Droplets

An experimental approach is presented that allows for the incorporation of large mass-to-charge ratio selected ions in liquid helium droplets. It is demonstrated that droplets can be efficiently doped with a mass-to-charge ratio selected amino acid as well as with the much bigger $m\approx 12000$ amu protein cytochrome C in selected charge states. The sizes of the ion-doped droplets are determined via electrostatic deflection. Under the experimental conditions employed, the observed droplet sizes are very large and range, depending on the incorporated ion, from ${10}^{10}$ helium atoms for protonated phenylalanine to ${10}^{12}$ helium atoms for cytochrome C. As a possible explanation, a simple model based on the size and internal energy dependence of the pickup efficiency is given.

Figure 1

Scheme of the experimental setup. Ions are brought into the gas phase via electrospray ionization, are mass-to-charge ratio selected by a quadrupole mass spectrometer and stored in an ion trap. Helium droplets can pick up those ions and after some distance downstream, the direct ion current is measured. Electric field deflection can be used to determine the droplet size distribution (see text).

Figure 2

(a) Time of flight profiles of CytC ions ($z=+14$) embedded in He droplets. The He droplets are produced at $P_0=30\mathrm{bar}$ and varying source temperatures $T_0$ from 5 K to 11 K. With increasing $T_0$ the peak shifts to earlier arrival times. A considerable portion of the peak widths can be attributed to the time response of the current amplifier. Measured velocities [(blue) points and curve] of ion-doped helium droplets as a function of source temperature $T_0$ (b) and pressure $P_0$ (c). Also shown are predictions for an ideal He gas expansion [(green) solid curve] as well as a liquid expansion [(red) dashed curve] following the Bernoulli expression.

Figure 3

Deflection profiles for three different charge states of CytC $z=+9$, $+14$, and $+17$ and protonated singly charged Phe at an electric field of $E=5.7\mathrm{kV}/\mathrm{cm}$ (diamonds) and at zero field (filled circles). Also shown are the respective fitted curves (solid lines).