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.

WAXS and 13C NMR study of Gluconoacetobacter xylinus cellulose in composites with tamarind xyloglucan.

- Model composites, produced using cellulose from stationary cultures of the bacterium Gluconoacetobacter xylinus and tamarind xyloglucan, were examined by wide-angle X-ray scattering (WAXS) and CP/MAS solid-state (13)C NMR spectroscopy. The dominant crystallite allomorph of cellulose produced in culture media with or without xyloglucan was cellulose I(alpha) (triclinic). The presence of xyloglucan in the culture medium reduced the cross-section dimensions of the cellulose crystallites, but did not affect the crystallite allomorph. However, when the composites were refluxed in buffer, the proportion of cellulose I(beta) allomorph increased relative to that of cellulose I(alpha). In contrast, cellulose I(alpha) remained the dominant form when cellulose, produced in the absence of xyloglucan, was then heated in the buffer. Hence the presence of xyloglucan has a profound effect on the formation of the cellulose crystallites by G. xylinus.

Using solid-state ¹³C NMR spectroscopy to study the molecular organisation of primary plant cell walls.

Studies of the mobilities of polysaccharides or parts of polysaccharides in a cell-wall preparation may give clues about the molecular interactions among the polysaccharides in the cell wall and the relative locations of polysaccharides within the cell wall. A number of solid-state (13)C NMR techniques have been developed that can be used to investigate different types of polysaccharide mobilities: rigid, semi-rigid, mobile, and highly mobile. In this chapter, techniques are described for obtaining spectra from primary cell-wall preparations using CP/MAS, proton-rotating frame, proton spin-spin, spin-echo relaxation spectra, and single-pulse excitation. We also describe how proton spin relaxation editing can be used to obtain subspectra for cell-wall polysaccharides of different mobilities.

Solid-state 13C-NMR spectroscopy shows that the xyloglucans in the primary cell walls of mung bean (Vigna radiata L.) occur in different domains: a new model for xyloglucan-cellulose interactions in the cell wall.

Xyloglucans (XG) with different mobilities were identified in the primary cell walls of mung beans (Vigna radiata L.) by solid-state 13C-NMR spectroscopy. To improve the signal:noise ratios compared with unlabelled controls, Glc labelled at either C-1 or C-4 with 13C-isotope was incorporated into the cell-wall polysaccharides of mung bean hypocotyls. Using cell walls from seedlings labelled with d-[1-13C]glucose and, by exploiting the differences in rotating-frame and spin-spin proton relaxation, a small signal was detected which was assigned to Xyl of XGs with rigid glucan backbones. After labelling seedlings with d-[4-13C]glucose and using a novel combination of spin-echo spectroscopy with proton spin relaxation-editing, signals were detected that had 13C-spin relaxations and chemical shifts which were assigned to partly-rigid XGs surrounded by mobile non-cellulosic polysaccharides. Although quantification of these two mobility types of XG was difficult, the results indicated that the partly-rigid XGs were predominant in the cell walls. The results lend support to the postulated new cell-wall models in which only a small proportion of the total surface area of the cellulose microfibrils has XG adsorbed on to it. In these new models, the partly-rigid XGs form cross-links between adjacent cellulose microfibrils and/or between cellulose microfibrils and other non-cellulosic polysaccharides, such as pectic polysaccharides.