611 Research Article Received: 20 June 2008 Revised: 2 October 2008 Accepted: 13 October 2008 Published online in Wiley Interscience: 19 January 2009 (www.interscience.wiley.com) DOI 10.1002/jctb.2089 Thermodynamic and NMR investigations on the adsorption mechanism of puerarin with oligo-β -cyclodextrin-coupled polystyrene-based matrix Li Yang, a,b Haiyang Zhang, a Tianwei Tan a* and Ata Ur Rahman a Abstract BACKGROUND: Adsorption of puerarin on native resin polystyrene (PS) and oligo-β -cyclodextrin-coupled matrix (PS-CDP) was studied for interactions between the adsorbents and the adsorbates. The sorption mechanism on PS-CDP was investigated using the isosteric heat approach and nuclear magnetic resonance (NMR) spectroscopy. RESULTS: The equilibrium adsorption data of puerarin on the two matrices PS and PS-CDP (polystyrene-based matrix before and after coupling by oligo-β -cyclodextrin) in the temperature range 288 – 318 K were well fitted to the Freundlich adsorption isotherm model. The energetic heterogeneity of the media was observed based on the result that the values of isosteric enthalpy were quantitatively correlated with the fractional loading of puerarin adsorption. The more heterogeneous surface of PS-CDP compared with PS was attributed to the complexation between puerarin and β -cyclodextrin (β -CD). NMR studies validated the formation of an inclusion complex puerarin/β -CD. CONCLUSION: Thermodynamic and NMR studies confirmed that multi-interaction cooperatively governed the isolation of puerarin from aqueous solution on PS-CDP matrix. c  2009 Society of Chemical Industry Keywords: adsorption mechanism; thermodynamics; isosteric enthalpy; surface energetic heterogeneity; matrix INTRODUCTION The characteristic property of β -cyclodextrin (β -CD), which is a torus-shaped cyclic oligosaccharide consisting of seven α-1,4- linked D-glucopyranose units, is the presence of an internal hydrophobic cavity with a remarkable capacity to form inclusion complexes with various molecules, especially aromatics. 1–3 This feature, known as a ligand, has been applied in coupling to silica gels for high-performance liquid chromatography (HPLC). Seeman et al. 4 and Lai and Ng 5 synthesized β -CD and its derivative-based chiral solid phases for separation of various optical isomers. Native β -CD can be linked with bifunctional or multifunctional crosslinking reagents such as epichlorohydrin in order to obtain β -CD polymer. Compared with bonded monomeric β -CD, higher selectivity can be obtained from poly-β -CD-substituted media because of the large β -CD group aggregates formed by polymerization of the monomer β -CD. 6 The use of water-soluble oligo-β -CD-substituted agarose gel media for the purification of puerarin and epigallocatechin gallate (EGCG) has previously been demonstrated. 6,7 Polymeric adsorbents have been broadly applied for the separation of natural products due to their high stability, adjustable surface area and porosity, and ease of modification and regeneration. 8,9 As has been reported, 10 interactions between solutes and sorbents can be optimized by chemical modification of the polymeric adsorbents. The inclusion interaction within β -CD cavities is one of the driving forces to be considered in the case of the separation of natural products, especially aromatic compounds. In our previous work, 11 oligo-β -CD-coupled polystyrene-based medium (PS-CDP) has been developed for isolation of the isoflavonoid puerarin, the most popular traditional Chinese herbal medicine. It is assumed that hydrophobic interaction, hydrogen bonding, π – π interaction and complexation resulting from the stacking of the phenolic groups of the adsorbed puerarin molecules and the pendant benzene rings of the β -CD matrix are involved in the sorption of puerarin on oligo-β -CD-coupled polymeric adsorbents from crude extract solution. 12–14 All of these interactions have an impact on the adsorption heat. In order to reveal the adsorption mechanism and clarify the adsorption process, the present study was designed to investigate the thermodynamic aspects of puerarin sorption on PS and PS-CDP. The isotherm, isosteric ∗ Correspondence to: Tianwei Tan, College of Life Science and Technology, Beijing University of Chemical Technology, Beijing 100029, PR China. E-mail: twtan@mail.buct.edu.cn a College of Life Science and Technology, Beijing University of Chemical Technology, Beijing 100029, PR China b College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, PR China J Chem Technol Biotechnol 2009; 84: 611–617 www.soci.org c  2009 Society of Chemical Industry