Catching proteins in liquid helium droplets

TL;DR

This paper introduces an experimental method for incorporating large ions, including proteins, into liquid helium droplets, enabling size determination and analysis of ion-doped droplets with potential applications in molecular spectroscopy.

Contribution

It demonstrates the first successful doping of large biomolecules like Cytochrome C into liquid helium droplets and provides a model explaining droplet size variations based on ion properties.

Findings

Droplets can be doped with amino acids and proteins up to 12,000 amu.

Droplet sizes range from 10^10 to 10^12 helium atoms depending on the ion.

A size- and energy-dependent pickup efficiency model is proposed.

Abstract

An experimental approach is presented that allows for the incorporation of large mass/charge selected ions in liquid helium droplets. It is demonstrated that droplets can be efficiently doped with a mass/charge selected amino acid as well as with the much bigger m$\approx$12 000 amu protein Cytochrome C in selected charge states. The sizes of the ion-doped droplets are determined via electrostatic deflection. Under the experimental conditions employed, the observed droplet sizes are very large and range, dependent on the incorporated ion, from 10$^{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.