Origins of Noncovalent Interactions in Lignocellulosic Biomass and Potential Implications on Recalcitrance

Noncovalent interactions (NCIs) between lignin and polysaccharides are increasingly being recognized as contributors to the structural integrity and recalcitrance of lignocellulosic biomass (LCB). In this work, density functional theory (M06-2X/6-311++G**) in combination with quantum theory of atoms in molecules (QTAIM) analyses at the MP2/6-311++G** level were employed to systematically analyze 189 physically bound complexes formed between the monolignols p-coumaryl (H), coniferyl (G), and sinapyl (S) alcohols and representative hemicellulose monosaccharides: mannopyranose, xylopyranose, glucuronic acid, and arabinofuranose. Calculated binding energies of these complexes range from -15.8 to -98.5 kJ/mol, with stability increasing with methoxy substitution on the lignin moiety and with the presence of a carboxylate functionality on the sugars. These results are consistent with experimental studies showing that LCB from genetically modified plants, with reduced methoxy in lignin and reduced acidic groups in sugars, are easier to break down when compared to the corresponding wild-type plants. The observations presented in this work, in combination with experimental evidence, suggest that reducing the methoxy content in lignin and the number of carboxylate groups in hemicellulose may be promising strategies for improving LCB valorization efficiency. Furthermore, charge transfer values extracted from QTAIM qualitatively correlate with the stabilization of the complexes, revealing that electron-deficient aromatic rings in lignin, such as those methoxy-substituted aromatic moieties in G- and S-lignin, and electron-rich sugars, such as glucuronic acid in side chains of hemicellulose, lead to the formation of strong hydrogen bonds and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>π</mml:mi></mml:mrow> <mml:annotation>$\pi$</mml:annotation></mml:semantics> </mml:math> -lone-pair interactions. Solvent screening computations also demonstrate that selective association of toluene, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>γ</mml:mi></mml:mrow> <mml:annotation>$\gamma$</mml:annotation></mml:semantics> </mml:math> -valerolactone, and tetrahydrofuran with lignin attenuates lignin-sugar NCIs, lowering the delocalization index (a QTAIM descriptor of electron sharing between species that correlates with the strength of interactions) between interacting monolignols and monosaccharides. These findings provide a detailed molecular-level description of the structural features that modulate NCIs in LCB. This study provides a rational basis for tuning lignin composition or for screening/designing solvent environments to mitigate biomass recalcitrance and advance cost-effective and efficient deconstruction of LCB.