Extended ensemble molecular dynamics study of cellulose I–ethylenediamine complex crystal models: atomistic picture of desorption behaviors of ethylenediamine

Cellulose I crystals swell on exposure to ethylenediamine (EDA) molecules to form a cellulose I–EDA complex, and successive extraction of EDA molecules converts the complex crystalline phase to either original cellulose I or cellulose III_I, depending on the treatment procedure. The present study reports the extended ensemble molecular dynamics (MD) simulation of the cellulose I–EDA complex models. An accelerated MD simulation allows most of the EDA molecules to desorb from the crystal model through a hydrophilic channel between the piles of cellulose chains, one at a time. Migration of a single EDA molecule along the channel is simulated by the adopted steered MD method combined with the umbrella sampling method to evaluate the potential of mean force (PMF) or free energy change on its movement. The PMF continues to increase during the migration of an EDA molecule to give a final PMF value of more than 30 kcal/mol. The PMF profiles are largely lowered by the removal of EDA molecules in the neighboring channels and by the widening of the channel. The former suggests that the EDA desorption cooperates with that in the neighboring channels, and, in the latter case, an EDA migration is efficiently promoted by solvation with water molecules in the expanded channel. We conclude that the atomistic picture of the EDA desorption behaviors observed in the crystal models is applicable to the real crystalline phase.