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Hydration of Folded and Unfolded Gas-Phase Proteins: Saturation of Cytochrome c and Apomyoglobin

We have examined the hydration of gas-phase cytochrome c and apomyoglobin and found that the average number of water molecules adsorbed under near-saturation conditions by unfolded conformations is substantially less than by folded ones. This suggests that cooperative effects, where water molecules interact strongly with more than one site on the protein, are very important in the solvation of the folded conformation. Solvent interactions play a critical role in determining the structures and properties of proteins in solution. For example, hydrophobic interactions are believed to provide the dominant force driving protein folding.1,2 Studies of the hydration of gas-phase proteins and peptides can provide information about solvent interactions that is not available from solution studies, such as thermodynamic information for the adsorption of specific water molecules. So far there have only been a few studies of hydrated proteins and peptides in the gas phase. Chait 3 and Williams4 and their collaborators have recorded mass spectra for partially dehydrated peptide and protein ions generated by electrospray ionization. Kebarle and collaborators 5 have measured∆G° at 293 K for the addition of the first few water molecules to several small peptides, the largest being Gly4H. We have recently reported measurements of ∆H° and∆S° for the first few water molecules adsorbed by bovine pancreactic trypsin inhibitor (BPTI), a 58-residue protein. 6 In this communication, we describe studies of the hydration of gasphase cytochromec and apomyoglobin in a broad range of (M + nH)n+ charge states. Previous studies have indicated that these proteins undergo unfolding transitions, somewhat analogous to acid denaturation in solution, as the charge increases. 7-11 Therefore, studies of folded and unfolded conformations are possible. The measurements were performed using an injected ion drift tube apparatus equipped with an electrospray source. 8 The number of water molecules adsorbed by the protein ions in the drift tube was determined from mass spectra of the ions exiting the drift tube. The drift tube was cooled to -20 °C to promote the hydration reactions. Drift time distributions were recorded by injecting 50μs pulses of ions into the drift tube and recording their arrival time at the detector. The filled circles in the upper half of Figure 1 show the average number of water molecules adsorbed by the (M + 4H)4+ to (M + 13H)13+ charge states of equine cytochrome c with 0.73 Torr of water vapor. The results shown in the figure were obtained with a helium buffer gas plus water vapor pressure of around 5 Torr and a drift field of 13.3 V cm-1. If the drift field is lowered by a factor of 2 so that the ions spend twice as long in the drift tube, the average numbers of adsorbed water molecules are not significantly different from those obtained with the higher drift field. This suggests that the hydration reactions