β-Cyclodextrin Mediated Purification of Sulforaphane: Selective Separation of a Critical Impurity—5-Vinyloxazolidine-2-thione

Sulforaphane (SFA), an isothiocyanate compound derived from cruciferous plants, has garnered significant attention owing to its cancer-preventive, anti-inflammatory, and antibacterial properties. However, the presence of complex impurities, including hydrolysis by-products of glucosinolates, structural analogues of SFA, and 5-vinyl-2-thioxazolidinone (VOT), in plant extracts significantly hinders both the efficiency of purification and the safety of application. In this study, 14 compounds were successfully isolated and identified from broccoli seed extracts using preparative high-performance liquid chromatography. Additionally, an efficient separation method was developed for removing the crucial impurity VOT based on the temperature-responsive properties of β-cyclodextrin (β-CD). The results demonstrated that the selective binding of β-CD to SFA at elevated temperatures achieved a VOT removal efficiency exceeding 97% following low-temperature precipitation. Additionally, the purity of SFA was enhanced from 55.8% to 90.1%. The separation mechanism at the molecular level was validated through density functional theory calculations. The binding energy of β-CD-SFA (-81.2   kJ/mol) was significantly higher than that of β-CD-VOT (-25.6   kJ/mol). The scanning electron microscopy and energy dispersive spectroscopy results confirmed the homogeneous distribution of SFA within the complexes. This study provides an innovative approach for the large-scale preparation of high-purity SFA and ensures its safe application in functional foods.