Roger H. Newman

Estimation of the lateral dimensions of cellulose crystallites using 13C NMR signal strengths

Differences in proton rotating-frame spin relaxation rates were exploited to edit the 13C NMR spectra of solid lignocellulosics, separating signals assigned to cellulose crystallites from signals assigned to amorphous material. Clusters of signals at 89 and 85 ppm were assigned to C-4 in the interiors and on the surfaces of cellulose crystallites, respectively. Relative signal areas were used to estimate the weight-averaged lateral dimensions of crystallites, using a model in which crystallites have approximately square cross sections. The same 10 samples were also characterized by wide-angle X-ray scattering (WAXS). There was a strong correlation (r2 = 0.988) between the two sets of lateral dimensions, but those estimated by WAXS were typically 10% lower than those estimated by NMR. The deviations were attributed to differences in molecular conformations between interior and surface chains, causing broadening of the WAXS peaks. In the case of an eleventh sample containing well-ordered xylan, the NMR and WAXS methods were in good agreement only after exclusion of a xylan signal at 82.6 ppm from the NMR data.

Linkages between phosphorus transformations and carbon decomposition in a forest soil

Phosphorus mineralization is chemically coupled with organic matter (OM) decomposition in surface horizons of a mixed-conifer forest soil from the Sierra Nevada, California, and is also affected by the disturbance caused by forest harvesting. Solution13C nuclear magnetic resonance (NMR) spectroscopy of NaOH extracts revealed a decrease of O-alkyl and alkyl-C fractions with increasing degree of decomposition and depth in the soil profile, while carbonyl and aromatic C increased. Solid-state13C-NMR analysis of whole soil samples showed similar trends, except that alkyl C increased with depth. Solution31P-NMR indicated that inorganic P (P1) increased with increasing depth, while organic-P (Po) fractions decreased. Close relationships between P mineralization and litter decomposition were suggested by correlations between P1 and C fractions (r = 0.82, 0.81, −0.87, and −0.76 for carbonyl, aromatic, alkyl and O-alkyl fractions, respectively). Correlations for diester-P and pyrophosphate with O-alkyl (r = 0.63 and 0.84) and inverse correlations with aromatics (r = −0.74 and −0.72) suggest that mineralization of these P fractions coincides with availability of C substrate. A correlation between monoester P and alkyl C (r = 0.63) suggests mineralization is linked to breakdown of structural components of the plant litter. NMR analyses, combined with Hedley-P fractionation, suggest that post-harvest buildup of labile P in decomposed litter increases the potential for leaching of P during the first post-harvest season, but also indicates reduced biological activity that transports P from litter to the mineral soil. Thus, P is temporarily stored in decomposed litter, preventing its fixation by mineral oxides. In the mineral horizons,31P-NMR provides evidence of decline in biologically-available P during the first post-harvest season.

Carbon-13 NMR distinction between categories of molecular order and disorder in cellulose

Differences between values of proton rotating-frame spin relaxation time constants can be exploited to separate a solid-state13C NMR spectrum of cellulose into subspectra of crystalline and noncrystalline regions. Variations in chemical shifts and13C spin-lattice relaxation time constants can then be used to study variations in molecular order and disorder within each of the two broader categories. Mechanical damage during Wiley milling increases the content of noncrystalline cellulose and changes the nature of molecular disorder within that category. Resolution enhancement of the subspectrum assigned to crystalline cellulose reveals pairs of signals at 83.9 and 84.9 ppm (cellulose I) or 86.8 and 88.3 ppm (cellulose II) assigned to C-4 on well-ordered crystal surfaces. A broader peak in the subspectrum of crystalline cellulose I is assigned to poorly-ordered surfaces. Relative proportions in Avicel microcrystalline cellulose were estimated as: 54% in crystal interiors, 22% on well-ordered surfaces, 8% on poorly-ordered surfaces, 16% in domains of disorder extending more than a few nanometres.

Solid-State 13C Nuclear Magnetic Resonance Characterization of Cellulose in the Cell Walls of Arabidopsis thaliana Leaves

Solid-state 13C nuclear magnetic resonance was used to characterize the molecular ordering of cellulose in a cell-wall preparation containing mostly primary walls obtained from the leaves of Arabidopsis thaliana. Proton and 13C spin relaxation time constants showed that the cellulose was in a crystalline rather than a paracrystalline state or amorphous state. Cellulose chains were distributed between the interiors (40%) and surfaces (60%) of crystallites, which is consistent with crystallite cross-sectional dimensions of about 3 nm. Digital resolution enhancement revealed signals indicative of triclinic and monoclinic crystalline forms of cellulose mixed in similar proportions. Of the five nuclear spin relaxation processes used, proton rotating-frame relaxation provided the clearest distinction between cellulose and other cell-wall components for purposes of editing solid-state 13C nuclear magnetic resonance spectra.

Crystalline forms of cellulose in softwoods and hardwoods

Abstract Resolution-enhanced 13C CP/MAS NMR spectra were obtained for woods from 7 different tree species. Ratios of signal strengths at 90.2 and 88.6 ppm were used to estimate relative proportions of Iα and Iβ crystalline forms of cellulose. These ratios were distinctly higher for softwoods than for hardwoods. The NMR results therefore support published X-ray diffraction evidence for differences between crystalline forms of cellulose in softwoods and hardwoods. Resolution of signals at 84.0 and 84.9 ppm provided evidence for a high degree of molecular order on the surfaces of cellulose crystallites in wood.

