Abstract
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 IIII, 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. Graphic abstract
Highlights
Among the versatile properties of cellulosic materials, crystal polymorphism has been attracting much attention in terms of both scientific and technological interests
The Accelerated molecular dynamics (aMD) parameters, the threshold boost energy, E, and the acceleration factor, a, are defined for each potential according to the Amber Tutorial (Pierce et al 2012; Salomon-Ferrer et al 2013b; Miao et al 2014), following: where Nresidues is the number of solute residues including EDA, Natoms is the total number of atoms, and Vdihedral_avrg and Vtotal_avrg are the average dihedral and total potential energies from conventional molecular dynamics (MD) calculations, respectively
We have reported desorption behaviors of guest EDA molecules from the cellulose I–EDA complex crystal models studied by adopting a combination of three extended ensemble MD methods
Summary
Among the versatile properties of cellulosic materials, crystal polymorphism has been attracting much attention in terms of both scientific and technological interests. The high-resolution crystal structures of the relevant cellulose allomorphs have exclusively been reported by groups in Tokyo, Grenoble, Los Alamos, and Oak Ridge, using synchrotron fiber diffraction analysis, sometimes combined with nuclear magnetic resonance (NMR) measurement and theoretical calculations (Nishiyama et al 2002; Wada et al 2004; Wada et al 2009a; Nishiyama et al 2010; Wada et al 2011; Sawada et al 2013) These allomorphs include cellulose Iα and Iβ, cellulose IIII, and cellulose I–ethylenediamine (EDA) and ammonia complexes. A time-resolved X-ray microscopy diffraction measurement detects that no intermediate phase appears during the crystalline conversion, which suggests the successive crystalline conversion from cellulose Iβ to the I–EDA complex (Nishiyama et al 2010)
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