Enhancing the OPLS-AA force field for cellulose Iβ: structural stability and surface functionalization capability with the CM5 charge model.

Molecular dynamics (MD) simulations of cellulose Iβ are essential for understanding its structural properties and enhancing its usability. However, MD models often deviate in reproducing experimental structural properties. This study aimed to 1) improve OPLS-AA force field in stabilizing the crystalline integrity of cellulose Iβ, and 2) overcome limitations in modeling surface-functionalized forms. To achieve this, we combined the CM5 charge model with the carbohydrate OPLS-AA force field, creating the CM5-OPLS model. Significantly improving upon the extreme instability of the original OPLS-AA model, the OPLS-CM5 model retained 90 % of tg conformations of primary alcohol groups, 90 % of intra-chain, and 64 % of inter-chain hydrogen bond populations along the simulation time. Accordingly, the OPLS-CM5 model reproduced the unit cell parameters with <1.5 % error compared to experimental data, outperforming other common carbohydrate force fields, such as CHARMM36 and GLYCAM06. Moreover, the OPLS-CM5 model also captured the thermal and mechanical properties of cellulose Iβs, and enabled successful modeling of surface-functionalized cellulose Iβ, previously limited in most models. Overall, the CM5-OPLS model proved itself as a rigorous candidate for studying cellulose Iβ, and modeling its surface-functionalized forms, advancing understanding of its properties and facilitating its modification for broader applications.