Standards of curcumin

Standards of curcumin (≥97%), demethoxycurcumin (≥94%), bisdemethoxycurcumin (≥94%), tetrahydrocurcumin (≥96%) and hexahydrocurcumin (≥90%) were purchased from Sigma Aldrich, Mumbai, India. The internal standard (IS), hesperetin was purchased from Sigma Aldrich, Steinheim. Octahydrocurcumin (≥92%) was prepared in-house and characterized by mass spectroscopy and melting point analysis. Curcumin-O-glucuronide (≥98%) and curcumin-O-sulfate (≥98%) were purchased from Clearsynth laboratories, Mumbai, India. The complete natural turmeric matrix formulation, Cureit™ (bioavailable form of curcumin) was obtained from Aurea Biolabs Pvt. Ltd., Cochin, India. Leucine enkephalin and sodium formate were purchased from Waters, Milford, MA, USA. All other solvents and chemicals were of LC-MS grade and were purchased from J.T. Baker, Center Valley, PA, USA. Millipore-MilliQ distilled water was utilized during the complete study.

All reagents used in this study were of analytical grade. Acetonitrile (ACN) and methanol (MeOH) were purchased from Fisher Scientific; Milli-Q water used to prepare buffer solutions was obtained by a Millipore^® purification system (Bedford, MA, USA). K₂HPO₄, phosphoric acid, sodium acetate, and acetic acid were purchased from Merck (Darmstadt, Germany); turmeric rhizome was purchased from the herbal drug market. Standards of curcumin, demethoxycurcumin, and bisdemethoxycurcumin (>98% in purity) were purchased from Sigma-Aldrich (St. Louis, MO, USA).

Ethyl acetate (ACS grade, ≥99%) (Sigma-Aldrich, Milan, Italy) was used in the extraction procedures. Acetonitrile CHROMASOLV^® (gradient grade, for HPLC, ≥99.9%) for HPLC analysis was purchased from Sigma-Aldrich, while Milli-Q H₂O was obtained in the laboratory using a Milli-Q Reference A+System (Merck Millipore). Standards of Curcumin (87.02% Curcumin, 12.98% other Curcuminoids) were purchased from Sigma Aldrich.