The role of hydroxyl groups in interchain interactions in cellulose Iα and Iβ

Complex networks of hydrogen bonds within the cellulose I_α and I_β contribute greatly to cellulose's anisotropic physical properties such as material stiffness. The interchain hydrogen bonding interactions through hydroxyl groups are isolated in each of the three lattice planes of the adjacent chains within the unit cell of two allomorphs of natural cellulose. In our density function theory study with dispersion corrected Perdew–Burke–Ernzerhof (PBE‐D2) functional, these hydroxyl groups participate in strong hydrogen bonding interactions (?24.8 and ?24.8 kcal/mol for cellulose I_α and I_β, respectively) in the side‐to‐side lattice plane. Unexpectedly, the hydroxyl groups also participate significantly in hydrogen bonding interactions (?11.0 and ?12.4 kcal/mol for cellulose I_α and I_β, respectively) in one of the diagonal lattice planes in both cellulose I_α and I_β. Both PM7 and PBE‐D2 method predict that the overall interaction is asymmetric and stronger in the right diagonal lattice plane. While hydrogen bonding interactions are strongest in side‐to‐side lattice plane as expected, the role of hydrogen bonding interactions for keeping the sheet together is more significant than previously thought.     

Structure conversions of cellulose IIII crystal models in solution state: a molecular dynamics study

This paper re-examines our previous molecular dynamics (MD) study on cellulose III_I crystal models with finite dimensions solvated in explicit water molecules. Eight crystal models, differing in a constituent lattice plane and dimensions, were studied. One calculation allowed for O–H and C–H bond stretching, and had a small time step of 0.5 fs. The other calculation adopted non-scaling factors of the 1–4 non-bonded interactions. As in our previous study, in the former MD calculations, six of the eight crystal models exhibited structure conversion with cooperative chain slippages generated by a progressive fiber bend. This converted the initial non-staggered chain packing of cellulose III_I into a near one-quarter staggering and gave the crystal model a triclinic-like configuration. In contrast, in the non-1–4 scaling MD calculations, all of the eight crystal models retained the initial cellulose III_I crystal structure. Another series of non-1–4 scaling MD calculations were performed for the four crystal models containing chains with a degree of polymerization (DP) of 40 at 370 K, which simulated hot water treatment to convert cellulose III_I to Iβ. Some of the hydroxymethyl groups irreversibly rotated from gt into tg conformation. This accompanied exchange of the intrasheet hydrogen bonding scheme along the (1 ?1 0) lattice plane from O2–O6 to O3–O6. The original corrugated (1 ?1 0) chain sheet was partly converted into a cellulose I-like flat chain sheet.     

Cellulose polymorphism study with sum-frequency-generation (SFG) vibration spectroscopy: identification of exocyclic CH2OH conformation and chain orientation

Sum-frequency-generation (SFG) vibration spectroscopy is a technique only sensitive to functional groups arranged without centrosymmetry. For crystalline cellulose, SFG can detect the C6H₂ and intra-chain hydrogen-bonded OH groups in the crystal. The geometries of these groups are sensitive to the hydrogen bonding network that stabilizes each cellulose polymorph. Therefore, SFG can distinguish cellulose polymorphs (I_β, II, III_I and III_II) which have different conformations of the exocyclic hydroxymethylene group or directionalities of glucan chains. The C6H₂ asymmetric stretching peaks at 2,944 cm^?1 for cellulose I_β and 2,960 cm^?1 for cellulose II, III_I and III_II corresponds to the trans-gauche (tg) and gauche-trans (gt) conformation, respectively. The SFG intensity of the stretch peak of intra-chain hydrogen-bonded O–H group implies that the chain arrangement in cellulose crystal is parallel in I_β and III_I, and antiparallel in II and III_II.     

Modeling of negative Poisson’s ratio (auxetic) crystalline cellulose I<Subscript>β</Subscript>

Energy minimizations for unstretched and stretched cellulose models using an all-atom empirical force field (molecular mechanics) have been performed to investigate the mechanism for auxetic (negative Poisson’s ratio) response in crystalline cellulose I_β from kraft cooked Norway spruce. An initial investigation to identify an appropriate force field led to a study of the structure and elastic constants from models employing the CVFF force field. Negative values of on-axis Poisson’s ratios ν ₃₁ and ν ₁₃ in the x ₁–x ₃ plane containing the chain direction (x ₃) were realized in energy minimizations employing a stress perpendicular to the hydrogen-bonded cellobiose sheets to simulate swelling in this direction due to the kraft cooking process. Energy minimizations of structural evolution due to stretching along the x ₃ chain direction of the ‘swollen’ (kraft cooked) model identified chain rotation about the chain axis combined with inextensible secondary bonds as the most likely mechanism for auxetic response.     

