Intramolecular proton transfer of serine in aqueous solution. Mechanism and energetics

Serine amino acid in aqueous solution is theoretically studied at the B3PW9 1/6-31 + G** level using a dielectric continuum: solvent model. Neutral and zwitterionic structures in the gas phase and in solution are described and the proton-transfer mechanism is discussed. A neutral conformation in whic h the carboxyl hydrogen atom is already oriented toward the amino group see ms to be the absolute energy minimum in the gas phase and the most stable n eutral form in solution. The absolute energy minimum in solution is a zwitt erionic form. The energy barrier for proton transfer is predicted to be ver y small, in particular when zero-point-energy contributions are added. Our calculations allow the dynamic aspects of the ionization mechanism to be di scussed by incorporating nonequilibrium effects.