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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 dm 3 /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

Spectroscopic studies on the molecular interactions of curcumin and piperine