CP/MAS Carbon-13 NMR Study of Spin Relaxation Phenomena of Cellulose Containing Crystalline and Noncrystalline Components

Comparison and assessment of methods for cellulose crystallinity determination

The degree of crystallinity in cellulose significantly affects the physical, mechanical, and chemical properties of cellulosic materials, their processing, and their final application. Measuring the crystalline structures of cellulose is a challenging task due to inadequate consistency among the variety of analytical techniques available and the lack of absolute crystalline and amorphous standards. Our article reviews the primary methods for estimating the crystallinity of cellulose, namely, X-ray diffraction (XRD), nuclear magnetic resonance (NMR), Raman and Fourier-transform infrared (FTIR) spectroscopy, sum-frequency generation vibrational spectroscopy (SFG), as well as differential scanning calorimetry (DSC), and evolving biochemical methods using cellulose binding molecules (CBMs). The techniques are compared to better interrogate not only the requirements of each method, but also their differences, synergies, and limitations. The article highlights fundamental principles to guide the general community to initiate studies of the crystallinity of cellulosic materials.

Lignocellulose molecular assembly and deconstruction properties of lignin-altered rice mutants

Bioengineering approaches to modify lignin content and structure in plant cell walls have shown promise for facilitating biochemical conversions of lignocellulosic biomass into valuable chemicals. Despite numerous research efforts, however, the effect of altered lignin chemistry on the supramolecular assembly of lignocellulose and consequently its deconstruction in lignin-modified transgenic and mutant plants is not fully understood. In this study, we aimed to close this gap by analyzing lignin-modified rice (Oryza sativa L.) mutants deficient in 5-HYDROXYCONIFERALDEHYDE O-METHYLTRANSFERASE (CAldOMT) and CINNAMYL ALCOHOL DEHYDROGENASE (CAD). A set of rice mutants harboring knockout mutations in either or both OsCAldOMT1 and OsCAD2 was generated in part by genome editing and subjected to comparative cell wall chemical and supramolecular structure analyses. In line with the proposed functions of CAldOMT and CAD in grass lignin biosynthesis, OsCAldOMT1-deficient mutant lines produced altered lignins depleted of syringyl and tricin units and incorporating noncanonical 5-hydroxyguaiacyl units, whereas OsCAD2-deficient mutant lines produced lignins incorporating noncanonical hydroxycinnamaldehyde-derived units. All tested OsCAldOMT1- and OsCAD2-deficient mutants, especially OsCAldOMT1-deficient lines, displayed enhanced cell wall saccharification efficiency. Solid-state nuclear magnetic resonance (NMR) and X-ray diffraction analyses of rice cell walls revealed that both OsCAldOMT1- and OsCAD2 deficiencies contributed to the disruptions of the cellulose crystalline network. Further, OsCAldOMT1 deficiency contributed to the increase of the cellulose molecular mobility more prominently than OsCAD2 deficiency, resulting in apparently more loosened lignocellulose molecular assembly. Such alterations in cell wall chemical and supramolecular structures may in part account for the variations of saccharification performance of the OsCAldOMT1- and OsCAD2-deficient rice mutants.

Altered lignocellulose chemical structure and molecular assembly in CINNAMYL ALCOHOL DEHYDROGENASE-deficient rice

Lignin is a complex phenylpropanoid polymer deposited in plant cell walls. Lignin has long been recognized as an important limiting factor for the polysaccharide-oriented biomass utilizations. To mitigate lignin-associated biomass recalcitrance, numerous mutants and transgenic plants that produce lignocellulose with reduced lignin contents and/or lignins with altered chemical structures have been produced and characterised. However, it is not fully understood how altered lignin chemistry affects the supramolecular structure of lignocellulose, and consequently, its utilization properties. Herein, we conducted comprehensive chemical and supramolecular structural analyses of lignocellulose produced by a rice cad2 mutant deficient in CINNAMYL ALCOHOL DEHYDROGENASE (CAD), which encodes a key enzyme in lignin biosynthesis. By using a solution-state two-dimensional NMR approach and complementary chemical methods, we elucidated the structural details of the altered lignins enriched with unusual hydroxycinnamaldehyde-derived substructures produced by the cad2 mutant. In parallel, polysaccharide assembly and the molecular mobility of lignocellulose were investigated by solid-state ¹³C MAS NMR, nuclear magnetic relaxation, X-ray diffraction, and Simon's staining analyses. Possible links between CAD-associated lignin modifications (in terms of total content and chemical structures) and changes to the lignocellulose supramolecular structure are discussed in the context of the improved biomass saccharification efficiency of the cad2 rice mutant.

