Proton transfer at the carboxylic sites of amino acids: A single water molecule catalyzed process

Ab initio calculations at MP2 level of theory were used to study the proton transfer at the carboxylic sites of amino acids, in the isolated, mono- and di-hydrated forms. In the case of water dimer, two interaction modes with glycine neutral structures (see Fig. 3) were explored, corresponding to the concerted and stepwise reaction pathways. Their transition states can be described as (H₂OHOH₂)⁺ [Fig. 4(a)] and (H₂O---HOH₂)⁺ [Fig. 4(b)], respectively. The energy analysis indicated that the concerted pathway is preferred. In the isolated, mono- and di-hydrated glycine complexes, the activation barriers of the proton transfer at the carboxylic sites were calculated to be 34.49, 16.59, and 13.36 kcal mol⁻¹, respectively. It was thus shown that the proton transfer is significantly assisted and catalyzed by water monomer so that it can take place at room temperature. Instead, the further addition of water molecules plays solvent effects rather than catalytic effects to this proton transfer process. The above results obtained with discrete water molecules were supported by the solvent continuum calculated data. It was also observed that the heavy dependence of the solvent continuum models on dipole moments may produce misleading results. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009