Hydration of protonated primary amines: effects of intermolecular and intramolecular hydrogen bonds

Abstract

Addition of individual water molecules to a number of protonated primary amines, RNH3+, was studied experimentally by measuring hydration equilibria in a mass spectrometer equipped with an ion mobility cell and theoretically using molecular mechanics and density functional theory calculations. Water binding sites and energies in RNH3+·(H2O)n were examined as a function of n and as a function of the nature of the R-group. R-groups ranged from simple alkyls to amide containing groups potentially able to form hydrogen bonds to the ammonium group. Effects of interest such as location and number of amide >C=O groups in R and the effect of size of R were investigated on carefully chosen molecules including alkylamines and lysine based systems, such as CH3(CH2)xNH2 (x = 0 and 9); acetylated lysine; and lysine containing peptides Ac–AAKAA and Ac–AxK (Ac = acetyl, A = alanine, K = lysine; x = 4, 6, 8). For ammonium groups without intramolecular hydrogen bonds it was found that three water molecules form hydrogen bonds to the three hydrogen atoms of the ammonium group filling the first solvation shell. The fourth and fifth water molecules add to water of the first solvation shell (alkylamines) or at a charge-remote site (peptides). Water binding energies in CH3NH3+·(H2O)n steadily decrease with increasing n (17, 14, 12, and 10kcal/mol for n = 1, 2, 3, and 4, respectively) due to increasing charge delocalization over the bound n − 1 water molecules, thereby obscuring the start of a new solvation shell at n = 4. A simple electrostatic model, based on natural bond analysis (NBO) derived atomic charges, reproduces this effect quantitatively. Hydration of systems RNH3+·(H2O)n with m = 1, 2, or 3 intramolecular hydrogen bonds is analogous to comparable systems R′NH3+·(H2O)n+m with no intramolecular hydrogen bonds. For instance, theory indicates that lysine, when N-acetylated on either one of the two amines, exhibits one strong intramolecular hydrogen bond. In these systems the RNH3+·(H2O)n…H2O binding energies are comparable to those of alkyl−NH3+·(H2O)n+1…H2O and NBO calculations confirm that intramolecular hydrogen bonds remove a similar amount of charge from the ammonium group as intermolecular hydrogen bonds. The pentapeptide (Ac–AAKAA)H+ was found to be a system with two intramolecular hydrogen bonds; the polypeptides (Ac–AxK)H+ (x = 4, 6, and 8) are systems with a fully self-solvated ammonium group. The binding energy of either charge-remote water or of water in a second solvation shell is ≤10kcal/mol. Larger values occur for smaller systems, e.g., for CH3NH3+·(H2O)3 + H2O ΔH° = −10kcal/mol, and smaller values occur for larger systems, e.g., for (Ac–A8K)H+ + H2O ΔH° = −5kcal/mol. A strong energy–entropy correlation of ΔS°n/ΔH°n=0.0018K−1 was experimentally found to hold for all hydration processes studied here.