Elucidation of the Relationship between Intrinsic Viscosity and Molecular Weight of Cellulose Dissolved in Tetra-N-Butyl Ammonium Hydroxide/Dimethyl Sulfoxide

The determination of molecular weight of natural cellulose remains a challenge nowadays, due to the difficulty in dissolving cellulose. In this work, tetra-n-butylammonium hydroxide (TBAH) and dimethyl sulfoxide (DMSO) aqueous solution (THDS) were used to dissolve cellulose in a few minutes under room temperature into true molecular solutions. That is to say, the cellulose was dissolved in the solution in molecular level, and the viscosity of the solution is linearly dependent on the concentration of cellulose. The relationship between the molecular weight of cellulose and the intrinsic viscosity tested in such dilute solutions has been established in the form of the Mark-Houwink equation, η=0.24×DP1.21. The value of 1.21 indicates that the cellulose molecules dissolve in THDS quite well. The cellulose dispersion in the THDS was proved to be in molecular level by atomic force microscope (AFM) and dynamic light scattering (DLS). The reliability of the established Mark-Houwink equation was cross-checked by the gel permeation chromatography (GPC) and traditional copper (II) ethylenediamine (CED) method. No considerate degradation was observed by comparing the intrinsic viscosity and the degree of polymerization (DP) values of the original with and the regenerated cellulose samples. The natural cellulose can be molecularly dispersed in the multiple-component solvent (THDS), and kept stable for a certain period. A time efficient and reliable method has been supplied for determination of the degree of polymerization and the molecular weight of cellulose.

Comparing the physiochemical parameters of three celluloses reveals new insights into substrate suitability for fungal enzyme production

Background

The industrial applications of cellulases are mostly limited by the costs associated with their production. Optimized production pathways are therefore desirable. Based on their enzyme inducing capacity, celluloses are commonly used in fermentation media. However, the influence of their physiochemical characteristics on the production process is not well understood. In this study, we examined how physical, structural and chemical properties of celluloses influence cellulase and hemicellulase production in an industrially-optimized and a non-engineered filamentous fungus: Trichoderma reesei RUT-C30 and Neurospora crassa. The performance was evaluated by quantifying gene induction, protein secretion and enzymatic activities.

Results

Among the three investigated substrates, the powdered cellulose was found to be the most impure, and the residual hemicellulosic content was efficiently perceived by the fungi. It was furthermore found to be the least crystalline substrate and consequently was the most readily digested cellulose in vitro. In vivo however, only RUT-C30 was able to take full advantage of these factors. When comparing carbon catabolite repressed and de-repressed strains of T. reesei and N. crassa, we found that cre1/cre-1 is at least partially responsible for this observation, but that the different wiring of the molecular signaling networks is also relevant.

Conclusions

Our findings indicate that crystallinity and hemicellulose content are major determinants of performance. Moreover, the genetic background between WT and modified strains greatly affects the ability to utilize the cellulosic substrate. By highlighting key factors to consider when choosing the optimal cellulosic product for enzyme production, this study has relevance for the optimization of a critical step in the biotechnological (hemi-) cellulase production process.

Electronic supplementary material

The online version of this article (10.1186/s40694-017-0039-9) contains supplementary material, which is available to authorized users.

Analysis of mercerization process based on the intensity change of deconvoluted resonances of (13)C CP/MAS NMR: Cellulose mercerized under cooling and non-cooling conditions

The area intensity change of C1, C4, and C6 in spectrum obtained by (13)C CP/MAS NMR and the mutual relationship between their changes were examined for cellulose samples treated with various concentrations of aqueous NaOH solutions under non-cooling and cooling conditions. The area intensity of C1-up and C6-down changed cooperatively with that of C4-down which corresponds to the crystallinity of samples: "-up" and "-down" are the up- and down- field component in a splitting peak of NMR spectrum, respectively. The intensity change of C1-up starts to decrease with decreasing in that of C4-down after that of C6-down is almost complete. These changes were more clearly observed for samples treated under cooling condition. It can be suggested that their characteristic change relates closely to the change in conformation of cellulose chains by induced decrystallization and the subsequent crystallization of cellulose II, and presumed that their changes at microscopic level relate to the macroscopic morphological changes such as contraction along the length of cellulose chains and recovery along the length.

