Free-Energy Changes of a Single Molecular Chain Peeling off from the Surface of the Native Cellulose Crystal Model in...

We report a molecular dynamics (MD) simulation study of the process of peeling off a single molecular chain from cellulose Iβ crystal models. This study serves as a model for crystal surface decrystallization. We combined two types of extended ensemble MD simulation methods: adaptive steered MD to define the chain peeling-off trajectory along the reaction coordinate and umbrella sampling MD to calculate the potential of the mean force and free-energy change along the chosen reaction coordinate. The crystal models were solvated with water, benzene, 10 wt % aqueous solution of NaOH, N,N-dimethylacetamide with 1 wt % LiCl, N-methylpyrrolidone with 1 wt % LiCl, or 1-allyl-3-methylimidazolium chloride (AMIMCl). Endothermic free-energy changes were required for the chain to peel off in all of the solvents. However, extremely low free-energy values were observed in the AMIMCl solvent over the entire chain peeling-off range. This suggests that the decrystallization scheme for the AMIMCl solvent differs from those of the other solvents. In water, the free-energy change required for the chain to peel off from the reducing end was slightly lower than that required for it to peel off from the nonreducing end. This is consistent with the direction of the chain peel off observed in physical and enzymatic decrystallization at the crystal surface. Other umbrella sampling simulations that allowed for the spatial motion of the peeled-off chain segment revealed that after linearly increasing, the potential of mean force values leveled off for all of the solvent systems except for the AMIMCl system.