Unravelling the nature of intra-molecular hydrogen bonds in curcumin using in-situ low temperature spectroscopic studies.

Curcumin, described as a wonder drug owing to various medicinal, viz. anticancer, antiviral, anti-inflammatory etc., properties, can also be seen as a model molecular system to study strong intra-molecular OH----O hydrogen bonds which govern its physico-chemical properties. The study of these hydrogen bonds is important to understand its binding characteristics. Here, we present systematic in-situ variable temperature studies of curcumin in the range 350-75 K using infrared spectroscopy to analyse the effects of external stresses on molecular structure and hydrogen bonding network. The results have been well supported by Raman spectroscopic studies. Our studies show striking difference in the nature of the two intra-molecular hydrogen bonds, generally considered equivalent, which form at the edges of the molecule. Also, the strongest intra-molecular hydrogen bond involving the enol group, present at the centre of the molecule, depicts a remarkable temperature induced strengthening upon cooling. The studies further indicate that the compound does not show any drastic structural transition in the measured temperature range. However, subtle spectral changes associated with reorientations of the hydrogen bonds are noticed across 210 K. These results will be useful to predict reaction pathways during chemical complexation of curcumin.