Different proton transfer channels for the transformation of zwitterionic alanine-(H₂O)(n=2-4) to nonzwitterionic alanine-(H₂O)(n=2-4): a density functional theory study.

We report here the various possibilities of proton transfer between the zwitterionic and the non-zwetterionic form of alanine (Ala) via (H₂O)(n=2-4) clusters by calculating the transition state structures of zwitterionic alanine (ZAla)-(H₂O)(n=2-4) and non-zwitterionic alanine (Ala)-(H₂O)(n=2-4) complexes at B3LYP/6-311++G(d,p) and CAM-B3LYP/6-311++G(d,p) level of theory. In order to determine the most feasible channel for proton transfer, the barrier energy corresponding to each channel was calculated. For the transformation of ZAla-(H₂O)(n=2) to Ala-(H₂O)(n=2), we identified eight channels for proton transfer. The lowest barrier energy (2.57 kcal mol⁻¹) channel, where ZAla-(H₂O)(n=2) transforms to Ala-(H₂O)(n=2) via triple proton transfer, is said to be the energetically most feasible channel. The values of barrier energy corresponding to the least energy pathway for proton transfer were calculated to be 1.14 and 9.82 kcal mol⁻¹ for n = 3 and n = 4 complexes, respectively, at B3LYP/6-311++G(d,p) level of theory. For complex n = 3, the structure where proton transfer takes place directly from -NH₃⁺ to -COO⁻ has the lowest energy pathway. However, the complexes for n = 2 and 3--the channels where proton transferred from -NH₃⁺ to -COO⁻ via two water molecules have the lowest barrier energy. For each n, the values of barrier energy calculated at CAM-B3LYP/6-311++G(d,p) level of theory were always less compared those calculated at B3LYP/6-311++G(d,p) level of theory. The value of rate constants corresponding to each proton transfer channel was also calculated.