Folded-chain structure of cellulose II suggested by molecular dynamics simulation

•Folded-chain crystal exists in regenerated cellulose based on the following results.•One glucose unit with boat conformer enables cellulose chain to form a hairpin turn.•Crystal parameters of the folded and extended-chain crystals are almost the same.•The folded-chain molecular sheet is as stable as extended chain molecular sheet. The folded chain structure was successfully modeled when one glucose residue with a B1,4 conformer was attached to the ends of up and down cellulose molecules without any changes in their molecular distance. We also confirmed that other conformers such as 1,4B, 1S3, 1S5, and 3S1, 5S1 could also form folded chain structures. Here, B and S were known to be boat and skew boat ring conformers. These ring conformers must be rearranged during molecular dynamics simulation into the most suitable conformation; therefore, B1,4 and 1,4B were used as the starting conformers and finally rearranged to 5S1 and 1S5 , respectively. We investigated the possibility of a folded-chain crystal of the cellulose II polymorph by molecular dynamics (MD) simulation. The molecular direction of cellulose chains in cellulose II is anti-parallel, which allows the crystal to have folded-chain packing. It is impossible for cellulose I to form such a structure due to its parallel up assembly. The folded-chain crystal of the cellulose II polymorph was suggested based on the following results: (1) the glucose residue with boat and skew boat ring conformations enabled the cellulose chain to form a hairpin turn; (2) the lattice parameters of the folded-chain crystal and original crystal were almost the same (deviations in the a, b, and γ parameters of both crystals were within 3%); (3) the folded-chain molecular sheet was as stable in a water medium as the extended-chain molecular sheet, and structural parameters such as the hydrogen bonding system and side chain conformation of both molecular sheets were almost the same, indicating that the folded-chain molecular sheet is an initial structure during crystallization of the folded-chain crystal.