Molecular Dynamics Simulations Suggest Folded Chain Ends Compatible with the Cellulose-II Fibril Structure

The structural feasibility of chain folding within an accepted cellulose-II structure is investigated in this study. We incorporate 3-residue and 5-residue folded glucosyl turns into a cellulose-II fibril crystal model to assess if the dominant structure can be retained. Ten candidate turn models were generated by metadynamics simulations and ranked by semiempirical quantum mechanical energies. Across >30 independent 300 ns molecular dynamics trajectories, the crystalline core retains its unit cell parameters, X-ray diffraction pattern, hydrogen bonding network, and canonical 4C1 ring conformation. This suggests that, should such folds be present, they need not compromise bulk crystalline integrity. No significant fibril twisting was observed in either the unfolded or chain-fold models. At the chain-folding ends of the fibril, high-energy boat and skew-boat conformations drive hydroxymethyl rotamer redistribution and noncanonical hydrogen bonding contacts. The greater structural flexibility of the 5-residue folded turns allows the chain-fold ends to access a broader conformational space, yet the structural perturbation remains localized within the same 3-residue region as the 3-residue folded turns. The 3-residue turn is therefore considered the more plausible chain-fold structure for cellulose-II, should such a structure actually exist in reality.