(400c) Thermodynamics of Lignin Dimers and ?-Cyclodextrin Complexes

Lignin is an organic structural polymer in and between the cell walls of plants. Due to lignin’s complex structure, a heterogeneous mixture of lignin derivatives are obtained when lignin is depolymerized. Developing methods to selectively separate, purify, and valorize lignin small derivatives is challenging. Cyclodextrins (CDs), particularly β-CD, are commonly used as chirally selective adsorbents for many applications including agriculture, pharmaceuticals, and analytical chemistry. CDs are shaped like truncated cones with a hydrophilic surface and a hydrophobic core, which provide host-guest size and structure selectivity.

Understanding the interactions of lignin-derived small molecules with CDs in solution will support our efforts to create selective immobilized CD-MSNPs, whose binding with small molecules can also be quantified with isothermal titration calorimetry (ITC). The present work aims to study binding of β-CD to three types of lignin dimers including two derivatives of βO4’ dimers of coniferyl alcohol (GG lignin dimer) as well as pinoresinol (G-ββ-G) using ITC. The equilibrium binding constant (K) and the thermodynamic binding energies (ΔH, ΔS, and ΔG) for dimer-β-CD complexes are determined from fitting binding models to the ITC data. The results of this work show that guiacylglycerol guaiacol ester with a hydroxypropenyl (HOC₃H₄-) tail (G-βO4’-G) binds to the β-CD through a different mechanism than pinoresinol and commercially available guaiacylglycerol-β-guaiacyl ether (G-βO4’-truncG) lignin dimer. The results for the G-βO4'-G are in good agreement with molecular simulations conducted for the same system. We also demonstrate that CD-lignin interactions can be used to develop selective separation strategies for lignin-derived small molecules, and investigated the surface modification techniques to attach β-CD directly to the silica MSNPs, with a goal of using the silica particles for selective capture of lignin-derived small molecules.