Screening of complex natural extracts by countercurrent chromatography using a parallel protocol.
In countercurrent chromatography (CCC) the choice of the liquid system is the heart of any separation. It corresponds to the selection of the mobile phase and the stationary phase at the same time. Any change in one phase composition induces a change in the other phase composition which renders the choice of the appropriate liquid system difficult and lengthy. A scale of compositions of the heptane-ethyl acetate-methanol-water quaternary liquid system was referred to by letters from A to Z and called the Arizona (AZ) liquid system. Each composition of the AZ system has the same heptane/ethyl acetate and methanol/water volume ratios. It is shown that there is a continuous polarity change from the hydrophilic A composition (ethyl acetate-water) to the hydrophobic Z (heptane-methanol) mixture by measuring the distribution constant K(D) of a known test mixture. For all compounds, the log K(D) is linearly increasing with the water content of the lower aqueous phase of the composition used. The slopes of the log K(D) versus percent H(2)O have very different values which means that the chromatographic selectivity changes with liquid system compositions. The AZ system was associated to the elution-extrusion method to design a procedure to identify rapidly the appropriate solvent composition able to fractionate correctly a complex natural extract. With the use of an integrated three-coil CCC column (40 mL each coil) able to test three AZ compositions in parallel, it is shown that the optimum AZ composition is found in half a day using less than a liter total volume of solvents. Two natural extracts are rapidly screened using the proposed protocol. An extract of Piper longum L. of intermediate polarity was fractionated in five usable portions using the 3/2/3/2 (Q) composition of the AZ system. A polar extract of Polygonum cuspidatum was also separated in five fractions using the 1/6/1/6 (D) composition. In both cases, a 140 mL CCC column was used for a direct scale-up transfer with the same liquid system. Purified fractions were subjected to an antioxidant activity assay and liquid chromatography with UV and mass spectrometry detection (LC-UV/MS) analysis to determine the molecular weight, number, and quantity of compounds in the active fractions. Four fractions of P. cuspidatum showed excellent antioxidant activity. They were rapidly produced at the milligram level by the 140 mL CCC column and fractionated by semipreparative high-performance liquid chromatography (HPLC) in individual compounds that were each identified by NMR and MS and reevaluated for confirmation of bioactivity. The rapid screening CCC protocol associated to the preparative capability of CCC allows for a fast identification and characterization of active compounds in natural products.
Lu
Yanbin
Y
Department of Chemistry, Zhejiang University, Hangzhou 310027, Zhejiang Province, China.
Berthod
Alain
A
Hu
Ruilin
R
Ma
Wenyan
W
Pan
Yuanjiang
Y