Mutarotational Kinetics and Glass Transition of Lactose

TL;DR

This study uses 13C NMR to investigate the mutarotation of lactose in the solid state, revealing temperature-dependent behavior and differences from aqueous solutions, with implications for understanding glass transition and molecular dynamics.

Contribution

First in situ, real-time measurement of lactose mutarotation in the solid state, highlighting the influence of physical state and temperature on chemical equilibrium.

Findings

Mutarotation initiates upon heating amorphous lactose.

Equilibrium reached near the glass transition temperature.

Final anomer ratio is 1:1, differing from aqueous solution.

Abstract

We report for the first time real time in situ and quantitative measurements of the mutarotation reaction of lactose in the solid state. The experiments have been performed by 13C NMR. We show that mutarotation is initiated on heating the amorphous state, and reaches chemical equilibrium close above the glass transition temperature Tg. We do not observe this transformation when starting from stable crystalline states. The final ratio of <alfa> and <beta> anomers is 1:1, which suggests that the energy profile of the mutarotation reaction pathway in the solid state is actually different from the mechanism proposed for aqueous solution. This chemical equipartition is reached before the crystallization into the corresponding 1:1 molecular compound. These new data clearly illustrate the interrelation between the chemical molecular properties, the physical state of the material, and the relaxational dynamics of the glass.