Modeling the Conformational Preference of the Lignocellulose Interface and Its Interaction with Weak Acids

We have examined the conformational space of model systems for the hydrogen-bonded and covalent linkages between the sugar and lignin components of lignocellulose. Specifically, glucose and paracoumaryl alcohol moieties are used in our models. Multistage screening protocols are used to identify and validate a set of lowest-energy isomers. We found a cost-effective screening process involving an initial screening with DFTB3/3ob using a 20 kJ mol–1 threshold, a refinement with the SCANh/6-31+G­(2d,p) method with a 10 kJ mol–1 cutoff, a third step at the DSD-PBEP86/ma-def2-TZVP//MS1-D3/6-31+G­(2d,p) level with the same 10 kJ mol–1 threshold, and a last step at the CCSD­(T)/CBS//B3LYP/cc-pVTZ level with a tighter 5 kJ mol–1 threshold. The use of machine learning (with the London and Axilrod–Teller–Muto potential) can further accelerate the screening process. In general, all low-energy conformers are characterized by hydrogen bonding between the sugar and lignin moieties. We examined the interactions of covalently bonded sugar–lignin models with weak acids (HSO3–, H2PO3–, HSeO3–, H2citrate–, etc.) and found that they interacted strongly with the oxygen of the sugar–O–lignin linkage. Our results suggest that acids such as dihydrogen citrate may be attractive alternatives to the commonly used HSO3– for lignocellulose processing.