Galactose isomerization to tagatose over MgBr2 follows a temperature-dependent reaction rate kinetics as predicted by first principles-based theories

Abstract

Tagatose, a classified low-calorie sugar, possesses anti-hyperglycaemic and prebiotic characteristics. Therefore, it is considered a potential food additive supplement (sweetener) for lifestyle health problems in humans. While many reports on its production via biological and chemical methods have been published, the thermodynamic behavior of its synthesis using galactose is rarely focused on. Herein, we report the MgBr2-mediated galactose interconversion to tagatose (single-step) and its kinetic mechanism. The oppositely charged counterions (Mg2+ and Br‒) can induce the reaction via different pathways in a parallel fashion, i.e., the Mg2+ behaving as a Lewis acid can enable via a 1,2-hydride shift pathway, whereas the Br‒ offering the Brønsted basic sites via a proton transfer pathway. This resulted in a tagatose production as high as 22% and 73% selectivity within 60 min at 120 °C in water. Furthermore, the reaction has yielded a single co-product formation (talose up to 7%). From the inverse-gated quantitative NMR analysis, the contribution of the counterions in the reaction is roughly 52%:48% by Mg2+:Br‒. The determination of the temperature-dependent reaction kinetics (elementary steps) was made by employing the first-principles-based theories (i.e., transition state-Eyring theory for the proton transfer pathway that involves no hydride shift and Marcus theory for the hydride shift pathway). The energy calculations of the pathways made via DFT modeling suggested the higher feasibility of a hydride shift mechanism of galactose to tagatose.