Synchrotron X-ray structures of cellulose I<Subscript>β</Subscript> and regenerated cellulose II at ambient temperature and 100 K

Synchrotron X-ray data have been collected to 1.4 Å resolution at the NE-CAT beam-line at the Advanced Photon Source from fibers of cellulose I_β and regenerated cellulose II (Fortisan) at ambient temperature and at 100 K in order to understand the effects of low temperature on cellulose more thoroughly. Crystal structures have been determined at each temperature. The unit cell of regenerated cellulose II contracted, with decreasing temperature, by 0.25%, 0.22% and 0.1% along the a, b, and c axes, respectively, whereas that of cellulose I_β contracted only in the direction of the a axis, by 0.9%. The value of 4.6×10^?5 K^?1 for the thermal expansion coefficient of cellulose I_β in the a axis direction can be explained by simple harmonic molecular oscillations and the lack of hydrogen-bonding in this direction. The molecular conformations of each allomorph are essential unchanged by cooling to 100 K. The room temperature crystal structure of regenerated cellulose II is essentially identical to the crystal structure of mercerized cellulose II.     

Partial crystalline transformation of solvated cellulose IIII crystals, reproduced by theoretical calculations

In hot-water molecular dynamics simulation at 370 K, four cellulose III_I crystal models, with different lattice planes and dimensions, exhibited partial crystalline transformations of (1 ?1 0) chain sheets, in which hydroxymethyl groups were irreversibly rotated from gt into tg conformations, accompanied by hydrogen-bond exchange from the original O3–O6 to cellulose-I-like O2–O6 bonds. The final hydrogen-bond exchange ratio was about 95 % for some of the crystal models after 50 ns simulation. The corrugated (1 ?1 0) chain sheet was converted to a cellulose-I-like flat chain sheet with a slightly right-handed twist. The 3D structures of the three types of isolated chain sheet models were optimized using density functional theory calculations to compare their stabilities without crystal packing forces. The cellulose Iβ (1 0 0) models were more stable than the cellulose III_I (1 ?1 0) models. The optimized structure of cellulose III_I (1 0 0) models deviated largely from the initial sheet form. It was proposed to the crystalline transformation from cellulose III_I to Iβ that conversion of the chain sheet structure first take place, followed by sliding of the chain sheet along the fiber axis.     

Anisotropy of the elastic properties of crystalline cellulose Iβ from first principles density functional theory with Van der Waals interactions

In spite of the significant potential of cellulose nanocrystals as functional nanoparticles for numerous applications, a fundamental understanding of the mechanical properties of defect-free, crystalline cellulose is still lacking. In this paper, the elasticity matrix for cellulose I_β with hydrogen bonding network A was calculated using ab initio density functional theory with a semi-empirical correction for van der Waals interactions. The computed Young’s modulus is found to be 206 GPa along [001] (c-axis), 98 GPa along [010] (b-axis), and 19 GPa along [100] (a-axis). Full compliance matrices are reported for 1.0, 1.5 and 2.0 % applied strains Color contour surfaces that show variations of the Young’s modulus and average Poisson’s ratio with crystallographic direction revealed the extreme anisotropies of these important mechanical properties. The sensitivity of the elastic parameters to misalignments in the crystal were examined with 2D polar plots within selected planes containing specific bonding characteristics; these are used to explain the substantial variability in the reported experimental Young’s moduli values. Results for the lattice directions [001], [010] and [100] are within the range of reported experimental and other numerical values.     

Theoretical investigation of azo dyes adsorbed on cellulose fibers: 1. Electronic and bonding structures

Structural, bonding and electronic characteristics of complexes of anthraquinone and 1-arylazo-2-naphtol dyes and cellulose I _β are studied using B3LYP density functional method with 6-31G** basis set based on the partially and fully optimized structures. Results reveal that for both partially and fully optimized complexes, there is a stabilizing attraction between dyes and cellulose surface. The hydrazone (Hy) tautomer in anionic state (Hy–SO₃ ^?) shows the strongest interaction with the cellulose surface. Natural bond orbital (NBO) and atoms-in-molecules (AIM) analyses have been carried out to study the nature of azo dyes-cellulose bonds in detail. According to NBO analysis, a remarkable charge transfer occurs between the –SO₃ ^? and –SO₃H functional groups of the dye and the cellulose surface which can be regarded as the main source of the large dye–cellulose interaction energy. AIM analysis confirms the existence of hydrogen and van der Waals bonds between the azo dyes and cellulose. Furthermore, a very good agreement is observed between the number of hydrogen bonding sites and dye–cellulose interaction energies.     

