Hitomi Miyamoto

Structural reorganization of molecular sheets derived from cellulose II by molecular dynamics simulations

We investigated structural reorganization of two different kinds of molecular sheets derived from the cellulose II crystal using molecular dynamics (MD) simulations, in order to identify the initial structure of the cellulose crystal in the course of its regeneration process from solution. After a one-nanosecond simulation, the molecular sheet formed by van der Waals forces along the (11 0) crystal plane did not change its structure in an aqueous environment, while the other one formed by hydrogen bonds along the (110) crystal plane changed into a van der Waals-associated molecular sheet, such as the former. The two structures that were calculated showed substantial similarities such as the high occupancy of intramolecular hydrogen bonds between O3(H) and O5 of over 0.75, few intermolecular hydrogen bonds, and the high occurrence of hydrogen bonding with water. The convergence of the two structures into one denotes that the van der Waals-associated molecular sheet can be the initial structure of the cellulose crystal formed in solution. The main chain conformations were almost the same as those in the cellulose II crystal except for a -16 degrees shift of phi (dihedral angle of O5-C1-O1-C4) and the gauche-gauche conformation of the hydroxymethyl side group appears probably due to its hydrogen bonding with water. These results suggest that the van der Waals-associated molecular sheet becomes stable in an aqueous environment with its hydrophobic inside and hydrophilic periphery. Contrary to this, a benzene environment preferred a hydrogen-bonded molecular sheet, which is expected to be the initial structure formed in benzene.

Supermolecular structure of cellulose/amylose blends prepared from aqueous NaOH solutions and effects of amylose on structural formation of cellulose from its solution

We previously proposed a mechanism for the structural formation of cellulose from its solution using a molecular dynamics (MD) simulation and suggested that the initial structure from its solution plays a critical role in determining its final structure. Structural changes in the van der Waals-associated cellulose molecular sheet as the initial structure were examined by MD simulation; the molecular sheet was found to be disordered due to maltohexaoses as an amylose model in terms of the hydrogen bonding system of cellulose. The structure and properties of cellulose/amylose blends prepared from an aqueous NaOH solution were examined experimentally by wide-angle X-ray diffraction and dynamic viscoelasticity measurements. The crystallinity of cellulose in the cellulose/amylose blend films was lower than that of cellulose film. The diffraction peaks of the cellulose/amylose blends were slightly shifted; specifically, (1 1 0) was shifted to a higher angle, and (1 1 0) and (0 2 0) were shifted to lower angles. These experimental results probably resulted from the disordered molecular sheet, as revealed by MD simulations.

Molecular dynamics simulation of the dissolution process of a cellulose triacetate-II nano-sized crystal in DMSO.

An understanding of the dissolution process of cellulose derivatives is important not only for basic research but also for industrial purposes. We investigated the dissolution process of cellulose triacetate II (CTA II) nano-sized crystal in DMSO solvent using molecular dynamics simulations. The nano-sized crystal consists of 18 CTA chains. During the 9 ns simulation, it was observed that one chain (C01) located at the corner of the lozenge crystal was solvated by the DMSO molecules and moved away from the remaining cluster into the DMSO solvent. The analysis showed that the breakage of the interaction between the H1, H3, and H5 hydrogens of the pyranose ring and the acetyl carbonyl oxygen in the C01 and C02 adjacent chains would be crucial for the dissolution of CTA. The DMSO molecules solvating around these atoms would prevent the re-crystallization of the CTA molecules and facilitate further dissolution.

Structural changes in the molecular sheets along (hk0) planes derived from cellulose Iβ by molecular dynamics simulations

We studied the stability of molecular sheets with four cellotetraoses in an aqueous environment by molecular dynamics simulation to identify the molecular details of first structure as one of the possibilities in the course of crystallization of cellulose I. After simulation, the molecular sheets formed by van der Waals forces along the (11̄0) and (110) crystal plane did not change their structures in an aqueous environment, whereas the other ones formed by hydrogen bonds along the (100) and (200) crystal plane changed into a van der Waals associated molecular sheet, similar to the former. These simulated molecular sheets formed by van der Waals forces were structurally stable in water because of their hydrophilic exterior and hydrophobic interior. Therefore, if the molecular sheet structures are formed in the real system, the sheets formed by van der Waals forces are probably the initial structure of crystallization. A close analysis indicated that these sheets could be classified into two groups in terms of the hydrogen bonding networks, camber angle, and main and side chain conformations. One group was the molecular sheets corresponding to the (110) after simulation. This sheet is probably rigid because intramolecular hydrogen bonds of the chains in the sheet are highly developed. The other group was the molecular sheets corresponding to (200), (100), and (11̄0) crystal plane: the chains in these sheets seemed to be rather flexible due to their moderately developed intramolecular hydrogen bonds.

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.

Molecular dynamics simulation of cellulose-coated oil-in-water emulsions

The behaviors of cellulose chains and cellulose mini-crystal in oil-in-water emulsions were studied by molecular dynamics simulations to investigate the coating states and the structural features of cellulose in these emulsions. In oil-in-water emulsion, dispersed cellulose chains gradually assemble during the progress of the simulation, eventually surrounding the octane droplet. In case of a cellulose mini-crystal, the cellulose chain at the corner of the crystal first contacts with the octane droplet through its hydrophobic surface. The other cellulose chains along the crystal plane then gradually move toward the octane molecules. In both emulsions, the cellulose was found to interact with both water and octane surfaces with specific conformations that allow the CH groups of the glucose rings to contact with octane molecules, while the OH groups of these rings contact with water molecules to form hydrogen bonds. The cellulose chains on the octane droplet also contact with each other through lateral hydrogen bonding between chains. These interactions stabilize the emulsion formed by cellulose molecules as surfactants.

