Abstract
The diarylheptanoid curcumin is the yellow coloring agent accumulated in the rhizome of the common spice turmeric (Curcuma longa L.). It has gathered a lot of pharmaceutical interest over the last decades due to some positive effects on human health. However, the use of curcumin as a drug is prevented by its low bioavailability and solubility in water. Interestingly, piperine, the pungent constituent of household pepper (Piper nigrum L.) is able to increase the bioavailability of curcumin up to 20-fold without any known adverse effects. The mechanism responsible for this piperine-based increase of curcumin bioavailability is, however, not fully understood. In a recent publication, a quantum chemical study suggested the formation of a molecular complex between curcumin and piperine being responsible for this effect. The present work now revealed that indeed a 1:1 complex formation can be observed in NMR titration experiments and by mass spectrometry, but the complex strength is rather low (K ~ 1.5 dm3/mol). Furthermore, it is shown that the presence of piperine does not increase the water solubility of curcumin, which makes it rather improbable that such a complex is the main reason for an enhanced curcumin bioavailability.Graphic abstract
Highlights
The diarylheptanoid curcumin (1), the yellow coloring agent of the rhizome of turmeric (Curcuma longa L.), is probably best known from its use in commercial curry mixtures
One interesting way to mitigate these problems is the addition of piperine (2). This is the pungent constituent of household pepper (Piper nigrum L.) and is able to increase the bioavailability of curcumin up to 20-fold with no known adverse effects [5,6,7]
Since the removal of these aggregates by centrifugation failed, we chose sterile filtering through a hydrophobic 0.2 μm PTFE filter. This resulted in a clear, non-yellow solution with a high reproducibility. While this method may not remove very small aggregates, we mainly focused on changes in curcumin (1) solubility in the presence of piperine (2) and not determining the absolute solubility of 1
Summary
The diarylheptanoid curcumin (1), the yellow coloring agent of the rhizome of turmeric (Curcuma longa L.), is probably best known from its use in commercial curry mixtures. The present work investigates the properties of such complex formation utilizing NMR titrations, mass spectrometry, and solubility measurements. NMR measurements were performed using methanol-d4 as solvent due to the low solubility of curcumin in water as well as in buffered aqueous solvents with defined pH values.
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