Crystalline cellulose elastic modulus predicted by atomistic models of uniform deformation and nanoscale indentation

The elastic modulus of cellulose Iβ in the axial and transverse directions was obtained from atomistic simulations using both the standard uniform deformation approach and a complementary approach based on nanoscale indentation. This allowed comparisons between the methods and closer connectivity to experimental measurement techniques. A reactive force field was used that explicitly describes hydrogen bond, coulombic and van der Waals interactions, allowing each contribution to the inter- and intra-molecular forces to be analyzed as a function of crystallographic direction. The uniform deformation studies showed that the forces dominating elastic behavior differed in the axial and transverse directions because of the relationship between the direction of the applied strain and the hydrogen bonding planes. Simulations of nanoscale indentation were then introduced to model the interaction between a hemispherical indenter with the $(1\bar{1}0)$ surface of a cellulose Iβ rod. The role of indenter size, loading force and indentation speed on the transverse elastic modulus was studied and, for optimized parameters, the results found to be in good agreement with experimentally-measured transverse elastic modulus for individual cellulose crystals.     

Cellulose‐Builder: A toolkit for building crystalline structures of cellulose

Cellulose‐builder is a user‐friendly program that builds crystalline structures of cellulose of different sizes and geometries. The program generates Cartesian coordinates for all atoms of the specified structure in the Protein Data Bank format, suitable for using as starting configurations in molecular dynamics simulations and other calculations. Crystalline structures of cellulose polymorphs Iα, Iβ, II, and III_I of practically any size are readily constructed which includes parallelepipeds, plant cell wall cellulose elementary fibrils of any length, and monolayers. Periodic boundary conditions along the crystallographic directions are easily imposed. The program also generates atom connectivity file in PSF format, required by well‐known simulation packages such as NAMD, CHARMM, and others. Cellulose‐builder is based on the Bash programming language and should run on practically any Unix‐like platform, demands very modest hardware, and is freely available for download from ftp://ftp.iqm.unicamp.br/pub/cellulose‐builder. © 2012 Wiley Periodicals, Inc.     

Young’s modulus calculations for cellulose I<Subscript>β</Subscript> by MM3 and quantum mechanics

Quantum mechanics (QM) and molecular mechanics (MM) calculations were performed to elucidate Young’s moduli for a series of cellulose I_β models. Computations using the second generation empirical force field MM3 with a disaccharide cellulose model, 1,4′-O-dimethyl-β-cellobioside (DMCB), and an analogue, 2,3,6,2′,3′,6′-hexadeoxy-1,4′-O-dimethyl-β-cellobioside (DODMCB), that cannot make hydrogen bonds reveal a considerable contribution of intramolecular hydrogen bonding to the molecular stiffness of cellulose I_β; the moduli for DMCB and DODMCB being 85.2 and 37.6 GPa, respectively. QM calculations confirm this contribution with modulus values of 99.7 GPa for DMCB and 33.0 GPa for DODMCB. However, modulus values for DMCB were considerably lower than values previously reported for cellulose I_β. MM calculations with extended cellulose chains (10–40 glucose units) resulted in modulus values, 126.0–147.5 GPa, more akin to the values reported for cellulose I_β. Comparison of the cellodecaose model, 1,4′-O-dimethyl-β-cellodecaoside (DMCD), modulus with that of its hydrogen bonding-deficient analogue, 2,3,6,2′,3′,6′-hexadeoxy-1,4′-O-dimethyl-β-cellodecaoside (DODMCD), corroborates the observed stiffness conferred by intramolecular hydrogen bonds; the moduli for DMCD and DODMCD being 126.0 and 63.3 GPa, respectively. Additional MM3 determinations revealed that modulus values were not strongly affected by intermolecular hydrogen bonding, with multiple strand models providing values similar to the single strand models; 87.5 GPa for a 7-strand DMCB model and 129.5 GPa for a 7 strand DMCD model.     

