Sucrose conformational polymorphism: A jigsaw puzzle with multiple routes to a unique solution

Scale-up sucrose crystals grown from a furfuryl alcohol-water sucrose solution at 60 °C exhibited a wide endothermic peak from 140° to 170 °C by differential scanning calorimetry (DSC) at 10 °C/min, whereas commercial sucrose crystals displayed only one sharp melting peak around 185° to 189 °C. Surprisingly, it was found that the inclusion of trace amounts of salts and water or degradation of sucrose did not offer a satisfying explanation for the occurrence of the wide low-melting peak from 140° to 170 °C. On the basis of powder X-ray diffraction (PXRD), single-crystal X-ray diffraction (SXD), solid-state nuclear magnetic resonance (SSNMR), Fourier transform infrared spectroscopy (FTIR), hot-stage optical microscopy (HSOM), variable temperature powder X-ray diffraction (VT-PXRD), electron spectroscopy for chemical analysis (ESCA), and solubility measurements, one possible explanation for the low-melting peak in differential scanning calorimetry (DSC) was because the sucrose crystals grown from the furfuryl alcohol-water sucrose solution at 60 °C consisted of sucrose molecules having different degrees of conformational disorders about the -CH₂-OH functional groups of the fructofuranose ring such that the inter-residue intramolecular hydrogen bonds between the hydroxyl groups and the glucopyranose ring oxygen were misaligned. The activation energy, E_a, and the enthalpy, ΔH, for the polymorphic transformation of the low melting metastable polymorph around 150° to 160 °C to the high melting thermodynamically stable form around 185 °C were 185.2 kJ/mol (i.e., 44.2 kcal/mol) and 2.9 kJ/mol (i.e., 0.7 kcal/mol), respectively. This polymorphic transformation could be kinetically controlled by mass-transfer through viscosity, solvent evaporation rate, and mixing. However, under the orchestration of intermolecular and intramolecular hydrogen bonds, the polymorphic transformation could be achieved by different pathways in different microdomains to produce various degrees of conformational polymorphs if the micromixing was poor. As a result, a mixture of crystals or microdomains with a wide range of melting temperatures and relatively high solubility were produced.