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

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.