Novel Catechin-Tiopronin Conjugates Derived from Grape Seed Proanthocyanidin Degradation: Process Optimization, High-Speed Counter-Current Chromatography Preparation, as Well as Antibacterial Activity.

Tiopronin, as a novel thiol-containing nucleophile, was introduced for depolymerizing polymeric proanthocyanidins from grape seed into catechins and three new proanthocyanidin-tiopronin degradation products: (+)-catechin-4β-S-tiopronin methyl ester (CT), (-)-epicatechin-4β-S-tiopronin methyl ester (ECT), and (-)-epicatechin gallate-4β-S-tiopronin methyl ester (ECGT). A Box-Behnken design was employed to optimize degradation conditions based on single-factor experiments to obtain target products. Each of the new degradation compounds was isolated by the high-speed counter-current chromatography combined with semipreparative high performance liquid chromatography in large amounts, and then, their structures were identified by 1H NMR, 13C NMR, 2D-NMR, as well as mass spectrometry analysis. The absolute configurations were further confirmed by comparison between the calculated electronic circular dichroism and experimental spectra. Further evaluation of antibacterial activities of these compounds showed that CT and ECT possessed more inhibiting capacity against Staphylococcus aureus and Escherichia coli than parent compound catechin and epicatechin. However, ECGT has no bacteriostatic capacity against these two bacteria.