Density Functional Theory Studies on Transimination of Vitamin B6 Analogues through Geminal Diamine Formation

Abstract

A theoretical study on the transimination reaction in the gas phase of a molecular complex that consists of a Schiff base formed from a vitamin B6 model compound, methylamine, and one water molecule is reported. In this nonenzymatic environment, DFT calculations (B3LYP/6-31+G*) with full optimization of each intermediate and transistion state structure and a further study of the topology of the charge density function according to Bader's AIM theory are carried out. This work has allowed the detailed outline of the energy profile of the reaction leading to the geminal diamine formation and the description of the geometries of all the structures involved. The reaction determining step is the addition of the incoming amine onto the Schiff base and the main intermediate of the overall reaction is a geminal diamine structure. The mechanism also points out the reactive role of one water molecule by allowing the proton transfer between distant sites of the molecular complex by means of the formation of a concerted transition state.