The degradation kinetics and mechanism of moringin in aqueous solution and the cytotoxicity of degraded products

In this work, we investigated the degradation of moringin (4-[(α-L-rhamnosyloxy)benzyl]-isothiocyanate), a major bioactive isothiocyanate (ITC) found in moringa seeds (Moringa oleifera Lam), at various food processing conditions. Moringin degrades rapidly to several water-soluble products via a pseudo-first-order kinetics. By analyzing the reaction products, the degradation mechanism was found to be through hydrolyzing to (A) 1-O-(4-hydroxymethylphenyl) α-L-rhamnopyranoside (rhamnobenzyl alcohol RBA) or (B) rhamnobenzylamine. The formed amine further reacts with moringin to form N,N′-bis{4-[(α-L-rhamnosyloxy)benzyl]}thiourea (di-rhamnobenzyl thiourea, DRBTU). In addition, moringin isomerizes to 4-[(α-L-rhamnosyloxy)benzyl]thiocyanate (RBTC), which further reacts with moringin to form S,N-bis{4-[(α-L-rhamnosyloxy)benzyl]}-dithiocarbamate (DRBDTC). Furthermore, pH was found to have an effect on the degradation of moringin. RBA and RBTC were major degraded products in neutral and acidic conditions while thiourea (DRBTU) was in alkaline condition. Although moringin showed higher cytotoxicity to cancer cells, its degraded products showed very weak or no activities, suggesting that the isothiocyanate group of ITCs is essential for their cancer chemoprevention activities.