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 approximate to 12 000 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.