Structural Micro-heterogeneities of Crystalline Iβ-cellulose

The crystal structure of the native Iβ-allomorph of cellulose [Nishiyama et al. 2002. J. Am. Chem. Soc. 124: 9074–9082] reveals subtle but significant conformational differences between the two different chains and also a multiplicity of positions of the hydrogen atoms of the HO₂ and HO₆ hydroxyl groups. Two structures differing in the hydrogen bonding networks were then proposed, however, the static or dynamic origin of the observed disorder remains to be specified. Molecular modelling was used to address this question: 18 minicrystal and 2 macrocrystal models of cellulose were generated differing by the initial orientations of the HO₂ and HO₆ hydroxyl groups; among which the two proposed structures (called N1 and N16) together with a random structure which respect the experimental percentage of hydroxyl hydrogen orientations. Results showed that only 10 of the studied combinations were stable, the major structure (N1) defined by crystallographers was estimated viable whereas not the minor one (N16). All the calculated data from the retained crystals, which describe the solid dimensions, the individual chain conformations and the supermolecular organisation, 1/ remained stable at their equilibrium value during the dynamics and 2/ were sensitive to the initial positions of the hydrogen atoms. Analysis of the hydrogen bonds revealed that sheet stacking might be stabilised by unexpected hydrogen bonds in addition to hydrophobic interactions. Our results thus favoured local disorders which involve a limited number of chains; they revealed the structural microheterogeneity of the Iβ-phase of cellulose and a complex disorder of its corresponding hydrogen bonding networks.     

Lateral thermal expansion of cellulose Iβ and IIII polymorphs

Measurements of the thermal expansion coefficients (TECs) of cellulose crystals in the lateral direction are reported. Oriented films of highly crystalline cellulose I_β and III_I were prepared and then investigated with X‐ray diffraction at specific temperatures from room temperature to 250 °C during the heating process. Cellulose I_β underwent a transition into the high‐temperature phase with the temperature increasing above 220–230 °C; cellulose III_I was transformed into cellulose I_β when the sample was heated above 200 °C. Therefore, the TECs of I_β and III_I below 200 °C were measured. For cellulose I_β, the TEC of the a axis increased linearly from room temperature at α_a = 4.3 × 10^?5 °C^?1 to 200 °C at α_a = 17.0 × 10^?5 °C^?1, but the TEC of the b axis was constant at α_b = 0.5 × 10^?5 °C^?1. Like cellulose I_β, cellulose III_I also showed an anisotropic thermal expansion in the lateral direction. The TECs of the a and b axes were α_a = 7.6 × 10^?5 °C^?1 and α_b = 0.8 × 10^?5 °C^?1. The anisotropic thermal expansion behaviors in the lateral direction for I_β and III_I were closely related to the intermolecular hydrogen‐bonding systems. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1095–1102, 2002     

Size,shape, orientation and crystallinity of cellulose I<Subscript>β</Subscript> by X-ray powder diffraction using a free spreadsheet program

X-ray powder diffraction is one of the most commonly used methods in cellulose science. This technique is used to identify the cellulose allomorphs, their crystallinity, and the size of their crystallites. In this paper, a novel model is introduced that implicitly takes into account the shape and size of cellulose I_β crystallites in the interpretation of powder diffractograms. Because of the limited amount of data in cellulose powder patterns, this model focuses on a small number of adjustable parameters. The method hypothesizes that cellulose I_β crystallites are straight crystalline rods with superelliptical cross-sections. This superellipse is a parametric curve that can, for example, describe various crystallite shapes as rectangles or ellipses. Additionally, preferred orientation along the (0 0 1) crystallographic planes can be modelled using the March–Dollase approach. The simulated background has a semi-empirical form. An initial model comprised cellulose I_β crystallites and the amorphous background. A second model comprised a biphasic distribution of crystallites and the same amorphous background. In this second model, large cellulose I_β crystallites coexisted with more slender crystallites, usually less than 20 Å in lateral size. Cellulose IV_I nanocrystals were selected as a modeling construct to represent these small and distorted forms of native cellulose. Both models produced simulations in excellent agreement with the experimental measurements.     

