Hydration - Solvation Shells & Charge Diffusion in Model Systems (Methylamine &
n-Decylamine)

    To obtain a basic understanding of the interactions between peptides and water, we have determined water binding energies and structures for the model compounds
CH₃NH₃⁺·(H₂O)_n and CH₃(CH₂)₉NH₃⁺·(H₂O)_n. Protonated alkylamines were chosen because they have a hydrophilic head (the -NH₃⁺ group) and a hydrophobic tail. In these systems, the protonated amines represent the charge site at the N-terminus of a peptide. The alkylamine results can be compared to those obtained for actual peptides to gain insight into peptide-water interactions.

Experimental and MM water binding energies for
CH₃(CH₂)₉NH₃⁺·(H₂O)_n and DFT geometries illustrating water coordination to the
-NH₃⁺ charge group in CH₃NH₃⁺. MM calculations suggest that three water molecules fill the first solvation shell of the charge site in alkylamines as indicated by the sudden drop in water binding energies between the third and fourth water. The theoretical geometries confirm this with only the first three water molecules coordinating directly to the
-NH₃⁺ group. In contrast, the experimental water binding energies decrease monotonically with increasing number of water molecules with no indication of solvation shells.

Comparison between experimental water binding energies and DFT electrostatic energies for alkylamines. The decrease observed in the experimental water binding energies with the addition of water to
n-decylamine is explained by electrostatic interactions. DFT calculations on CH₃NH₃⁺ indicate that as more water is added, some of the charge on the -NH₃⁺ group is transferred to the water molecules. This delocalization of the charge over the -NH₃⁺ group and the water molecules solvating it results in formation of a weaker bond with the next water molecule added.