Experimental (FTIR, BDS) and theoretical analysis of mutarotation kinetics of d-fructose mixed with different alcohols in the supercooled region

Abstract

The mutarotation kinetics of pure molten D-fructose and its binary mixture with alcohols (i.e., sorbitol and maltitol) have been reported using Fourier Transform Infrared (FTIR), Broadband Dielectric Spectroscopy (BDS) and Density Functional Theory (DFT) calculations. The time evolution of the integrated intensity and structural relaxation time acts as a suitable dynamical observable to monitor the progress of the reaction in FTIR and BDS, respectively, leading to the construction of kinetic curves for the process. The bands at 776 cm⁻¹ (ν_β) and 990 cm⁻¹ (ν_α) indicate the respective vibrations originating from the β and α-isomers of D-fructose. The rate constants estimated from FTIR, (k_IR) are shorter than those obtained from BDS, (k_BDS) studies and the activation energies determined from both spectroscopies differ by more than 30 kJ mol⁻¹. This is interpreted in the context of specific reaction pathways that contribute to the rate constants and drive mutarotation. Additionally, it is found that the rate of mutarotation depends on the system studied i.e., in binary mixtures consisting of sorbitol and D-fructose it becomes faster, while in solid dispersion with maltitol it gets slower. This fact can be explained by taking into account the viscosity of the system. In addition, natural bond orbital (NBO) calculations and an electrostatic potential surface (EPS) analysis were carried out to gain insight into the atomic charge distribution and description of the possible interactions between D-fructose and alcohol molecules. We note that there are some differences between both systems under examination in the strength of H-bonds. However, the impact of these interactions on the progress on mutarotation is not as significant as viscosity.