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 of water occurs around protonated primary alkylamines where the protonated amine replaces one of the water molecules in the clathrate structures, which encapsulate one or more neutral water molecules. This observation supports the conjecture that the doubly protonated cyclic decapeptide gramicidin S with 40 water molecules attached, the most significant magic number observed in mass spectra at various delay times, has both protonated ornithine residues solvated by pentagonal dodecahedron clathrate structures. Other peptides such as doubly protonated bradykinin do not exhibit any specific solvation during the freeze-drying process. Studies of model compounds are presented which reveal other interesting aspects of water structure around singly and multiply charged ions with low extents of hydration, including the observation of neutral clathrates attached to a “spectator” ion.