LignAmb25: A comprehensive AMBER force field addressing lignin's structural and chemical diversity

LignAmb25 is a comprehensive force field for lignin molecular-dynamics simulations implemented natively within the AMBER package. The force field includes parameters for all common monolignol units (p-coumaryl, coniferyl, caffeyl, and sinapyl alcohol) and their associated linkages (β-O4, β-5, β-β, β-1, 5-5, 5-O4, α-O4, benzodioxane [BDO], and dibenzodioxocin [DBDO]) along with less commonly encountered units such as tricin, spirodienones, and hydroxystilbenes. This enables simulations of both softwood and hardwood lignin structures with compositions that would be difficult to isolate experimentally. Force-field parameters were initially derived from the GAFF2 force field and systematically optimized using quantum-mechanical calculations at the ωB97X-D4/def2-TZVPP level of theory on conformer ensembles derived via the Conformer-Rotamer Ensemble Sampling Tool/Commandline Energetic Sorting (CREST/CENSO) conformational sampling toolchain. Partial atomic charges were derived using the restrained electrostatic potential (RESP) methodology, consistent with AMBER conventions. Experimentally measured crystal structures of lignin simulated with LignAmb25 accurately retain their packing based on calculations of the root-mean-square deviation (RMSD) and density error compared to the deposited crystal structure, thereby exceeding the performance of the lignin force field for CHARMM. Additionally, LignAmb25 is shown to reliably estimate the enthalpy of vaporization and the absolute hydration free energy of lignin-related compounds. The LignAmb25 force field is provided in two variants: LignAmb25^Solo, a standalone version not meant for use with other biomolecular force fields that focuses on accurate modeling of lignin-solvent interactions, and LignAmb25^HF, a version that is compatible with all other major biomolecular force fields in the AMBER molecular-dynamics suite. This includes force fields of the GLYCAM (carbohydrates), ff19SB (proteins), and LIPID (lipids) families, as well as the DNA and RNA force fields routinely used in AMBER. The LignAmb25 force field will be distributed as of AMBER 26.