Neutron crystallographic and molecular dynamics studies of the structure of ammonia-cellulose I: rearrangement of hydrogen bonding during the treatment of cellulose with ammonia

The hydrogen bond arrangement in a complex of cellulose with ammonia has been studied using neutron crystallography in combination with molecular dynamics simulations. The O6 atom of the hydroxymethyl group is donor in a highly occupied hydrogen bond to an ammonia molecule. This rotating ammonia molecule is donor in partially occupied and transient hydrogen bonds to the O2, O3 and O6 atoms of the hydroxyl groups of other chains. The hydrogen atom bound to the O3 atom is disordered but it is almost always involved in some type of hydrogen bonding. It is donated in a hydrogen bond most of the time to the O5 atom on the same chain. However, it also rotates away from this O5 atom to be donated to an ammonia molecule part of the time. On the other hand the hydrogen atom bound to the O2 atom is free from hydrogen bonding most of the time. It is donated in a hydrogen bond to the O6 atom on a neighboring chain only with a relatively small probability. These results provide new insights into how hydrogen bonds are rearranged during the conversion of cellulose I to cellulose III_I by ammonia treatment.     

Molecular dynamics simulation of dissociation behavior of various crystalline celluloses treated with hot-compressed water

The dissociation behavior of the crystalline cellulose polymorphs I_β, II, III_I, and IV_I (Cell I_β, etc.) at 503 K and 100 bar was studied by molecular dynamics simulation, and the mechanism of the experimental liquefaction during treatment with hot-compressed water was elucidated. The results showed that the mini-crystals of Cell I_β and Cell IV_I exhibited similar resistance to dissociation, which implies the occurrence of crystal transformation from Cell IV_I to Cell I. On the other hand, the mini-crystal of Cell II gradually dissociated into the water environment with the progress of time in the simulation. The water molecules gradually penetrated the Cell II crystal, especially along the (1 \(\overline{1}\) 0) crystal plane. In contrast, the dissolution behavior differed for the surface and the core areas of the mini-crystal of Cell III_I. The cellulose chains on the surface were dissociated into the water environment, whereas the ordered structure of the chains in the core region was maintained for the entire simulation period. The detailed investigation showed that the core part of Cell III_I was transformed into Cell I at an early stage of the simulation: Cell I is resistant to dissociation of the structure even in the hot-compressed water environment. It can be confirmed that the stability of these four crystals under high temperature and pressure conditions follows the order: Cell II < III_I < IV_I ≈ I_β.     

Iα → Iβ transition of cellulose under ultrasonic radiation

Aqueous suspensions of dispersed Glaucocystis cellulose microfibrils were sonicated at 4 °C for 3 h, using 24 kHz ultrasonic waves. This treatment induced a variety of ultrastructural defects, as the microfibrils became not only shortened, but also presented substantial damage materialized by kinks and subfibrillation. Upon analysis by X-ray diffraction and ¹³C solid-state NMR spectroscopy, it was found that the initial sample that contained 90 % of cellulose I_α allomorph became, to a large extent, unexpectedly converted into the I_β phase, while the loss of crystallinity was only moderate during the sonication treatment.     

Study on inclusion interaction of hydrophilic 2-chloromandelic acid with hydroxypropyl-β-cyclodextrin

The inclusion interaction between hydroxypropyl-β-cyclodextrin (HP-β-CD) and hydrophilic 2-chloromandelic acid (CMA) was studied by ultraviolet (UV) absorption spectrophotometer. A reliable determination of the complex stoichiometry was provided by the continuous variation technique. ¹H NMR spectrum and Thermo-gravimetric/differential thermal analyzer (TG/DTA) techniques were explored to further characterize the inclusion complex, and molecular modeling was used to investigate the mechanism of inclusion interaction. The results showed that HP-β-CD reacted with R,S-CMA to form inclusion complexes, with 1:1 stoichiometry and inclusion stability constants K_R and K_S were 24 and 39 L/mol determined from UV data by the method of Benesi-Hildebrand’s. Molecular modeling confirmed experimental observation and indicated that the hydrogen bonding interaction plays an important role in the interactive inclusion between HP-β-CD and CMA. Besides, compared with the HP-β-CD, molecular modeling showed R, S-CMA interact with β-CD through different binding modes, in which Vander Waals is the main intermolecular force between β-CD and R-CMA (or S-CMA) while without obvious hydrogen bonding interaction.     

Complexation-induced conformational regulation of ferrocene-dipeptide conjugates to nucleate γ-turn-like structure

The complexation of the ferrocene-dipeptide conjugate bearing one dipeptide chain of heterochiral sequence (-l-Ala-d-Pro-NHPy) with PdCl₂(MeCN)₂ was demonstrated to afford the 2:1 trans palladium complex, which is present in the pseudo-helical conformation and γ-turn-like structure in the crystal structure through complexation and intramolecular hydrogen bonding. Furthermore, the left-handed pseudo-helical molecular arrangement was formed through a network of intermolecular hydrogen bonds.