LysH(+)center dot(H2O)(n-1) + H2O reversible arrow LysH(+)center dot(H2O)(n) [GRAPHICS] Using a mass spectrometer equipped with a drift cell, water binding energies of protonated arginine (ArgH(+)) and protonated lysine (LysH(+)) were determined in equilibrium experiments and supplementary calculations at. the B3LYP/6-311++G** level of theory. The binding energy of the first water molecule was measured to be 10.3 and 10.9 kcal/mol for ArgH(+) and LysH(+), respectively. Water binding energies decrease with increasing degree of hydration reaching values of 6-7 kcal/mol for the fourth and fifth water molecule. Theory reproduces this trend of decreasing binding energies correctly and theoretical. water binding energies agree with experiment quantitatively within 2 kcal/mol. Lowest-energy theoretical structures of ArgH(+) and LysH(+) are characterized by protonated side chains and neutral a-amino and carboxyl groups which form intramolecular hydrogen bonds to the ionic group (charge solvation or CS structures). The salt bridge (SB) structures with two cationic groups (side chain and a-amine) and one anionic group (carboxyl) are 13.1 and 9.3 kcal/mol higher in energy for ArgH(+) and LysH(+), respectively. Theory indicated that the first water molecule binds to the ionic group of the CS structures of ArgH(+) and LysH(+). With increasing degree of hydration intramolecular interactions are replaced one by one with water bridges with water inserted into the intramolecular hydrogen bonds. Whereas the global minima of ArgH(+)center dot(H2O)(n) and LysH(+)center dot(H2O)(n), n < 7, were calculated to represent CS structures, 7-fold hydrated CS and SB structures, ArgH(+)center dot(H2O)(7) and LysH(+)-(H2O)(7), are nearly isoenergetic (within <1 kcal/mol).