Concerted transfer and transfer direction of three protons in the protonated amino-acid dimers

Several dimers involving two molecule species, which are linked by a proton bridge, are discussed at the B3LYP/6-311++G(3df,3pd)//B3LYP/6-31++G∗∗ level. These molecule species include glycine(s), an alanine, a glyglycine peptide, and a water molecule. Results reveal that the most stable protonated glycine can stabilize a zwitterionic glycine (ZW) in the gas phase, but their complex (dimer) would degenerate into the most stable structure, in which two neutral glycines (NE) are linked by a proton, barrierlessly if the zero point vibrational energies are included. In the degeneration process, three intramolecular proton (hydrogen) transfers are involved and concerted, and the transfer directions of two protons (hydrogens) respective at the two amino terminals of two ZW species point to the corresponding carboxyl oxygens. The bridge proton locates at the middle site of two carboxyl oxygens of two NE species in the most stable dimer. If the two reactant species linked by the bridge proton are unequal, then the bridge proton would transfer to the larger species with small energy barrier. The induced proton (hydrogen) transfers or excursions would also be concerted. The dipole interaction from a water molecule is less than from a glycine for a protonated glyglycine, but enough to induce the hydrogen (proton) at amine terminal nitrogen of the glyglycine transferring to the opposite carboxyl oxygen.