Complexation of Lignin Dimers with β‑Cyclodextrin and Binding Stability Analysis by ESI-MS, Isothermal Titration Calorimetry, and Molecular Dynamics Simulations

Lignin
derived from lignocellulosic biomass is the largest source
of renewable bioaromatics present on earth and requires environmentally
sustainable separation strategies to selectively obtain high-value
degradation products. Applications of supramolecular interactions
have the potential to isolate lignin compounds from biomass degradation
fractions by the formation of variable inclusion complexes with cyclodextrins
(CDs). CDs are commonly used as selective adsorbents for many applications
and can capture guest molecules in their internal hydrophobic cavity.
The strength of supramolecular interactions between CDs and lignin
model compounds that represent potential lignocellulosic biomass degradation
products can be characterized by assessing the thermodynamics of binding
stability. Consequently, the inclusion interactions of β-CD
and lignin model compounds G-(β-O-4′)-G, G-(β-O-4′)-truncG (guaiacylglycerol-β-guaiacyl ether), and G-(β–β′)-G
(pinoresinol) were investigated empirically by electrospray ionization
mass spectrometry and isothermal titration calorimetry, complemented
by molecular dynamics (MD) simulations. Empirical results indicate
that there are substantial differences in binding stability dependent
on the linkage type. The lignin model β–β′
dimer showed more potential bound states including 1:1, 2:1, and 1:2
(guest:host) complexation and, based on binding stability determinations,
was consistently the most energetically favorable guest. Empirical
results are supported by MD simulations that reveal that the capture
of G-(β–β′)-G by β-CD is promising
with a 66% probability of being bound for G-(β-O-4′)-truncG compared to 88% for G-(β–β′)-G
(unbiased distance trajectory and explicit counting of bound states).
These outcomes indicate CDs as a promising material to assist in separations
of lignin oligomers from heterogeneous mixtures for the development
of environmentally sustainable isolations of lignin compounds from
biomass fractions.