Cell wall changes in ripening kiwifruit: 13C solid state NMR characterisation of relatively rigid cell wall polymers

Cell wall material was isolated from the outer pericarp of kiwifruit at harvest and at several ripening stages following a postharvest ethylene treatment. Solid state 13C NMR spectra showed no evidence for changes in the nature of the cellulose crystallites or the polysaccharides adhering to crystallite surfaces even in cell wall material isolated from fruits in which cell wall dissolution was extreme. Nuclear spin relaxation experiments showed that pectin retained in the cell wall became ‘softened’ in the early stages of ripening, prior to solubilisation later in ripening. Chemical data showed that this ‘softening’ of pectin was not accompanied by any marked change in its chemical composition. Fruit firmness was directly proportional to the amount of non-cellulosic matter that remained sufficiently rigid to respond to the cross-polarisation NMR pulse sequence. The results support the idea that pectin solubilisation in ripening fruit may in part be a consequence of cell wall swelling rather than a direct cause of it.

Interactions between locust bean gum and cellulose characterized by13C n.m.r. spectroscopy

Abstract Molecular interactions between locust bean gum (LBG) and cellulose crystallite surfaces appear to involve most mannosyl residues of the mannan backbone, not just the small proportion contained in long segments which lack galactosyl residues. This conclusion is based on: (1) relative strengths of 13 C n.m.r. signals at 102.2 ppm in the cross-polarization (CP) spectrum and 101.3 ppm in the single-pulse excitation (SPE) spectrum, assigned to mannosyl C-1 in bound and non-bound segments of LBG, respectively; (2) displacement of a 13 C n.m.r. signal assigned to mannosyl C-4 to 81 ppm, indicating a change of conformation in the mannan backbone relative to a gel phase; (3) similarities between proton spin relaxation time constants for cellulose and LBG, indicating proton spin diffusion between polymers in close contact; (4) broadening of CP n.m.r. signals at 83.9 and 85.0 ppm, assigned to C-4 of cellulose chains exposed on crystallite surfaces. The galactosyl C-1 signal appears at 99.9 ppm in the SPE n.m.r. spectrum of the LBG-cellulose complex but shows a poor response to CP n.m.r., indicating that galactosyl residues are not fixed in rigid conformations relative to the mannan backbones.

Solid-State 13C NMR Study of a Composite of Tobacco Xyloglucan and Gluconacetobacter xylinus Cellulose: Molecular Interactions between the Component Polysaccharides

To investigate possible molecular interactions between xyloglucans (XGs) and cellulose in plant cell walls, a model composite was produced using cellulose from the bacterium Gluconacetobacter xylinus and XG from the walls of a tobacco cell-suspension culture that had been incubated with (13)C-labeled glucose. Solid-state (13)C NMR with cross-polarization (CP) and magic-angle spinning (MAS) was used in combination with proton spin-relaxation editing to separate signals from crystalline (rigid) and less rigid domains of the composite. Signals from XG were confined to subspectra of less rigid domains, with no detectable signals from XG attached to surfaces of cellulose crystallites. Signal displacements indicated XGs were more rigid than the mobile coil (twisted backbone) conformation expected for unattached XGs. Similar (13)C chemical shifts were observed in a single-pulse excitation experiment. The results were not compatible with extensive hydrogen bonding between XG and cellulose, but were consistent with a composite structure in which cellulose crystallites were embedded in a matrix of XG with a semirigid (straightened backbone) conformation, that is, a matrix that is partly ordered rather than amorphous.

Crystalline forms of cellulose in the silver tree fern Cyathea dealbata

Solid-state 13C NMR spectroscopy was used to characterize fibrous material cut from the midrib of a fern frond. Signals associated with cellulose crystallites were separated from those associated with the lignin--hemicellulosic matrix by exploiting differences in proton rotating-frame relaxation time constants. Heights of signals at 90.2 and 88.5 ppm, assigned to C-4 in cellulose Iα and Iβ, indicated similar proportions of the two crystalline forms. This observation conflicts with a suggestion that plant celluloses can be grouped into the two categories of Iα-rich and Iβ-rich.

Solid-state n.m.r. determination of residual unsaturation in styrene-cured polyester resins

Abstract A resin containing fumarate, isophthalate and propylene glycol structural units was cured with styrene and characterized by 13C solid-state nuclear magnetic resonance spectroscopy. The degree of residual polyester unsaturation was estimated from relative areas of peaks at 172 ppm (carboxylic carbon in saturated structural units derived from fumarate) and 165 ppm (carboxylic carbon in isophthalate and unreacted fumarate structural units). Decreasing the styrene content below 47% by weight resulted in residual polyester unsaturation. Results were interpreted in terms of a statistical model for copolymerization, providing evidence for low reactivity of fumarate structural units relative to diethyl fumarate and drawing attention to differences in reactivity between the resin system used in the present work and those used in earlier studies.