13C high-resolution solid state NMR studies of the structure and dynamics of 1,6:3,4-dianhydro-2-O-tosyl-beta-D-galactopyranose

13C CP/MAS, dipolar dephasing MAS and theoretical GIAO calculations were employed to assign 13C resonances to the molecular structure of 1,6:3,4-dianhydro-2-O-tosyl-beta-D-galactopyranose 1. From spinning sideband intensities, employing the graphical method of Herzfeld and Berger the 13C delta(ii) parameters for aromatic residue were calculated. The experimental data were compared with computed results obtained by means of the B3PW91 hybrid method and 6-311G (df, p) basis set. The X-ray geometry of 1 with the correlated position of hydrogen atoms was taken as input data for theoretical calculations. As concluded from Cambridge Crystallographic Database (CSD) search, there are two reports describing the X-ray studies of 1 that show the slightly different geometry of the compound under investigation. This work shows that such discrepancies in geometry can generate differences between computed 13C delta(ii) parameters up to 6 ppm. 13C T1 and 1H T1rho relaxation times reveal that 1 is very rigid in crystal lattice. This structure is characterized by extremely long 1H T1rho, found to be in range ca. 200 ms.

Proton spin-lattice relaxation time measurements of solid wood and its constituents as a function of pH: part II

The proton spin-lattice relaxation times (T1H) for isolated cuoxam lignin and fully bleached cellulose were measured as a function of pH in the solid-state. These experiments provided the opportunity to examine for possible macromolecular connectivities that may be present between lignin and carbohydrates within softwood. These studies have shown that the molecular mobilities of the isolated polymeric constituents of wood are affected by the ionization of their functional groups at different pHs. The shapes of the plots of T1H as a function of pH for the two isolated polymers were dramatically different, while those for softwood were similar. This fact provides evidence supporting the notion that in wood, lignin and carbohydrates are intimately associated with each other.

CP/MAS 13C-NMR spectroscopy applied to structure and interaction studies on cellulose I

Solid-state Cross-Polarization Magic Angle Spinning Carbon-13 Nuclear Magnetic Resonance (CP/MAS 13C-NMR) has been used to investigate the structure and interactions of cellulose I. The use of spectral fitting for the extraction of information from CP/MAS 13C-NMR spectra is reviewed and results obtained are discussed. Examples are shown where the method has been used to monitor the structural changes occurring in wood cellulose during kraft pulping. The effects observed on the cellulose and hemicelluloses are further investigated using a model system. Assignments of signal intensities originating from xylan-cellulose interactions are made.

13C MAS NMR studies of the effects of hydration on the cell walls of potatoes and Chinese water chestnuts

13C NMR with magic angle spinning (MAS) has been employed to investigate the cell walls of potatoes and Chinese water chestnuts over a range of hydration levels. Both single-pulse excitation (SPEMAS) and cross-polarization (CPMAS) experiments were carried out. Hydration led to a substantial increase in signal intensities of galactan and galacturonan in the SPEMAS spectra and a decrease in line width, implying mobilization in the backbone and side chains of pectin. In CPMAS spectra of both samples, noncellulose components showed signal loss as hydration increased. However, the signals of some galacturonan in the 3(1) helix configuration remained in the spectra even when the water content was as high as 110%. Cellulose was unaffected. It is concluded that the pectic polysaccharides experience a distribution of molecular conformations and mobility, whereas cellulose remained as typical rigid solid.

Fine structure in cellulose microfibrils: NMR evidence from onion and quince

It has been controversial for many years whether in the cellulose of higher plants, the microfibrils are aggregates of 'elementary fibrils', which have been suggested to be about 3.5 nm in diameter. Solid-state NMR spectroscopy was used to examine two celluloses whose fibril diameters had been established by electron microscopy: onion (8-10 nm, but containing 40% of xyloglucan as well as cellulose) and quince (2 nm cellulose core). Both of these forms of cellulose contained crystalline units of similar size, as estimated from the ratio of surface to interior chains, and the time required for proton magnetisation to diffuse from the surface to the interior. It is suggested that the onion microfibrils must therefore be constructed from a number of cellulose subunits 2 nm in diameter, smaller than the 'elementary fibrils' envisaged previously. The size of these subunits would permit a hexagonal arrangement resembling the cellulose synthase complex.