Freeze-Dried Biomolecules: FT-ICR Studies of the Specific Solvation of Functional Groups and Clathrate Formation Observed by the Slow Evaporation of Water from Hydrated Peptides and Model Compounds in the Gas Phase

Abstract

Solvent evaporation from extensively hydrated peptides and selected model compounds formed by electrospray ionization has been examined using an external ion source Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. Water evaporation from the clusters, formed at room temperature by appropriate operation of an electrospray ion source, is initially rapid and results in evaporative cooling of the clusters to a temperature around 130−150 K, determined by the balance between evaporative cooling and heating by background blackbody radiation. In this “freeze-drying” process, it is observed that the kinetics of solvent evaporation and the cluster size distributions are highly dependent on the nature of the core ion in the cluster. In agreement with earlier studies of the hydrated proton, pure water clusters exhibit special stability characteristic of clathrate formation where, for example, a hydronium ion is encapsulated by a pentagonal dodecahedron of twenty water molecules. Similar clustering...