A Study of Phenolic-Carbohydrate Linkages in the Gramineae

Wallace, Graham (1989) A Study of Phenolic-Carbohydrate Linkages in the Gramineae. PhD thesis, University of Glasgow.

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The complex phenolic polymer lignin is generally accepted as having a strong influence over the biodegradability of plant cell walls. The precise mechanisms of this phenomenon, however, are not well understood and a number of factors may be of particular importance i.e the size and structure of the lignin molecules, their distribution within the cell wall and their interactions with other cell wall components as lignin-carbohydrate complexes. To date, relatively little is known about these factors. The purpose of this work was to study the interactions between phenolics and carbohydrate in graminaceous cell walls. Barley straw (Hordeum vulgare c. v Golden Promise) and a 'non-mesophyll' preparation of perennial ryegrass (Lolium perenne c. v. Perma) cell walls were subjected to the fractionation scheme demonstrated in Figure 1. The carbohydrate and phenolic components of the resulting fractions were analysed by a number of 'classical' wet chemical techniques and several physical methods (Pyrolysis-MS (and GC-MS); NMR and Infrared spectroscopy). It was found that oxalic acid hydrolysis solubilised much of the arabinose and ferulic acid content of both. A high proportion of the phenolic content (phenolic acids and lignin) was solubilised from ryegrass, whereas much less was solubilised from barley straw. Fragments, consisting of ferulic acid ester linked through the O-5 of arabinose and p-coumaric acid ester linked to arabinose were isolated and characterised. Extraction of the hydrolysed materials with dimethyl sulphoxide (DMSO) was found to solubilise a high molecular weight lignin- carbohydrate complex, the carbohydrate component consisting, predominantly, of a beta 1-4 xylan with small amounts of mixed linked beta D-glucan. The LCC's differed considerably in their response to alkali hydrolysis, suggesting differences in content of ester and ether linked phenolics. Enzymic hydrolysis of the residues with a commercial cellulase preparation, 'Driselase' was then found to solubilise much of the remaining sugar to leave a lignin 'core'. Solid state NMR also suggested differences in ester/ether linked phenolics between barley and ryegrass. Analytical pyrolysis techniques were then used for further structural analysis. Time resolved platinum filament pyrolysis-mass spectrometry (electron impact) suggested differences in the structure of the lignin moieties in the DMSO soluble and residue fractions. These differences were characterised by thioacidolysis analysis, which showed that the DMSO soluble lignin (type 1 lignin) consists of either small and/or highly branched molecules with a high condensed lignin content (90%), and is associated with a pentosan fraction whereas the residual lignin (type 2 lignin), in ryegrass at least, is either large and/or straight chained lignin, have a much lower condensed lignin content (59%) and is associated with a pentosan/hexosan fraction. It is also suggested that two different xylan components are present in the starting materials. It was suggested that the two different lignin moieties may have originated from different areas within the cell wall, type 1 lignin originating from the secondary cell wall and type 2 lignin from primary layers of secondary thickened cell walls.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Plant sciences
Date of Award: 1989
Depositing User: Enlighten Team
Unique ID: glathesis:1989-78004
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 30 Jan 2020 15:44
Last Modified: 30 Jan 2020 15:44
URI: https://theses.gla.ac.uk/id/eprint/78004

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