Curcumin is a component of the Indian spice turmeric (Curcumin longa) which belongs to the family of gingers called the Zingiberaceae. Curcumin is one of three curcuminoids present in turmeric, which are responsible for the yellow color of the herb. The other two curcuminoids are called demethoxycurcumin and bisdemethoxycurcumin. Commercial curcumin contains 77% curcumin, 17% demethoxycurcumin and 3% bisdemethoxycurcumin.
Curcumin is an orange–yellow crystalline powder that is soluble in ethanol, dimethylsulfoxide, and acetone but insoluble in ether and water. It was first isolated by Vogel in 1815. In 1870, it was obtained in crystalline form and indentified as 1, 6-heptadiene-3,5-dione-1,7-bis(4-hydroxy-3-methoxyphenyl)-(1E,6E) or diferuloylmethane. The compound has a molecular formula of C21H20O6, melts at 183°C and its molecular weight is 368.37 g/mol.
The biosynthesis of curcumin has been difficult for researchers to determine. Two mechanisms were first proposed in 1973 by Roughly and Whiting who described the following pathways:
- The first suggested that a chain extension reaction by cinnamic acid and 5 malonyl-CoA eventually arylized and formed the curcuminoid.
- The second suggested that two cinnamate units are coupled by malonyl-CoA.
Each of these mechanisms begins with cinnamic acid which is derived from the amino acid phenylalanine. However, it is rare for plants to use cinnamic acid as a starting point in biosynthesis, with, p-Coumaric acid being the molecule more commonly used for this initial stage.
It was only very recently, in 2008, that a route of biosynthesis was proposed that was backed by experiments. Labelling studies suggested the first mechanism was correct but observations concerning how the functional groups become part of the curcuminoid supported the second theory more strongly. Therefore, Roughly and Whiting’s second proposed mechanism of biosynthesis was accepted as correct.