Hydration - The Effect of the Peptide's Charge State

    ESI on peptides can produce ions with a distribution of charge states. For instance, the 13-residue peptide neurotensin is formed in the +1, +2 and +3 charge states under our experimental conditions. The charge state of the peptide, not surprisingly, effects its hydration.

ESI mass spectra of (a) singly-, (b) doubly- and (c) triply-protonated neurotensin recorded after exposure to 1.3 Torr of water vapor at 260 K. The labels above the peaks indicate the number of water molecules coordinated to the peptide. At each charge state, the maximum number n_max of waters adding to the peptide is roughly proportional to the number of charges z on the peptide.

Maximum number n_max of water molecules adding to the corresponding peptide with charge state z at 1.3 Torr of water vapor and 260 K. Mass spectra were measured for a variety of small peptides. In all cases, the n_max/z ratio was found to be approximately 6. This result indicates that the charge is the dominant factor in the hydration process of these peptide ions and, without further evidence, seems to suggests that the water molecules form a cluster around each charge on a peptide as observed for n-decylamine.

Molecular mechanics structures for bare and hydrated protonated dialanine. Despite the apparently simple correlation between the number of charge sites and the maximum number of water molecules adding to a peptide ion, extensive MM analysis indicates the processes directing hydration of peptide ions is complex. Unlike alkylamines, peptides have hydrophilic groups other than the charge site. Not only do these other groups compete with the charge sites for water, they compete with water to solvate the charge sites.