Thermal Response in Crystalline Iβ Cellulose:  A Molecular Dynamics Study

Abstract

The influence of temperature on structure and properties of the cellulose Ibeta crystal was studied by molecular dynamics simulations with the GROMOS 45a4 force-field. At 300 K, the modeled crystal agreed reasonably with several sets of experimental data, including crystal density, corresponding packing and crystal unit cell dimensions, chain conformation parameters, hydrogen bonds, Young's modulus, and thermal expansion coefficient at room temperature. At high-temperature (500 K), the cellulose chains remained in sheets, despite differences in the fine details compared to the room-temperature structure. The density decreased while the a and b cell parameters expanded by 7.4% and 6%, respectively, and the c parameter (chain axis) slightly contracted by 0.5%. Cell angles alpha and beta divided into two populations. The hydroxymethyl groups mainly adopted the gt orientation, and the hydrogen-bonding pattern thereby changed. One intrachain hydrogen bond, O2'H2'...O6, disappeared and consequently the Young's modulus decreased by 25%. A transition pathway between the low- and high-temperature structures has been proposed, with an initial step being an increased intersheet separation, which allowed every second cellulose chain to rotate around its helix axis by about 30 degrees . Second, all hydroxymethyl groups changed their orientations, from tg to gg (rotated chains) and from tg to gt (non-rotated chains). When temperature was further increased, the rotated chains returned to their original orientation and their hydroxymethyl groups again changed their conformation, from gg to gt. A transition temperature of about 450 K was suggested; however, the transition seems to be more gradual than sudden. The simulated data on temperature-induced changes in crystal unit cell dimensions and the hydrogen-bonding pattern also compared well with experimental results.

Unique Aspects of the Structure and Dynamics of Elementary Iβ Cellulose Microfibrils Revealed by Computational Simulations

“…Chain tilt occurred from the beginning of our simulations, consistent with observations in other MD simulations of cellulose using different force fields, crystalline models, and finite models (Bergenstråhle et al, 2007;Matthews et al, 2011aMatthews et al, , 2012Zhang et al, 2011;Chen et al, 2014). When microfibrils were viewed from the nonreducing end, origin layers had a clockwise tilt (positive) compared with the crystal structure, while the center layers had an anticlockwise tilt (negative; Fig.…”

“…A 36-chain, hexagonal-shaped model was suggested by Ding and Himmel (2006) and thought to be an optimal model from transmission electron microscopy and atomic force microscopy studies (Ding et al, 2014), because the cellulose synthase rosette complex from which cellulose is produced in plants is thought to be composed of 36 cellulose synthase catalytic proteins (Herth, 1983). Other 36-chain complexes have been suggested that have a square shape, in which the hydrophobic (100) and hydrophilic (010) faces are exposed to solvent (Mazeau, 2005;Matthews et al, 2006;Bergenstråhle et al, 2007), and a diamond-like shape where two hydrophilic (110 and 1-10) faces are exposed (Matthews et al, 2006;Zhang et al, 2011;Chen et al, 2014). However, there is now evidence from Rotation about the O5-C5-C6-O6 dihedral allows for three different conformations of the C6 exocyclic group of each Glc monomer.…”

“…One alternative is to use computational approaches, with numerous studies published in which molecular dynamics (MD) simulations have been performed on cellulose microfibrils, generally with a 36-chain model (Mazeau, 2005;Matthews et al, 2006Matthews et al, , 2011aMatthews et al, , 2011bMatthews et al, , 2012Bergenstråhle et al, 2007;Gross and Chu, 2010;Zhang et al, 2011;Chen et al, 2014). These simulations have investigated the structure and dynamics of the microfibrils, temperature dependence, H-bonding patterns, and effect of solvent water, and compared different carbohydrate force fields.…”

Temperature Dependence of Hydroxymethyl Group Rotamer Populations in Cellooligomers

“…Specifically, numerous studies use MD simulations to study the influence of temperature on, e.g., structure and properties of cellulose , 16,17,18 and shorter oligosaccharides, 19,20,21 the interactions between carbohydrates and other molecular species, 22,23,24 and, not the least, dissolution of cellulose in, e.g., ionic liquids, 25,26 and water. 27 Recently, Shen et al 28 published an extensive simulation study with the GLYCAM04 parameter set (available for download at www.glycam.org) in which the conformational flexibility of shortchain cellooligomers was investigated as a function of temperature.…”

Biomass Pyrolysis Kinetics: A Review of Molecular-Scale Modeling Contributions

“…Significant observations include an "untwisting" due to the formation of a three-dimensional hydrogen bonding network near the center chains of the 36-chain model (DP 40), for which the authors suggest arise from conformational changes of the hydroxymethyl groups. Bergenstråhle et al also studied the influence of temperature on the properties of the cellulose Iβ crystal using molecular dynamics simulation with the GROMOS 45a4 force field and found that at room temperature, the crystal structure could be sufficiently reproduced in terms of crystal density, unit cell dimensions and packing, whereas above 450 K, changes in the orientation of the hydroxymethyl group caused a shift in hydrogen bonding patterns and unit cell dimensions (Bergenstråhle et al, 2007).…”