Hydration - The Role of the Charge-Carrying Group

    Another factor that can influence hydration is the identity of the peptide's charge-carrying group. Gas-phase peptide cations are typically protonated at the N-terminus and/or at basic amino acid residues such as arginine. Temperature-dependent equilibrium studies indicate that arginine-containing peptides have somewhat smaller water binding energies than peptides without arginine.

Experimental water binding energies for compounds with amine and guanidine charge groups. Dialanine is protonated at the N-terminus (structure on left) and forms a 14.8 kcal/mol bond with water as does n-declyamine. In both cases, water binds to the
-NH₃⁺ group. Protonated arginine (structure on right), however, forms a weaker bond with water at only 9.0 kcal. Arginine is protonated at the guanidine group in the side chain instead of at the amine group as in dialanine and n-decylamine. Because of this, the charge is delocalized over more atoms and the electrostatic interaction with water is weakened. A similar bond energy is obtained for the arginine methyl ester, which is also protonated at the guanidine group.