Characterization of some cell wall components of untreated and ozone-treated cotton stalks

Untreated (CS) and ozone-treated (OCS) cotton stalks were studied to determine the effects of ozonation on the content of water-soluble phenolics, on the chemical features of cell wall (CW) matrix complexes (MC), and on in situ structural and chemical properties as reflected in solid-state cross-polarization/magic angle spinning (CP/MAS) 13C NMR spectroscopy. The concentrations of water-soluble total monomeric phenolics were 20.1 and 48.2 mg/100 g of dry matter in the CS and OCS materials, respectively. The major increase was in protocatechuic acid (PA), whose concentration increased nearly 10-fold. Matrix complexes extracted by N NaOH from neutral detergent fiber (NDF) of CS and OCS pretreated by ball-milling plus Trichoderma reesei cellulase were analyzed by high-performance size exclusion chromatography (HPSEC) to assess the molecular weight distribution. Two-thirds of the MC of CS were in the molecular weight range 800-10 000 with a weight-average molecular weight (Mw) of 8500. The MC isolated from OCS showed a more dispersed pattern, with a higher proportion of molecules in the range higher than 10 000 and a (Mw) of 15 000. The MC isolates from CS and OCS contained 54% and 39% carbohydrates, respectively, with xylose as the major component and uronic acid as the major potentially branching unit. Solid-state CP/MAS 13C NMR spectroscopy of NDF preparations of CS and OCS showed that the 89 and 65 ppm peaks (the 'crystallinity peaks') were affected by ozonation. The intensity of the signals found in the 160-110 ppm chemical region, assigned to aromatic carbons, declined. The quantitative data on carbon distribution in the various chemical entities of the NDF samples analyzed by solid-state CP/MAS 13C NMR showed a decline in the proportion of aromatic carbons from 13% in CS to 7.4% in OCS. This is the first evidence to show that CS lignin degradation by ozone is mediated via ring cleavage.