Influence of dyestuffs on the crystallinity of a bacterial cellulose and a regenerated cellulose

The crystal structures of bacterial cellulose (BC) obtained by cultivation of an Enterobacter species CJF 002 stock under the presence of direct, acid, and basic dyestuffs were examined. Optical microscopic observation showed that direct and basic dyestuffs stained BC samples but acid dyestuff did not. This suggests that direct and basic dyestuffs are contained within the resulting BC samples. Analysis of wide angle x-ray diffraction (WAXD) data indicates that direct dyestuffs inhibited crystallization of BC at dyestuff concentration in culture media (Cdye) of more than 0.05 wt% with lower angle shift of the diffraction peak for the (200) plane of BC, but almost no influence on BC crystallization in the case of basic dyestuff was observed. In addition, we investigated the crystallinity of regenerated cellulose (RC) from a cuprammonium solution and the reaction of RC with the dyestuffs. The dyestuffs had almost no impact on the crystallinity of RC even in cases where the samples showed staining. It was found that the apparent crystallite size of (110) and (020) in the RC samples with dyestuffs were slightly lower than that in the RC blank sample, while the apparent crystallite size of ( 1 1 ¯ 0 ) in the RC samples with dyestuffs retain the values at the same level as the RC blank sample. These results suggest that the cellulose molecular sheets held together by van der Waals interactions were the basic structure formed from RC and they probably retain their structure in the cuprammonium solution at relatively high concentrations of cellulose.

Structure of cellulose/direct dye complex regenerated from supercritical water

The regeneration of cellulose from supercritical water in the presence of direct dyes was studied by small- and wide-angle synchrotron X-ray scattering and cryo-transmission electron microscopy to understand the effects of dyes on the structure formation of cellulose. In addition, the interactions between cellulose and the direct dyes were characterized using molecular dynamics simulations. Peaks corresponding to cellulose II crystals were observed in the wide-angle X-ray diffraction pattern of cellulose regenerated from supercritical water without dyes, whereas these peaks were not observed in the diffraction patterns of samples with direct dye (Direct Red 28 or Direct Blue 1). This result indicated that the direct dyes prevented the crystallization of regenerated cellulose. The results of the molecular dynamics simulations indicated that the planes of glucose rings interacted with the aromatic moieties of the dyes and that the sulfonate groups of the dye molecules interacted with the hydroxyl groups of cellulose. In addition, the CH groups of the glucose rings and aromatic moieties of the dyes (e.g., naphthalene and biphenyl moieties) interacted weekly. When cellulose regenerates from solution, cellulose sheet structures formed via hydrophobic interactions appear as the initial structure. The direct dyes were found to affect the formation of this cellulose sheet structure because cellulose molecularly dissolved in supercritical water. In the Kratky plots for small-angle X-ray scattering, a peak was clearly observed for the cellulose and cellulose/DR28 samples in the region of smaller q (<0.5), indicating that the nanoscale assembly structures dispersed in these systems. Bundled sheet-like and twisted ribbon-like structures were observed in the supernatants of the cellulose and cellulose/DR28 samples. These dispersed structures were considered to be intermediates in the structural formation of cellulose.

Cross-sectional Distribution of Crystalline and Fibril Orientations of Typical Regenerated Cellulose Fibers in Relation to their Fibrillation Resistance

The structural distributions and their formation mechanisms, on both the morphological and crystallographic scales of two types of regenerated cellulose fiber, cuprammonium rayon (Cupra) and solvent spun rayon (Lyocell), were studied in relation to fibrillar formation using transmission electron microscopy and electron micro-diffraction analysis. Cupra had a multilayer structure. The outer layer of the fiber had a network structure without distinct fibrils, but with high crystallinity (χc) and crystalline orientation (F c), whereas the center of the fiber had a well-developed fibrillar structure with an amorphous state. The high χc in the outer layer probably resulted from the release of both copper and ammonia from the fiber during coagulation. The fibrillar structures in the center could have been formed by the deformation of the network structure along the elongation direction before crystallization took place. Lyocell was crystallographically and morphologically uniform along the fiber radius, with a highly developed fibrillar structure with high χc and F c. The type of fibrillar formation was closely correlated with the morphological distribution, that is, fibrillation started from layers with a well-developed fibrillar structure.

Molecular dynamics simulation of dehydration in cellulose/water crystals

It is well known that there are two crystal types for a cellulose/water complex: Na–cellulose IV and cellulose II hydrate. The water molecules in these crystals are released with increasing temperature and the hydrated models are then transformed into cellulose II crystals. Dehydration can be observed even when the cellulose/water crystal is soaked in water. In this study, we investigated the dehydration process in Na–cellulose IV and cellulose II hydrate using molecular dynamics simulations. In both crystals, as simulation time progressed, the water molecules were gradually released from the crystal while forming hydrogen bonds with the hydroxyl groups of cellulose. Interestingly, water molecules were released from the spaces between the molecular sheets formed by hydrophobic interactions. All hydroxyl groups existed on the surface of the molecular sheets. Therefore, it is reasonable to suggest that water molecules pass between the sheets. For Na–cellulose IV, the ratio of gt conformations was high. In addition, it was observed that water molecules were in the space between O6 and O3 on each adjacent molecular sheet forming hydrogen bonds during simulation. On the other hand, the gg conformations were mainly observed in cellulose II hydrate probably because of the presence of the O6···O6 intermolecular hydrogen bond. During the simulation, the water molecules were present in the space between O2 and O3 while forming hydrogen bonds.