Insights on the mechanism of proton transfer reactions in amino acids

In the present work, the joint use of the potential energy, the reaction electronic flux profiles and NBO analysis along the intrinsic reaction coordinate within the framework of the reaction force analysis allows us to gain insights into the mechanism of the proton transfer process in amino acids. The reaction was studied in alanine and phenylalanine in the presence of a continuum and with addition of one water molecule acting as a bridge, the results were compared to those of tryptophan. The bridging water molecule stabilizes the zwitterionic form and increases the reaction barriers by a factor of two. This result is interpreted in terms of the energy required to bring the amino acid and the water molecule closer to each other and to promote the proton transfer through the reordering of the electron density. Furthermore, the bridging water molecule induces a concerted asynchronous double proton transfer, where the transfer of the carboxyl hydrogen atom is followed by the second proton transfer to the ammonium group. In addition, a second not intervening water molecule was added, which changes the proton acceptor and donor properties of the reactive water molecule modulating the reaction mechanism. The aforementioned methods allow us to identify the order of the transferred protons and the asynchronicity, thereby, evolving as promising tools to not only characterize but also manipulate reaction mechanisms.