Existence of β-diketone form of curcuminoids revealed by NMR spectroscopy.

Curcumin (CUR), demethoxycurcumin (DMC) and bisdemethoxycurcumin (BDMC) - the class of natural compound derived from turmeric can exist as keto-enol and β-diketone tautomer form. The structure and dynamics of particular relevance CUR is reported in prior studies, whereas DMC and BDMC, by far, have not been scrutinized. In the present studies, we have investigated the detailed molecular structure of CUR, DMC and BDMC by employing NMR spectroscopy as a key tool. The bridging carbon as methylene in β-diketone form and methine in keto-enol form shows significant difference in NMR spectrum. The results justified that Curcuminoids (CC) has nearly 3% of β-diketone tautomer in DMSO solvent at 298 K. Further, results revealed that β-diketone form was favoured in alkaline pH condition whereas acidic and neutral pH conditions favour keto-enol tautomer. However, at higher temperature equilibrium shift towards β-diketone tautomer. Moreover, this is the first report by NMR for observing the presence of β-diketone tautomer.